Progression and transformation of chronic lymphocytic leukemia/small lymphocytic lymphoma and B-cell prolymphocytic leukemia: Report from the 2021 SH/EAHP Workshop

Abstract

Objectives

Session 3 of the 2021 Workshop of the Society for Hematopathology/European Association for Haematopathology examined progression and transformation of chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL) and B-cell prolymphocytic leukemia (B-PLL).

Methods

Thirty-one cases were reviewed by the panel. Additional studies such as immunohistochemistry and molecular genetic testing, including whole-exome sequencing and expression profiling, were performed in select cases.

Results

Session 3 included 27 CLL/SLL cases and miscellaneous associated proliferations, 3 cases of B-PLL, and 1 case of small B-cell lymphoma. The criteria for ­accelerated CLL/SLL are established for lymph nodes, but extranodal disease can be diagnostically challenging. Richter transformation (RT) is a broad term and includes true transformation from original CLL/SLL clone(s) and clonally unrelated neoplasms. The morphologic, immunophenotypic, and genetic spectrum is diverse with classical and highly unusual examples. T-cell proliferations can also be encountered in CLL/SLL. B-cell prolymphocytic leukemia is a rare, diagnostically challenging disease due to its overlaps with other lymphoid neoplasms.

Conclusions

The workshop highlighted complexity of progression and transformation in CLL/SLL and B-PLL, as well as diagnostic caveats accompanying heterogeneous presentations of RT and other manifestations of disease progression. Molecular genetic studies are pivotal for diagnosis and determination of clonal relationship, and to predict response to treatment and identify resistance to targeted therapy.

Key Points.

• Histologic criteria for accelerated chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL) are defined for lymph nodes and best applied to excision or incisional biopsy, whereas accelerated CLL/SLL is challenging to diagnose in extranodal sites.

• Richter transformation is a broad term and includes true transformation derived from an original malignant clone(s) and unrelated neoplasms arising de novo, a distinction with a prognostic impact.

• Genetic testing allows us to differentiate between related and de novo transformation, as well as determine prognosis and therapy resistance, and therefore is required for optimal patient care.

INTRODUCTION

Chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL) is the most common chronic leukemia in the Western hemisphere, with an age-adjusted incidence of 4.9 cases per 100,000.^1,2 In the white population, the disease affects middle-age and elderly individuals, with a median age of 70 years at diagnosis. The incidence and median age at presentation can be lower in other ethnic groups.³ In general, the incidence increases with age, and family members of patients with CLL/SLL are at higher risk related to genetic susceptibility.⁴ The CLL and SLL are considered different presentations of a single entity with minimal clinicobiological differences when patients with advanced-stage CLL are studied.⁵ Chronic lymphocytic leukemia is defined by 5 × 10⁹/L or more monoclonal B cells with CLL immunophenotype in peripheral blood and with bone marrow involvement, and SLL primarily presents with lymphadenopathy with absent or minimal lymphocytosis and no cytopenias related to bone marrow involvement. Recommended initial diagnostic testing includes morphologic or histopathologic examination; immunophenotyping; fluorescence in situ hybridization (FISH) to determine the presence of trisomy 12, del(11q), del(13q), and del(17p); and genetic testing to evaluate TP53 status and somatic hypermutations of the variable regions of immunoglobulin heavy chains (IGHV).^6-8 The latter allows the division of CLL/SLL into 2 groups: IGHV mutated and IGHV unmutated. The most widely accepted hypothesis is that the IGHV-mutated group is derived from germinal center cell–experienced memory B cells, and IGHV-unmutated CLL arises in either naive or germinal center–independent memory B cells.⁹ The recommended diagnostic testing facilitates a diagnosis and also allows us to predict a response to chemoimmuno- and targeted therapy. Nevertheless, a significant heterogeneity exists even within patient groups defined by the markers discussed above. Not surprisingly, prediction of disease biology and patient outcome can be improved by immunogenetic analysis of B-cell receptor (BCR). The BCR plays a critical role in the pathogenesis of CLL/SLL, as monoclonal B cells are dependent on BCR signaling upon engagement by foreign antigens or autoantigens or through homotypic interactions.^10-12 In a proportion of patients, CLL cells show a remarkable BCR restriction, so-called stereotypic BCRs, which correlate with distinct clinicopathologic profiles and suggest that an exposure to an antigen may play a role in the pathogenesis and progression of the disease.^13,14 Approximately 40% of patients can be assigned to one of the immunogenetic groups that show homogeneous disease biology, clinical presentation, and outcome including response to therapy and risk of transformation to higher-grade disease.^12,15 Therefore, it has been recently recommended to add immunogenetic analysis to the diagnostic workup of patients with CLL/SLL.

The evolution of CLL/SLL starts with an asymptomatic precursor stage, monoclonal B-cell lymphocytosis (MBL) Figure 1. It has been hypothesized that persistent stimulation by foreign antigens or autoantigens leads to polyclonal expansion, stepwise acquisition of chromosomal abnormalities and recurrent mutations, and transitioning to an oligoclonal/monoclonal state.¹⁴ Monoclonal B-cell lymphocytosis is defined as less than 5 × 10⁹/L of monoclonal B cells in peripheral blood in the absence of lymphadenopathy or organomegaly (on physical examination or imaging) or disease-related cytopenias or symptoms. Using routine flow cytometry, MBL is detected in 3% to 20% of healthy individuals worldwide, and its frequency increases with age. A similar frequency of tissue-based MBL has been reported by Habermehl et al.¹⁶ The immunophenotype of the MBL varies; however, the CLL-like MBL occurs more frequently in individuals of European descent.^17-22 The genetic abnormalities, including driver mutations, are similar in MBL and CLL, but the frequency of genetic aberrations and the number of cases with stereotyped BCRs are lower in MBL. The progression to overt CLL correlates with expansion of clones preexisting in MBL.²³ Essentially, all patients with CLL are shown to harbor MBL clones prior to overt presentation, and the progression to CLL is partially dependent on the monoclonal B-cell count.²⁴ Individuals with high-count MBL (>500/μL) progress to CLL at a higher rate of 1% to 5% per year compared with low-count MBL, which shows progression at 1% to 2% per year.^25-27 Moreover, in the general population, low-count MBL may be stable over extended periods of time.²⁸ Therefore, it remains to be proven if low-count MBL represents a step in the progression to CLL or is a signature of immunologic senescence linked to chronic antigenic stimulation.

FigURE 1.

Evolution and progression of chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL): persistent antigenic stimulation leads to polyclonal expansion and through acquisition of genetic lesions to low- and high-count monoclonal B-cell lymphocytosis (MBL). Low-count MBL may remain stable over time or progress to high-count MBL, an immediate precursor to CLL/SLL. Further evolution may include accelerated-phase/prolymphocytic progression and/or Richter transformation (RT) to diffuse large B-cell lymphoma (DLBCL), high-grade B-cell lymphoma (HGBCL), classic Hodgkin lymphoma (cHL), and other less common forms of transformation such as plasmablastic lymphoma and B-lymphoblastic leukemia/lymphoma (adapted from Vardi et al¹⁴).

The natural history of untreated CLL has been shown to follow 2 patterns: a logistic growth with a lower number of clonal and subclonal drivers seen more commonly in the IGHV-mutated cases and an exponential growth with higher numbers of driver events and with more significant changes in clonal fractions.²⁹ Not surprisingly, patients with an exponential growth pattern required therapy more frequently than those characterized by a logistic growth. Clonal evolution is also appreciated following treatment, either in a form of clonal selection for chemotherapy-resistant clones, such as those with abnormalities in DNA repair pathways, TP53 and ATM, or with mutations in therapeutic targets, such as the Bruton tyrosine kinase (BTK) gene, BCR signaling pathway (PLCG2), or BCL2 homology region 3 (BH3), in patients receiving ibrutinib and venetoclax, respectively.^29-33 Select subclones have been shown to exist in pretreatment samples and can expand or acquire a second hit under therapy pressure.²⁹

A proportion of patients with CLL/SLL experience a significant progression or transformation of the disease. The most common, seen in approximately 10% of patients with CLL/SLL, is the transformation to a lymphoma with a more aggressive phenotype, typically diffuse large B-cell lymphoma (DLBCL) or classic Hodgkin lymphoma (cHL). This phenomenon is referred to as Richter transformation (RT) and was originally described by an American pathologist, Maurice Richter, nearly 100 years ago. Richter transformation can occur in untreated and treated patients, and it can be clonally related to original CLL/SLL. The DLBCL type of RT has to be differentiated from accelerated-phase CLL/SLL. Other patterns of RT include high-grade B-cell lymphoma, plasmablastic lymphoma, and B-lymphoblastic leukemia/lymphoma. T-cell lymphomas have also been described in patients with CLL/SLL and more likely represent independent neoplasms arising in a setting of immune dysfunction, immunosuppression, or genetic predisposition. In session 3 of the 2021 Workshop of the Society for Hematopathology/European Association for Haematopathology, we discussed common and uncommon forms of CLL/SLL and B-cell prolymphocytic leukemia (B-PLL) progression and transformation, as well as their histologic and clinical mimickers Table 1, Table 2, Table 3, and Table 4.

TABLE 1.

Accelerated Chronic Lymphocytic Leukemia/Small Lymphocytic Lymphoma and Classical Examples of Richter Transformation

Case No.	Age, y/Sex	Panel diagnosis	Interesting features
Accelerated CLL/SLL
245	56/F	Accelerated CLL/SLL (lymph node)	Highly complex karyotype with MYC alterations and a complete TP53 inactivation, unmutated IGHV in a case of accelerated CLL/SLL. Patient responded to ibrutinib and R-venetoclax.
241	69/M	Accelerated CLL/SLL (skin)	Multiple cutaneous lesions showing aCLL/SLL, IGHV unmutated. Peripheral blood with numerous prolymphocytes. Despite several poor prognostic factors, the patient responded well to venetoclax with resolution of skin lesions and no evidence of CLL/SLL in bone marrow.
Classical Richter transformation
152	72/M	RT-DLBCL	DLBCL morphology in lymph node in a patient with IGHV-unmutated CLL/SLL; peripheral blood and bone marrow showing both small- to medium-sized and large neoplastic lymphoid cells. Highly complex karyotype; del(17p13.1) and TP53 mutation detected at the time of RT likely contributed to disease progression.
219	54/M	RT-DLBCL	Stepwise acquisition of genetic alterations, including BTK C481S, loss of CDKN2A, NOTCH1 rearrangement, and t(2;12)(p12p13.3), IGK::CCND2. The latter is a recurrent alteration described primarily in cyclin D1–negative mantle cell lymphoma and is exceedingly rare in RT.
115	65/M	RT-HGBCL, DH	RT with diffuse large B-cell lymphoma morphology and double-hit MYC and BCL2 rearrangements in a patient with a prior diagnosis of unmutated IGHV CLL.
19	80/F	RT-HGBCL, DH	Leukemic presentation of double-hit lymphoma with MYC and BCL6 rearrangements without extramedullary involvement. CLL/SLL population was present in a background.
206	79/F	1.RT-HGBCL 2.RT-HGBCL	Progression of λ light chain–restricted CLL to 2 distinct λ light chain–restricted B-cell lymphomas with distinct immunophenotype and MYC abnormalities. Both lymphomas showed private and shared mutations. The latter included genes involved in DNA repair such as TP53 and MRE11. The heavy chain usage was identical in samples 1 and 2.
103	51/F	High-grade blastic transformation with plasmacytoid dendritic cell and T-cell immunophenotype	Aggressive leukemic transformation with unusual immunophenotype during ibrutinib maintenance, in a patient who previously experienced typical RT. Neoplastic cells showed plasmacytoid dendritic cell and T-cell markers and shared the same IgH gene rearrangement with the CLL clone.
62	75/M	EBV reactivation in CLL/SLL with morphology of EBV+ DLBCL	EBV reactivation in an immunocompromised patient with CLL/SLL manifesting as EBV+ DLBCL.
41	45/F	CLL with focal EBV reactivation	Focal areas of necrosis and EBV positivity in a patient with CLL/SLL, most likely related to immunosuppression.
122	50/M	RT-cHL, type I	History of cHL (nodular sclerosis, cervical lymph node) and treatment-refractory CLL/SLL prior to the current presentation. Diagnosis most consistent with EBV-positive type I RT-cHL in a retroperitoneal lymph node. Molecular genetic studies to demonstrate clonal relationship to the original CLL/SLL would be helpful.
127	75/M	RT-cHL, type I	Prior diagnosis of untreated CLL/SLL. Patient presented with EBV-positive type I RT-cHL in an inguinal lymph node and was treated with radiotherapy. Three years later presented with recurrence of type I RT-cHL in a cervical lymph node and was treated with systemic immunochemotherapy.
142	54/M	RT-cHL with progression from type I to type II	Progression from type I to type II morphology in RT-cHL. EBER in situ hybridization was negative, whereas LMP1 immunostain was positive in HRS cells.
163	73/M	cHL, most likely types I and II	Simultaneous presentation with EBV-positive RT-cHL involving lymph nodes and bone marrow, with the latter showing a type II pattern. This case illustrates difficulties in RT-cHL pattern assignment in core needle biopsies.
93	57/M	cHL, type II focal (associated with necrosis)	Untreated biclonal CLL/SLL with transformation to EBV-positive RT-cHL, clonally related per IGH gene rearrangement. Lambda light chain–restricted CLL clone expanded over time and gave rise to RT, as demonstrated by the HRS cells positive for λ light chain.
183	72/M	RT-cHL transforming from CLL/SLL with subclonal cyclin D1 rearrangement	Initial core needle biopsy specimen suspicious for cHL. Lymph node excision demonstrated EBV-negative cHL and SLL. Three discrete clonal B-cell populations were seen by flow cytometry, including one with CLL/SLL-type immunophenotype. CCND1 rearrangement was demonstrated by FISH in a minor subset of the CLL/SLL clone. IgH/CCND1 translocation in CLL is rare and is more commonly reported after alkylating agent therapy.
78	76/F	Coexistent RT-DLBCL and RT-cHL, type II	Previously treated unmutated IGHV CLL (ibrutinib and venetoclax), presenting with lymphadenopathy and lytic lesions in axial and appendicular skeleton. RT-DLBCL in the occipital soft tissue biopsy specimen and both RT-cHL and RT-DLBCL in bone marrow. Sequencing performed on occipital biopsy and bone marrow biopsy specimens showed different IGH-VDJ usage and 2 different DLBCL subtypes (GCB, genetic cluster 3 and ABC, genetic cluster 5, respectively).
73	71/F	RT-DLBCL and RT-cHL, type II	Unmutated IGHV CLL, previously treated, transforming to EBV-negative RT-cHL presenting as multifocal bone lesions with subsequent nodal RT-DLBCL and recurrent nodal RT-cHL.

aCLL, accelerated chronic lymphocytic leukemia; cHL, classic Hodgkin lymphoma; CLL/SLL, chronic lymphocytic leukemia/small lymphocytic lymphoma; DH, double-hit; DLBCL, diffuse large B-cell lymphoma; EBV, Epstein-Barr virus; FISH, fluorescence in situ hybridization; HBGCL, high-grade B-cell lymphoma; HRS, Hodgkin and Reed-Sternberg; RT, Richter transformation.

TABLE 2.

Uncommon Examples of CLL/SLL Transformation

Case No.	Age, y/Sex	Panel diagnosis	Interesting features
62	75/M	EBV reactivation in CLL/SLL with morphology of EBV+ DLBCL	EBV reactivation in an immunocompromised patient with CLL/SLL manifesting as EBV+ DLBCL.
185	79/F	RT-PBL	Concurrent presentation of CLL/SLL with plasmacytic differentiation and lymph node–based RT-PBL diagnosed on pleural effusion. No bone lesions detected. Shared mutations (TP53 and KMT2D) were detected in bone marrow (lower VAFs) and RT-PBL, but IGH::MYC was only seen in PBL. Partial response to myeloma-type treatment.
174	63/M	Plasmablastic myeloma	Previously treated longstanding CLL presenting with new-onset renal failure, IGM κ monoclonal protein, and lymphadenopathy. Bone marrow showed plasmablastic plasma cell myeloma with IGH::MYC. IGH gene rearrangement studies suggested a clonal relationship between CLL and plasmablastic myeloma.
37	68/M	Burkitt-like lymphoma with 11q aberration	Unusual transformation consistent with Burkitt-like lymphoma with 11q aberration showing a clonal relationship with original CLL (IGH gene rearrangement studies). NGS of CLL and RT sample showed numerous shared and private mutations. Private mutations acquired in RT included TP53, DDR2, and PIK3R1.
143	88/F	CLL/SLL and subsequent B-ALL	Presentation of B-lymphoblastic leukemia/lymphoma with IGH::MYC in a soft tissue mass. CLL/SLL and B-ALL did not share trisomy 12 or MYC::IGH abnormalities but showed similar IGHV usage.
217	77/M	CLL/SLL transformation to B-ALL	Clonally related transformation to B-ALL in a patient treated with duvelisib. Clonal relationship confirmed by IGH gene rearrangements studies, IGVH sequencing, and shared somatic mutations.
103	51/F	High-grade blastic transformation with plasmacytoid dendritic cell and T-cell immunophenotype	Aggressive leukemic transformation with unusual immunophenotype during ibrutinib maintenance, in a patient who previously experienced typical RT. Neoplastic cells showed plasmacytoid dendritic cell and T-cell markers and shared the same IgH gene rearrangement with the CLL clone.

B-ALL, B-lymphoblastic leukemia/lymphoma; CLL/SLL, chronic lymphocytic leukemia/small lymphocytic lymphoma; DLBCL, diffuse large B-cell lymphoma; EBV, Epstein-Barr virus; NGS, next-generation sequencing; PBL, plasmablastic lymphoma; RT, Richter transformation; VAF, variant allele frequency.

TABLE 3.

T/NK-Cell Proliferations in a Setting of CLL/SLL and Miscellaneous Workshop Cases

Case No.	Age, y/Sex	Panel diagnosis	Interesting features
T/NK-cell proliferations associated with CLL/SLL
47	57/M	Atypical T/NK-cell proliferation in a setting of CLL/SLL	Patient with refractory CLL/SLL presenting with clinical features concerning for hemophagocytic syndrome. Bone marrow demonstrated CLL/SLL and large highly atypical lymphoid cells, most likely NK cells, which persisted after therapy over a period of several months.
223	75/M	ALK-negative ALCL and residual CLL/SLL	Unusual case of ALK–, ALCL arising in a setting of CLL/SLL. Co-occurrence of T-cell lymphomas, and CLL/SLL has been rarely reported.
Miscellaneous cases
41	45/F	CLL with focal EBV reactivation	Focal areas of necrosis and EBV positivity in a patient with CLL/SLL, most likely related to immunosuppression.
162	85/F	Small B-cell lymphoma with aggressive genetic features	Morphologic and immunophenotypic features consistent with small B-cell lymphoma in a patient with an aggressive clinical course. Complex karyotype including IGH::ETV6 rearrangement, high mutational burden.
250	89/M	Composite lymphoma, CLL/SLL, and mantle cell lymphoma	Patient with a prior diagnosis of stable untreated CLL/SLL presenting with progressive lymphocytosis, anemia, and thrombocytopenia. Development of second, clonally unrelated lymphoma, mantle cell lymphoma with leukemic presentation.
258	76/M	Therapy-related acute leukemia	Morphologic features consistent with acute leukemia, likely therapy related. Additional immunophenotyping recommended to confirm immunophenotype.

ALCL, anaplastic large cell lymphoma; CLL/SLL, chronic lymphocytic leukemia/small lymphocytic lymphoma; EBV, Epstein-Barr virus; NK, natural killer.

TABLE 4.

B-Cell Prolymphocytic Leukemia Cases

Case No.	Age, y/Sex	Panel diagnosis	Interesting features
256	66/M	Consistent with B-PLL	Morphologic features suggestive of B-PLL, unusual immunophenotype. Bone marrow presentation is clonally related to blastoid proliferation seen in lymph nodes.
66	68/M	B-PLL	Presentation with both peripheral blood and lymph node involvement. Complex karyotype with t(8;22)(q24;q11.2), trisomy 12, gains of BCL2 and BCL6, and MYC variant.
201	59/F	B-PLL with clonal evolution and acquired resistance to ibrutinib	Morphologic progression to large cell morphology and clonal evolution with increased VAF of TP53 R248W, MYD88 L265P (along with increased copy number), and TP63 R337Q and acquisition of several mutations enhancing BCR signaling, proliferation, and conferring BTK resistance (BTK C481S). Decreased CD58 expression seen late in the disease may be linked to immune evasion.

BCR, B-cell receptor; B-PLL, B-cell prolymphocytic leukemia; VAF, variant allele frequency.

CLL/SLL IN ACCELERATED PHASE

It has long been appreciated that proliferation centers play an important role in the biology of CLL/SLL, and expanded proliferation centers have been historically considered a harbinger of poor outcome.^34-37 Nevertheless, the features of proliferation centers associated with adverse outcome were not described until 2010. The CLL/SLL with prominent proliferation centers, termed accelerated CLL/SLL (aCLL/SLL), has been defined for nodal sites and includes a presence of prominent proliferation centers broader than a 20× field or showing high proliferative activity (>2.4 mitoses or >40% Ki67 proliferation rate per proliferation center).³⁵ Despite well-delineated criteria, the accelerated phase may be challenging to diagnose, and alternative approaches, such as artificial intelligence–assisted mapping of proliferation centers, have been suggested to aid in diagnosis.^38-40 In peripheral blood, most CLL cases present with less than 15% prolymphocytes, and a cutoff of 15% prolymphocytes has been suggested as a prolymphocytic progression.^41,42 Whereas the exact prolymphocyte percentage defining prolymphocytic progression may be still discussed, it is well accepted that higher numbers of prolymphocytes are associated with adverse outcome, including shorter progression-free and overall survival.⁴³

Workshop cases 245 and 241 illustrate challenges in making a diagnosis of aCLL/SLL Table 1. The patient in case 245 was a 56-year-old woman with a prior diagnosis of de novo diffuse large B-cell lymphoma treated with R-CHOP (rituximab, cyclophosphamide, doxorubicin, vincristine and prednisone). She experienced a relapse with an axillary lymph node biopsy specimen showing aCLL/SLL Figure 2. Review of prior diagnosis of DLBCL was confirmed to represent aCLL/SLL. Evaluation of bone marrow and peripheral blood smear concurrent to the axillary lymph node biopsy specimen showed a CLL immunophenotype with medium- to large-sized lymphoid cells with round to oval nuclei, open chromatin, and prominent nucleoli. Cytogenetic and molecular genetic studies showed a complex karyotype with deletion of chromosome 17p, multiple 8q24 regions with extra copies of nonrearranged MYC, heterozygous deletion of 13q14.3, high-level amplification of rearranged MYC gene, and unmutated IGHV gene. Both lymph node and peripheral blood sample showed TP53 mutation and TP53 deletion consistent with a complete TP53 inactivation. The patient responded to ibrutinib and R-venetoclax, which has been previously reported to be effective in patients with aCLL/SLL. This case illustrates several points, including the importance of an accelerated-phase diagnosis, which is managed differently from typical RT. Lymph node excision or incisional biopsy is critical, as the application of diagnostic criteria for aCLL/SLL is more challenging with the limited material available in core needle biopsy specimens and is not applicable to fine needle aspiration.³⁷ Last, the definition of accelerated phase is only applicable to lymph nodes; therefore, concurrent bone marrow and peripheral blood evaluation was challenging in this case, considering the unclear relationship between aCLL/SLL in lymph nodes and increased prolymphocytes in peripheral blood and bone marrow. Similar diagnostic challenges can be encountered in extranodal sites, as demonstrated in case 241, which showed cutaneous CLL/SLL involvement with increased medium-sized lymphoid cells with high proliferative activity in a 69-year-old man with a longstanding history of treated CLL/SLL Figure 2. Molecular studies performed on peripheral blood showed unmutated IGHV, IGHV4-34 family, and 2 pathogenic NOTCH2 mutations, features associated with poor prognosis. Considering morphologic features and high proliferative activity, this case most likely represents aCLL/SLL; nevertheless, diagnostic criteria for accelerated phase are not defined for extranodal sites.

FigURE 2.

Accelerated-phase chronic lymphocytic leukemia/small lymphocytic lymphoma. A, Case 245: lymph node showing expanded proliferation centers composed of prolymphocytes and paraimmunoblasts. Sheets of large lymphoid cells are not identified. B, Case 241: dense lymphoid proliferation of medium-sized lymphoid cells in the dermis. C, Case 241: higher magnification shows predominantly medium-sized lymphoid cells with irregular nuclei and relatively open chromatin. D, Case 241: high proliferative activity demonstrated by Ki67 immunohistochemical stain.

CLASSICAL RICHTER TRANSFORMATION

Richter transformation occurs in 2% to 10% of patients with CLL/SLL, with a frequency of approximately 1% per year dependent on the patient population investigated.^44-46 It occurs primarily in patients treated with immunochemotherapy, with a median time of 4 years from the original CLL/SLL diagnosis, and is characterized by a dismal survival of less than 1 year. Richter transformation is typically suspected in patients with rapidly progressive lymphadenopathy, B symptoms, increased lactate dehydrogenase (LDH), and elevated C-reactive protein in the absence of infection. Histologic confirmation of RT is pivotal, and positron emission tomography/computed tomography–guided biopsy should be performed in all cases, even in patients with an extremely high maximum standardized uptake value.^47,48 Fine needle aspiration is not recommended.^37,49,50 Mimickers of RT include infections and pseudoprogression on immunotherapy.

The most common histologic types of RT comprising 90% of cases are diffuse large B-cell lymphoma (RT-DLBCL) and high-grade B-cell lymphoma (RT-HGBCL).⁵⁰ Classic Hodgkin lymphomas represent 10% of RT cases.⁵¹ Most cases of RT-DLBCL are of activated B-cell type, while approximately 20% show a germinal center cell–like immunophenotype.⁵² Approximately 70% to 80% of RT-DLBCLs are clonally related to CLL/SLL, as determined by identical IGH-VDJ gene rearrangements. The remaining cases of DLBCL arising in patients with CLL/SLL are clonally unrelated and most likely represent de novo disease. The determination of a clonal relationship between CLL/SLL and transformed disease is clinically significant due to differences in prognosis and management. Clonally related RT-DLBCL represents particularly aggressive disease frequently resistant to chemotherapy.^53,54

Clinical and laboratory parameters associated with a higher risk of RT include higher-stage disease, lymphadenopathy, and high LDH. Genetic risk factors include inactivation of TP53 and CDKN2A genes, NOTCH1 mutations, trisomy of chromosome 12, 11q deletion, complex karyotypes, unmutated IGHV status, and stereotyped BCR subset 8. Not surprisingly, RT is associated with clonal evolution, including increased number of mutations, copy number changes, and structural alterations.^55,56 Most patients with RT harbor mutations in genes involved in proliferation/cell cycle regulation, chromatin modification, and MYC, nuclear factor (NF)–κB, and NOTCH pathways.⁵⁷TP53 and CDKN2A/B abnormalities are seen in 50% of patients, and NOTCH1 mutations and MYC abnormalities are seen in 30% each. These abnormalities can also be seen in CLL/SLL cells prior to RT; however, loss of CDNK2A/B is most often acquired during transformation. Interestingly, RT clones can be detected at the time of original diagnosis of CLL/SLL and expand under a pressure of therapy.^55,56 Conversely, preliminary studies of unrelated RT-DLBCL show an absence of DLBCL clones in the CLL/SLL phase.⁵⁸

The workshop included a spectrum of RT cases illustrating diverse clinical and histologic presentations and genetic abnormalities Tables 1 and 2. There were 2 classical DLBCLs (cases 152 and 219) and 5 HGBCLs (cases 37, 115, 206, 19, and 103) Figure 3. Case 152 represented a typical RT-DLBCL in a cervical lymph node of a 72-year-old man with a 3-year history of IGHV-unmutated CLL/SLL with no abnormalities detected by CLL FISH panel. The patient was treated with bendamustine and rituximab. Upon transformation, a complex karyotype with loss of 17p and TP53 Y234S mutation was detected in peripheral blood and bone marrow. The complex karyotype showed overlapping clones suggestive of clonal evolution. Case 219 illustrated a stepwise development of resistance to immunochemotherapy and acquisition of genetic lesions in a ­patient with a longstanding CLL/SLL. After 7 years of watchful waiting, the patient required therapy with ibrutinib, at which time, trisomy 12 and deletion of 17p13 were demonstrated. Treatment with venetoclax was initiated upon resistance to ibrutinib. The RT-DLBCL diagnosed on bone marrow examination showed BTK C481S, loss of CDKN2A, NOTCH1 rearrangement, and t(2;12)(p12;p13.3)/IGK-CCND2, with a high mutational burden of 5.6 mutations per megabase Figure 3. The CCND2 rearrangements are rarely seen in RT and have been previously reported in 1 case.⁵⁹ The constellation of genetic changes seen in this case is typical for patients treated with ibrutinib and, with the exception of BTK mutation, overlaps with those seen after conventional immunochemotherapy. BTK C481S mutations prevent ibrutinib binding, leading to continuing BCR and NF-κB signaling. In case 219, resistance to ibrutinib manifested clinically 2 years prior to RT. In general, BTK mutations occur frequently in ibrutinib-treated patients, and their contribution to RT requires further study. Another pattern of progression and a potential diagnostic pitfall can be seen upon cessation of ibrutinib therapy. Several reports described a disease flare with DLBCL morphology upon ibrutinib withdrawal.^60-62 This has been attributed to an expansion of B-cell clones released from BCR inhibition and typically responds to reintroduction of ibrutinib.

FigURE 3.

Morphologic spectrum of Richter transformation (RT). A, Case 219: RT presenting as diffuse large B-cell lymphoma in bone marrow. Note blastoid morphology. B, Case 219: bone marrow aspirate smear. C, Case 115: RT with MYC and BCL2 rearrangements showing immunoblastic morphology. D, Case 206: high-grade B-cell lymphoma with IGH::MYC rearrangement.

MYC oncogene is another cell cycle–related gene commonly altered in RT. MYC abnormalities frequently coexist with TP53 and CDKN2A in RT and have been reported in 4 additional workshop cases, all with high-grade morphologic features. Two cases of HGBCL were classified as double-hit lymphoma based on FISH demonstrating MYC and BCL2, as well as MYC and BCL6 rearrangements (cases 115 and 19, respectively). Case 115 represented a typical patient with unmutated IGHV CLL treated with obinutuzumab and ibrutinib. Richter transformation was histologically confirmed on a biopsy specimen of a cervical lymph node Figure 3. The patient described in case 19 had a newly diagnosed HGBCL in the bone marrow with a small CLL population in the background. There was no documented history of CLL/SLL, and the evaluation was limited. The RT with double-hit phenotype is uncommon and has been reported in approximately 10% of patients.⁵² Interestingly, neither double/triple-hit nor double expressor status influenced survival. Two additional unusual cases demonstrated MYC rearrangements. The patient in case 206 had a longstanding history of CLL/SLL treated with obinutuzumab and ibrutinib and developed 2 sequential HGBCLs Figure 3. The first presentation of HGBCL with IGH::MYC rearrangement was followed by an HGBCL with deleted MYC. Both lymphomas showed 13q14.3 deletion by FISH, and the original peripheral blood CLL sample was positive for 13q14.3 and 11q22.3 deletions. Per NGS performed during workshop review, both HGBCL samples showed shared and private mutations, as well as identical IGH-VDJ usage, confirming their clonal relationship. The comparison to the original CLL/SLL was not available. An additional patient with a history of CLL and prior RT-DLBCL, case 103, developed blastic transformation with an unusual immunophenotype showing plasmacytoid dendritic cell and T-cell markers. The lesion showed loss of ATM and clonal IGH gene rearrangement identical to the original CLL, as well as MYC rearrangement and additional mutations in BCOR, MYC, and NRAS.

Approximately 25% of RT-DLBCLs show Epstein-Barr virus (EBV) expression, but the reported frequency varies.^52,63 These cases display latency program II or III, are more frequently derived from IGHV-mutated CLL, and are not clonally related to the ­original CLL. The involvement of EBV is not surprising considering immune dysfunction associated with CLL/SLL and frequent immunosuppressive therapy. Monitoring of EBV levels has been suggested to prevent EBV reactivation and development of EBV-positive RT-DLBCL. Two workshop cases illustrated EBV reactivation in patients with CLL/SLL. Case 62 with morphology of RT-DLBCL showed positivity for EBV, including intermediate to early BZLF1 lytic protein indicative of intratumoral EBV replication Figure 4. Epstein-Barr virus levels by quantitative polymerase chain reaction significantly decreased after therapy with rituximab. Case 41 represented an axillary lymph node with CLL/SLL immunophenotype and focal areas of necrosis (<5% of the biopsy specimen) associated with EBER positivity Table 3. The clinical information for this case was limited; nevertheless, the diagnosis was most consistent with CLL/SLL with focal EBV reactivation. Interestingly, several oncogenic mutations (HOXB13, NOTCH1, NUP214, SMARCE1, TFRC) and IGH::MYC rearrangement were detected by NGS.

FigURE 4.

Epstein-Barr virus reactivation in chronic lymphocytic leukemia/small lymphocytic lymphoma (case 62). A, Diffuse effacement of nodal architecture by large lymphoid cells with pleomorphic, immunoblastic features and small lymphocytes with round nuclei, condensed chromatin, and inconspicuous nucleoli. B, BZLF1 immunohistochemical stain. C, EBER in situ hybridization. D, LMP1 immunohistochemical stain.

Epstein-Barr virus has also been shown to play a role in the cHL type of RT. Two morphologic patterns have been described: type I with Hodgkin and Reed-Sternberg (HRS) cells interspersed within a CLL background and type II with HRS cells in a polymorphous background similar to that seen in de novo cHL.⁵¹ The number of HRS cells in type I varies from rare to 20 per high-power field (median, 4); therefore, a close inspection at high power is recommended. Type II RT-cHL can present as scattered foci of cHL within a CLL/SLL background (“moth-eaten” pattern) or as large fields of RT-cHL clearly demarcated from CLL/SLL proliferation (segregated pattern). As illustrated in workshop cases, there is a histologic continuum between types I and II. Both patterns can be clonally related or unrelated to CLL/SLL.^51,64,65 Both types of RT-cHL are typically EBV positive, and their overall survival is similar and improved when treated with ABVD (adriamycin, bleomycin, vinblastine, and dacarbazine)-based regimens.^66,67 Workshop cases illustrated type I and II RT-cHL and included interesting cases demonstrating coexistence or transition between the 2 patterns. Type I with HRS cells displaying a typical cHL immunophenotype and EBV positivity was shown in an inguinal lymph node in case 127 and in a retroperitoneal lymph node in case 122 Figure 5. The latter patient had a history of nodular sclerosis cHL; however, details and timing of the presentation in relation to the CLL/SLL diagnosis were not available. Type I with a synchronous or metachronous pattern II is illustrated in cases 142 and 163. Case 142 demonstrated a transformation of CLL/SLL into type I RT-cHL and subsequently a type II pattern in a 53-year-old man who received intensive chemoimmunotherapy, including chimeric antigen receptor T-cell infusions for CLL/SLL. In case 163, a core needle biopsy specimen of a retroperitoneal lymph node demonstrated a predominantly type I pattern with focal HRS cells with neighboring T cells and epithelioid histiocytes. Increased histiocytes and rare cases with nonnecrotizing granulomata have been previously described in pattern I RT-cHL.⁵¹ Eosinophils and plasma cells were not prominent in the case 163 lymph node sample, and the authors highlighted a difficulty in assigning a pattern in a core needle biopsy sample. Type II RT-cHL was more clearly visualized in the bone marrow in this case. Focal evolution to type II RT-cHL was seen in an interesting patient with biclonal CLL, κ and λ positive, likely due to light chain switch (case 93) Figure 6. A single IGH gene rearrangement proved a common origin of both CLL clones and cHL. The latter originated from a λ light chain–restricted CLL clone, as demonstrated by immunohistochemistry Figure 6. Case 183 demonstrated RT-cHL and 3 distinct clonal B-cell populations, including CLL/SLL with subclonal CCND1 rearrangement. CCND1 rearrangement has been previously reported in CLL cases treated with alkylating agents and is rare at the time of presentation.^68,69 The HRS cells in this case were negative for EBER and for CCND1 rearrangement.

FigURE 5.

Type I Richter transformation classic Hodgkin lymphoma (cHL). A, Generous core needle biopsy specimen facilitated diagnosis (case 122). B, Hodgkin and Reed-Sternberg (HRS) cells interspersed within chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL) population (case 127). C, PAX5 expression in HRS cells and background CLL/SLL (case 122). D, Similar to de novo cHL cases, HRS cells in case 122 were positive for LEF1.

FigURE 6.

Type II Richter transformation classic Hodgkin lymphoma (cHL) arising in biclonal chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL) (case 93). A, Hodgkin and Reed-Sternberg (HRS) cells embedded in a polymorphous background. B, PAX5 is positive in HRS cells and shows a paucity of CLL/SLL cells. C, CD30 immunohistochemical stain highlights HRS cells. D, The λ light chain immunostain is positive in HRS cells (arrows). Clonal relationship with λ light chain–restricted B CLL/SLL lymphocytes was confirmed by IGH gene rearrangement studies.

A rare RT with composite DLBCL and cHL was illustrated in cases 73 and 78. The patient in case 78 had a longstanding history of CLL with unmutated IGHV and deleted 11q22.3 and 13q14 Figure 7. The patient was heavily pretreated, progressing on ibrutinib therapy, and presented with bony lesions and retroperitoneal and pelvic lymphadenopathy. The biopsy specimen of the occipital lesion showed GCB-type DLBCL, genetic subgroup 3,⁷⁰ whereas bone marrow examination showed EBV-negative cHL and focal DLBCL. Sequencing of the bone marrow sample showed a profile consistent with ABC-type DLBCL, genetic cluster 5. Sequencing of the occipital lesion and bone marrow demonstrated private and shared mutations, as well as different IGH-VDJ usage. Altogether, these data suggest RT with 2 distinct DLBCLs and cHL. Case 73 demonstrated transformation into DLBCL (unclassified cell of origin, genetic subgroup 2) and cHL at different time points and sites. Reports of a composite RT including DLBCL and cHL are exceedingly rare.^71,72

FigURE 7.

Composite Richter transformation (RT), diffuse large B-cell lymphoma (DLBCL), and classic Hodgkin lymphoma (cHL) presenting as bone marrow involvement, bony lesions, and lymphadenopathy (case 78). Bone marrow demonstrated involvement by RT-cHL (A) and focal perivascular aggregates of large lymphoid cells consistent with RT-DLBCL (B). C, Occipital lesion was composed of large lymphoid cells with no evidence of RT-cHL.

RICHTER TRANSFORMATION WITH UNUSUAL FEATURES

Plasmablastic lymphomas (PBLs) have rarely been reported as a manifestation of CLL/SLL transformation.^72-76 Most patients have been previously treated for CLL/SLL, but rare untreated cases have also been reported. The immunophenotype is similar to that of de novo PBL with both EBV-positive and EBV-negative cases and frequent MYC rearrangements. Cases of RT-PBL can be clonally unrelated or related to preexistent CLL. It has been hypothesized that EBV-positive cases may be associated with iatrogenic immunosuppression. The workshop case 185 represents an interesting pleura- and lymph node–based plasmablastic transformation of CLL with plasmacytic differentiation. This EBV-negative PBL showed κ light chain restriction and shared TP53 and KMT2D mutations, which were also seen in the bone marrow CLL/SLL, suggestive of a clonal relationship. The IGH::MYC fusion was detected only in PBL. Another patient with a longstanding history of heavily treated CLL/SLL developed lymphadenopathy, renal failure, and a significant IgM κ monoclonal protein (case 174) Figure 8. The peripheral blood and bone marrow showed extensive involvement by plasmablasts positive for IGH::MYC. In addition, flow cytometry showed a background CLL population. The IGH gene rearrangement studies performed on a bone marrow with plasmablastic proliferation and a prior spleen sample involved by CLL suggested a clonal relationship, but trisomy 12 and deletion 17p, seen previously in the CLL clone, were not present in plasmablastic myeloma. The patient responded to dexamethasone, fractionated cyclophosphamide, adriamycin, and bortezomib.

FigURE 8.

Unusual plasmablastic myeloma clonally related to previously diagnosed chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL) (case 174). Plasmablastic proliferation was seen on bone marrow clot section (A) and aspirate smear (B). Similar population was seen in peripheral blood smear (not shown). Neoplastic cells were positive for CD138 (C) and CD56 (D) and negative for CD20 and PAX5 (not shown).

Case 37 included a series of 3 RT cases with features of Burkitt-like lymphoma with 11q aberration (high-grade B-cell lymphoma with 11q aberrations, aka large B-cell lymphoma with 11q aberration).^42,77,78 The patient presented in the workshop was a 68-year-old man with a 10-year history of CLL and current cecal mass. The histology showed a highly proliferative diffuse infiltrate of large atypical lymphoid cells with a germinal center cell–like immunophenotype, negative for MYC, BCL2, and BCL6 rearrangements and with gains of 11q23 and losses of 11q24 demonstrated by FISH Figure 9. The original CLL and current RT showed clones with identical IGH rearrangements. Sequencing of the CLL (bone marrow) and HGBCL sample showed numerous shared and private mutations previously reported in CLL/SLL, DLBCL, and a few seen in Burkitt lymphomas and Burkitt-like lymphomas with 11q aberration. Private mutations acquired in RT included TP53, DDR2, and PIK3R1.

FigURE 9.

Richter transformation presenting as Burkitt-like lymphoma with 11q aberration (case 37). A, Diffuse proliferation of medium- to large-sized lymphoid cells and frequent tingible-body macrophages. B, Fluorescence in situ hybridization demonstrates proximal gain and distal loss of 11q (green 11q23 region, blue 11q24 region, red CEN11).

Transformation of CLL/SLL to B-lymphoblastic leukemia/lymphoma (B-ALL) has been reported in rare cases. It can represent a second, clonally unrelated neoplasm or a clonally related transformation.^79-82 Careful attention to the immunophenotype is recommended to exclude high-grade B-cell lymphoma with TdT expression. The workshop included 1 case of CLL/SLL with subsequent B-ALL with a low-level IGH::MYC rearrangement (case 143) and a case of clonally related CLL and B-ALL occurring in the setting of therapy with a PI3K-δ inhibitor, duvelisib, discussed below (case 217). A 77-year-old patient was diagnosed with CLL/SLL on the lymph node biopsy specimen.⁸³ Further workup demonstrated 11q23 deletion and several pathogenic mutations, including TP53 L265P (variant allele frequency [VAF] 2.0%), TP53 G244D (VAF 4.0%), and ATM M753fs*12 (VAF 7.4%), and a deletion of exons 7 to 9 of the BIRC3 gene. The patient had been treated with duvelisib for nearly 3 years and developed progressive anemia and thrombocytopenia. The bone marrow examination demonstrated B-ALL with TP53 L265P mutation (VAF 87.8%), TP53 G244D (VAF 2.0%), CXCR4 R338* (VAF 6.8%), NF1 splice-site 5960_60061118del165 (VAF 78.6%), and CDKN2A/B loss. Polymerase chain reaction for IGH gene rearrangements and IGHV sequencing confirmed a common clonal origin. The reported incidence of transformation in patients with CLL on PI3K inhibitors is low.⁴⁷ Nevertheless, duvelisib can potentially contribute to transformation, as it has been shown that PI3K inhibition leads to genomic instability through activation of activation-induced cytidine deaminase.⁸⁴

T-CELL PROLIFERATIONS AND T-CELL LYMPHOMAS IN CLL/SLL

Alterations of T-cell compartments have been extensively studied in CLL/SLL.^85,86 T cells constitute critical components of the leukemia microenvironment and play an important role in disease pathophysiology. Various subsets of CD4-positive T cells have been implicated in the activation, proliferation, and inhibition of apoptosis of leukemic B cells. On the contrary, CD8-positive T cells show markers of functional exhaustion and therefore are less efficient in controlling proliferation of CLL/SLL cells. Clonal or oligoclonal T-cell expansions are common, especially in cytotoxic T cells, and can persist or increase throughout a course of the disease.^87,88 T cells are also influenced by both traditional chemotherapy and novel kinase inhibitors. It has not been investigated if expanded T-cell clones are related to rare T-cell lymphomas arising in a setting of CLL/SLL. Other factors such as increased susceptibility to develop secondary clonally unrelated malignancies, immune suppression, or genetic predisposition may play a role in T-cell lymphoma pathogenesis. T-cell lymphomas occurring in CLL/SLL frequently have a cytotoxic phenotype and are predominantly classified as peripheral T-cell lymphomas, ALK-positive and ALK-negative anaplastic large cell lymphomas (ALCLs, ALK+ and ALK–), and angioimmunoblastic T-cell lymphoma.⁸⁹ In addition, the incidence of cutaneous T-cell lymphomas is increased in male patients with CLL/SLL.⁹⁰ Both in lymph nodes, skin, and other extranodal sites, T-cell lymphomas may present as composite lymphomas accompanying CLL/SLL or occur independently. The workshop included 2 cases of T-cell proliferations arising in a context of CLL/SLL: case 47 represented an atypical T/natural killer (NK)–cell proliferation, and case 223 was diagnosed as ALCL, ALK– Table 3. Case 47 was a 57-year-old man with refractory CLL who had clinical and laboratory features concerning for hemophagocytic syndrome. The bone marrow demonstrated CLL/SLL and a prominent population of large atypical lymphoid-appearing cells positive for CD3 (weak cytoplasmic), CD8, CD2, and CD56 and negative for other T-cell markers and EBER in situ hybridization. TCR gene rearrangement studies showed a polyclonal pattern. The atypical NK/T-cell proliferation persisted after the patient’s counts normalized following venetoclax, rituximab, and steroid therapy and could not be further characterized. Case 223 represented ALCL, ALK– presenting in a 75-year-old man in a cervical lymph node a few months after a diagnosis of CLL. The clonal relationship could not be established due to a lack of material for further studies. Anaplastic large cell lymphomas are one of the most common T-cell lymphomas in patients with CLL/SLL.^91,92 They can occur in untreated patients, in a proportion of patients simultaneously with a diagnosis of CLL/SLL, and up to 8 years posttherapy. Both lymph node involvement and extranodal presentations have been reported.

B-CELL PROLYMPHOCYTIC LEUKEMIA

B-cell prolymphocytic leukemia is described as a rare neoplasm of mature B cells of unknown origin.⁹³ It involves primarily peripheral blood, bone marrow, and spleen. It has been defined by more than 55% prolymphocytes, which are medium-sized lymphoid cells with a round nucleus, moderately condensed chromatin, distinct “punched-out” nucleolus, and a moderate amount of basophilic cytoplasm. Due to cytomorphologic features overlapping with other lymphocytic leukemias and a lack of specific immunophenotypic or cytogenetic/molecular genetic features, the diagnosis of this entity can be challenging and has been an area of vigorous debate. The most recent edition of World Health Organization classification no longer recognizes B-PLL as a distinct entity. Most cases previously classified as B-PLL fall under a variant of mantle cell lymphoma, prolymphocytic transformation of CLL/SLL, or a splenic B-cell lymphoma/leukemia with prominent nucleoli.⁴¹ The International Consensus Classification includes B-PLL, but it recommends a rigorous diagnostic workup to exclude transformation from CLL/SLL, mantle cell lymphoma, and splenic marginal zone lymphoma before rendering a diagnosis of B-PLL.⁷⁶

B-cell prolymphocytic leukemia was initially described as an aggressive variant of CLL, and indeed, other entities such as CLL with increased prolymphocytes, leukemic mantle cell lymphoma, and splenic marginal zone lymphoma have to be excluded due to a significant overlap in clinical presentation and morphologic, immunophenotypic, and, to some extent, genetic features. The ­overlap with CLL with increased prolymphocytes is the most challenging, but based on a very limited number of cases, the gene expression profiles of these 2 entities appear to be distinct.⁹⁴ In another study, however, there was clustering and overlap of B-PLL with CLL and leukemic and nodal mantle cell lymphoma.⁹⁵ The most recent study of a sizable cohort of cases identified frequent complex karyotypes; mutations of TP53, BCOR, and MYD88; and abnormalities of the MYC gene, and it established 3 prognostic groups based on the MYC and TP53 involvement.⁹⁶ This study represents a valuable contribution to the knowledge base; nevertheless, the controversies will likely persist due to the low frequency of B-PLL and a significant overlap with genetic features of other entities.

Three cases of B-PLL were submitted to the workshop Table 4. Case 256 highlights diagnostic difficulties, as it demonstrated morphologic heterogeneity and molecular genetic findings not frequently reported in B-PLL. Peripheral blood showed medium- to large-sized lymphoid cells with scant cytoplasm, and a proportion of cells displayed prominent nucleoli. The lymph node showed a diffuse proliferation of medium to large blastoid cells with irregular nuclei and was interpreted as blastoid transformation of B-PLL by the author. The flow cytometry immunophenotype was nonspecific with coexpression of CD5, CD10, and bright λ light chain and negative CD23 and FMC7 antigens. The karyotype was complex with no rearrangements of MYC, BCL2, or BCL6 genes. The FISH analysis demonstrated deletions of 13q14.2, 11q22.3 (ATM), and TP53. The deletion of 11q22.3 (ATM) has been reported only in rare cases of B-PLL. The sequencing performed by the panel demonstrated high VAF TP53 mutation and identical IGH-VDJ, confirming a clonal relationship of bone marrow and lymph node disease. The case showed IGHV4-34 usage, which has been reported in both CLL and B-PLL.⁹⁶ Taken together, the classification of this case is challenging; nevertheless, B-PLL cannot be excluded. Case 66 represented B-PLL in a 68-year-old patient with a WBC count of 45.7 × 10⁹/L with 30% of medium-sized atypical lymphoid cells with round nuclei, prominent nucleoli, and relatively scant cytoplasm. A diffuse to vaguely nodular proliferation of similar lymphoid cells was seen in a lymph node. Immunohistochemistry and flow cytometry showed positivity for CD19, CD20 (dimmer and brighter populations), dim CD200, partial CD5, and CD23. Similar to CD20, 2 populations with dimmer and brighter expression of κ light chain were seen. The karyotype was complex with t(8;22)(q24;q11.2), trisomy 12, and gains of BCL6 and BCL2. MYC mutation was also detected. An interesting and well-documented example of a morphologic and genetic progression was presented in case 201. The 59-year-old male patient had leukocytosis with an absolute lymphocyte count of 39 × 10⁹/L with medium-sized lymphocytes with prominent nucleoli, anemia, thrombocytopenia, and hepatosplenomegaly. Immunophenotype included expression of brighter CD20, CD79b (SN8 clone), CD22, and bright κ light chain. CD5, CD23, CD10, FMC7, and cyclin D1 were negative. The karyotype was complex. The patient has been treated with ibrutinib and rituximab. After initial remission, the patient relapsed twice with stepwise acquisition of mutations and numerical abnormalities conferring proliferative advantage and resistance to ibrutinib. As disease progressed, the VAF of several mutations increased, including TP53 R248W, MYD88 L265P (along with increased copy number), and TP63 R337Q, and new pathogenic mutations emerged, including BTK C481S. The decreased expression of CD58 late in the disease course was demonstrated by RNA sequencing and likely contributed to immune evasion. The sequential genetic changes and transition to large cell morphology defined progression of the disease, clonal evolution, and acquired resistance to Bruton tyrosine kinase inhibitor therapy under therapy pressure.

CONCLUSIONS

Session 3 of the 2021 Workshop of the Society for Hematopathology/European Association for Haematopathology highlighted a heterogeneous spectrum of progression and transformation of CLL/SLL and B-PLL. Diagnostic challenges included interpretation of accelerated CLL/SLL in extranodal sites, unusual presentations of RT, and T-cell proliferations arising in a setting of CLL/SLL. Molecular genetic testing is required for diagnosis and prognostication, to provide additional clues to the origin and clonal dynamics of progression, identity of transformation, and help in predicting response to therapy.

Diagnosis of B-PLL remains controversial and difficult due to overlap with other lymphoid leukemias, including CLL/SLL with increased prolymphocytes. Advanced molecular genetic testing will likely help define this rare entity.

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Conflict of interest disclosure

The authors have nothing to disclose.

Funding

Supported by National Cancer Institute, National Institutes of Health (P30 CA008748; A.S.D., A.D., W.X.).