Primary nervous system lymphoma in cats

Abstract

Here we characterize the neuroanatomic distribution, neuropathology, and immunophenotype of 10 cases of primary nervous system lymphoma in cats. Cases were retrospectively searched from 2 academic institutions. Selected cases were reviewed and subjected to immunohistochemistry (IHC) for CD3, CD20, and Pax5. The mean age of affected cats was 9.1 y, and no sex or breed predilection was observed. The most common clinical sign was ataxia (8 cases). Gross changes reported in 8 cases consisted of white-to-tan masses (7 cases) or swelling (1 case) within the neuroparenchyma (5 cases) or epidural spaces (3 cases). Histologically, intraparenchymal lymphomas occurred in the gray and white matter or perivascular spaces (7 cases); extraparenchymal lymphomas (6 cases) consisted of neoplastic cell infiltration of the perivascular spaces in the leptomeninges, choroid plexus, or epidural spaces. Nerve lymphomas were diffusely infiltrative. Tumors occurred in the brain (4 cases), spinal cord and nerves (3 cases), spinal cord (2 cases), and brain, spinal cord, and nerves (1 case). IHC was consistent with a B-cell lymphoma in 5 cases and with a T-cell lymphoma in 5 cases.

Lymphoma is a malignant neoplasm of lymphocytes that occurs in many animal species and organ systems. ¹⁵ In cats, lymphoma is the most prevalent hematopoietic neoplasm worldwide, ⁸ occurring mainly as gastrointestinal, mediastinal, multisystemic, and extranodal lymphoma, among others. ¹⁵ Common types of extranodal lymphoma in cats consist of nasal, renal, ocular, cutaneous, and central or peripheral nervous system (CNS and PNS) lymphoma. ¹⁵ Most lymphomas occurring in the nervous system are part of multisystemic lymphoma.^10,11 Primary nervous system lymphoma is described uncommonly, either as sporadic single case reports in the brain,^2,13 spinal cord,^2,13 and/or nerves,^3,6,9,12 or, more commonly, as part of case series describing primary and secondary lymphomas of the nervous system.^5,10,11 Although a few studies have characterized these neoplasms more thoroughly, ¹⁶ most of these investigations have focused on the clinical aspects of the disease.^16,17 Studies assessing the diagnostic neuropathology features of primary CNS and PNS lymphoma in cats are still scarce in the veterinary literature. Here we describe the neuroanatomic distribution, neuropathology, and diagnostic immunohistochemistry (IHC) of 10 primary nervous system lymphomas in cats diagnosed over a period of 16 y.

Cases of lymphoma in cats were identified retrospectively from the archives of the Athens Veterinary Diagnostic Laboratory (College of Veterinary Medicine, University of Georgia, Athens, GA, USA) and the New York State Animal Health Diagnostic Center (College of Veterinary Medicine, Cornell University, Ithaca, NY, USA) between January 2005 and December 2020. Submission forms and autopsy reports were reviewed for cases in which tumors affected exclusively the CNS (brain and spinal cord) and/or the PNS (nerve roots and nerves). Archived slides were retrieved, and H&E–stained tissue sections were selected for cases that had the following CNS or PNS tissue sections available: telencephalon (including cerebrum and basal nuclei), thalamus, mesencephalon, brainstem, and cerebellum; cervical, thoracic, and lumbar spinal cord; and affected nerve segments.

Formalin-fixed, paraffin-embedded tissues were immunolabeled with CD3, CD20, and Pax5 for diagnostic confirmation and characterization of the immunophenotype of neoplastic lymphocytes. All IHCs were performed on an automated stainer (Bond Max automated staining system; Leica). The following antibodies were used: Bond ready-to-use (RTU) CD3 (LN10, PA0553; Leica); CD20 (PA5-16701; Thermo Fisher/Invitrogen); and Bond RTU Pax5 (1EW, PA0552; Leica). All IHC stains were performed in the automated stainer using predetermined settings: 1) slides dewaxed with Bond dewax solution (Leica); 2) heat epitope retrieval with Bond epitope retrieval solution 2 (Leica) for 20 min (CD3) or 40 min (Pax5), or heat epitope retrieval with Bond epitope retrieval solution 1 (Leica) for 30 min for CD20; 3) peroxide block (Leica) for 5 min; d) application of antibody (CD3, RTU, 15 min; CD20, 1:400, 15 min; Pax5, RTU, 60 min); 4) secondary antibody applied as PV-HRP anti-mouse IgG reagent (PV6114; Leica) for 10 min (CD3) or 30 min (Pax5); and Bond polymer refine detection kit (Leica) for 10 min (CD20); 5) Bond polymer refine detection kit (Leica) for 10 min (CD3, CD20) or Bond polymer refine red detection (Leica) for 15 min (Pax5); 6) all sections were then stained for 5 min with hematoxylin. Positive and negative control tissues consisted of normal feline lymph node.

After diagnostic confirmation and immunophenotyping, lymphomas were characterized as CNS lymphomas (when affecting the brain and/or the spinal cord), PNS lymphomas (when affecting nerves), or CNS and PNS lymphomas (when concomitantly affecting the CNS and PNS). CNS lymphomas were classified as intraparenchymal or extraparenchymal. The distribution of neoplastic lymphocytes in intraparenchymal lymphomas was characterized as neuroparenchymal (neoplastic lymphocytes in the gray or white matter) or perivascular (neoplastic lymphocytes surrounding parenchymal, choroid plexus, or leptomeningeal vessels). Extraparenchymal lymphomas were characterized as intradural or extradural (epidural). ⁸ PNS lymphomas were classified as endoneurial, perineurial, epineurial, or diffuse (when involving all 3 nerve compartments). The histologic classification and grading of lymphomas were assessed according to the World Health Organization criteria adapted for use in domestic animals. ²⁰ In summary, lymphomas were classified as diffuse or nodular; small, intermediate, or large; and low-, intermediate-, or high-grade, according to the mitotic activity assessed in 2.37 mm ² (equivalent to 10 FN22 40× fields). When assessing mitosis, areas of necrosis and inflammation (if present) were avoided, and the mitotic count was performed in tumor areas with the highest mitotic activity.

Ten cases met the criteria for inclusion in our study (Tables 1, 2). The average age of affected cats was 9.1 y, and there was no sex or breed predisposition. Feline immunodeficiency virus (FIV) or Feline leukemia virus (FeLV) status was reported only for case 6 (FIV-negative and FeLV-positive) and case 7 (FIV-positive). Reported clinical signs varied greatly, but ataxia was described in 8 of 10 cases. Neurologic examination and/or magnetic resonance imaging (MRI) with determination of the neurolocalization of the lesion were performed in cases 3 and 9. Eight cats were euthanized because of a poor prognosis; 2 cats died spontaneously. Gross changes were reported in 8 cases and consisted mainly of a white-to-tan soft mass (7 cases) or segmental swelling (1 case) within the neuroparenchyma (5 cases) or epidural space (3 cases). Tumors occurred in the brain (4 cases), spinal cord and nerves (3 cases), spinal cord (2 cases), and brain, spinal cord, and nerves (1 case). Intraparenchymal lymphomas (7 cases; Fig. 1) were characterized by neoplastic cell infiltration of the gray and white matter (Fig. 2) or parenchymal perivascular spaces (Fig. 3). In contrast, extraparenchymal lymphomas (6 cases; Fig. 4) consisted of neoplastic cell infiltration of the perivascular spaces in the leptomeninges (Fig. 5) or rarely the choroid plexus or epidural spaces (Fig. 6). All lymphomas affecting nerves were diffuse (Fig. 7) and accompanied by neoplastic infiltration of the spinal cord and/or brain.

Table 1.

Signalment, clinical signs, outcome, and gross changes of primary nervous system lymphoma in 10 cats.

Case	Age (y)	Sex	Breed	Main clinical signs	MRI	Outcome	Gross neuropathology changes
1	1	CM	DLH	Ataxia, progressive pelvic paraparesis	NR	E	Well-demarcated, pale-tan, soft epidural spinal cord mass (T3-T5)
2	12	SF	DLH	Ataxia, anisocoria	NR	E	NR
3	13	SF	DSH	Ataxia	Mass (L4-S2)	E	Segmental spinal cord swelling (L)
4	14	CM	DSH	Decerebrate rigidity	NR	E	Poorly demarcated, white-to-tan, soft parenchymal mass (right cerebellar peduncle)
5	11	CM	Siamese	Ataxia, seizures, circling	NR	E	Poorly demarcated, tan, soft parenchymal mass (right olfactory and frontal lobes)
6	4	SF	DSH	Ataxia, seizures, rotatory nystagmus, opisthotonus, loss of menace response	NR	D	Poorly demarcated, tan, soft leptomeningeal and parenchymal mass (parietal and frontal lobes)
7	8	CM	DSH	Ataxia, pelvic paraparesis	NR	E	Poorly demarcated, tan, soft epidural spinal cord mass (C4-C6)
8	13	SF	DSH	Ataxia, thoracic paraparesis, loss of pelvic limb reflex, urinary incontinence, thoracolumbar pain	NR	E	Poorly demarcated, tan, soft parenchymal mass (caudal equina and sciatic nerves)
9	7	CM	DLH	Bilateral patellar luxation and muscle atrophy	Muscle atrophy (right pelvic limb)	E	Well demarcated, white-to-tan, soft epidural spinal cord mass (L6-L7)
10	8	CM	DSH	Ataxia, circling, decreased postural reactions, disorientation	NR	D	NR

CM = castrated male; D = spontaneous death; DLH = domestic longhair; DSH = domestic shorthair; E = euthanasia; MRI = magnetic resonance imaging; NR = none reported; SF = spayed female.

Table 2.

Distribution of neoplastic lymphocytes, histology, immunohistochemistry, and diagnosis of primary nervous system lymphoma in 10 cats.

Case	Distribution of neoplastic cells				PNS	Histology and IHC											Diagnosis
	CNS					Neoplastic infiltration					Cell and nuclear features			IHC
	CNS location		CNS compartment			Parenchymal		Perivascular			CS	NS	MC	CD3	CD20	Pax5
	Brain	Spinal cord	IP	EP		GM	WM	NP	CP	LM
1	No	T3-T5	No	Yes (ED)	No	−	−	−	−	−	S	S	1	+	−	−	Diffuse, low-grade, small lymphocytic TCL
2	Widespread	Widespread	Yes	Yes	Spinal and optic nerves	+	+	+	+	+	S-M	I	35	+	−	−	Diffuse, low-grade, PTCL-NOS
3	No	L-S	Yes	Yes (ID)	Sacral nerves	+	+	+	−	+	S-M	S	2	+	−	−	Diffuse, low-grade, small lymphocytic TCL
4	Right cerebellar peduncle	No	Yes	No	No	+	+	+	−	+	S-M	L	19	+	−	±	Diffuse, low-grade, PTCL-NOS
5	Right olfactory and frontal lobes	No	Yes	No	No	+	+	+	−	+	S-M	L	21	−	+	+	Diffuse, low-grade, lymphoblastic BCL
6	Frontal and parietal lobes	No	Yes	No	No	+	+	+	−	+	S	S	4	−	+	+	Diffuse, low-grade, TCRLBCL
7	No	C4-C6	Yes	Yes (ED)	No	+	+	+	−	+	S-M	I-L	12	+	+	+	Diffuse, low-grade, TCRLBCL
8	No	L-S	No	Yes (ID)	Cauda equina and sciatic nerves	−	−	−	−	+	M	I-L	20	−	+	+	Diffuse, low-grade, lymphoblastic BCL
9	No	L6-L7	No	Yes (ED)	Sciatic nerves	−	−	−	−	−	S	I-L	2	+	+	+	Diffuse, low-grade, TCRLBCL
10	Widespread	No	Yes	No	No	+	+	+	−	+	M	L	23	+	−	−	Diffuse, low-grade, PTCL-NOS

BCL = B-cell lymphoma; CNS = central nervous system; CP = choroid plexus vessels; CS = cytoplasm size; ED = epidural; EP = extraparenchymal; GM = gray matter; I = intermediate; ID = intradural; IHC = immunohistochemistry; IP = intraparenchymal; L = large; LM = leptomeningeal vessels; M = moderate; MC = mitotic count in 2.37 mm ² or 10 FN22 40× fields; NP = neuroparenchymal vessels; NS = nuclear size; PNS = peripheral nervous system; PTCL-NOS = peripheral T-cell lymphoma not otherwise specified; S = small; TCL = T-cell lymphoma; TCRLBCL = T-cell–rich, large B-cell lymphoma; WM = white matter; − = negative; + = positive.

Figures 1–7.

Nervous system lymphoma in cats. Figure 1. Mass of neoplastic lymphocytes (arrows) effaces and expands the right basal nuclei and internal capsule, case 5. H&E. Figure 2. Neoplastic lymphocytes infiltrate and partially disrupt the spinal cord white matter, case 2. H&E. Figure 3. Neoplastic lymphocytes infiltrate and expand the perivascular spaces within the right cerebellar peduncle parenchyma, case 4. H&E. Figure 4. Neoplastic lymphocytes diffusely infiltrate and expand the spinal cord leptomeninges, case 2. H&E. Figure 5. Neoplastic lymphocytes infiltrate and expand the perivascular spaces within the telencephalic leptomeninges, case 2. H&E. Figure 6. Clusters of neoplastic lymphocytes infiltrate and expand the epidural adipose tissue around the thoracic spinal cord, case 1. H&E. Figure 7. Neoplastic lymphocytes diffusely infiltrate the endoneurium, perineurium, and epineurium in a spinal nerve, case 2. H&E.

Neoplastic lymphocytes were arranged in characteristic sheets supported by a fine fibrovascular stroma or preexisting tissues. The mitotic activity in all cases varied from 1 to 35 mitoses in 2.37 mm ² , which is consistent with low-grade lymphomas. Peripheral T-cell lymphomas not otherwise specified (3 cases) and B-cell lymphomas (2 cases) consisted of neoplastic lymphocytes with scant eosinophilic cytoplasm and round-to-oval, intermediate-sized nuclei with dispersed chromatin, 1–3 nucleoli, and moderate anisokaryosis (Suppl. Fig. 1). Neoplastic T lymphocytes had membranous immunolabeling for CD3 (Suppl. Fig. 2). Neoplastic B lymphocytes had membranous immunolabeling for CD20 (Suppl. Fig. 3) or nuclear immunolabeling for Pax5. T-cell–rich, large B-cell lymphomas (3 cases) had scattered neoplastic cells with abundant eosinophilic cytoplasm and round-to-indented nuclei with finely stippled chromatin and one nucleolus surrounded by numerous small lymphocytes (Suppl. Fig. 4). The large neoplastic lymphocytes had membranous immunolabeling for CD20 or nuclear immunolabeling for Pax5 (Suppl. Fig. 5). The small reactive lymphocytes had membranous immunolabeling for CD3 (Suppl. Fig. 6). Small lymphocytic T-cell lymphomas (2 cases) were characterized by neoplastic cells with scant eosinophilic cytoplasm and small, round nuclei with dense chromatin and limited anisokaryosis (Suppl. Fig. 7); these cases were immunopositive for CD3 (Suppl. Fig. 8).

Lymphoma accounts for the most common neoplasm of the spinal cord and the second most common neoplasm of the brain in cats.^8,10 However, because these tumors typically occur as part of widespread lymphoma,^8,10,16 there is little information in the literature regarding the pathology and immunophenotype of primary lymphoma of the nervous system in cats.

Primary CNS lymphoma accounts for <3% of all primary brain and spinal cord tumors of cats. ² Based on our archives search, ~406 cases of feline lymphoma with multiple organ involvement were diagnosed at both institutions during the study period. Of these, 32 cases (7.8%) had concurrent nervous system involvement and only 10 (2.4%) were restricted to the brain, spinal cord, or nerves, supporting the rare nature of primary nervous system lymphoma in cats. However, these numbers should be interpreted with caution because CNS or PNS tissues may not have been collected or trimmed in all cases, depending on the reported clinical signs in each submission. As observed in our cases, adult individuals are more commonly affected, and clinical signs vary according to the neurolocalization of neoplastic lymphocytes.^2,13 A few studies indicate that CNS lymphoma tends to occur in younger cats compared to other CNS neoplasms. ¹⁰ However, this characteristic is associated with both primary and secondary CNS lymphomas ¹⁰ and may not be true for primary CNS lymphomas, as suggested by the age of our affected cats (average age = 9.1 y). Not surprisingly, cats with CNS lymphoma have the shortest survival time after diagnosis. ¹⁷ In our cases, 8 of 10 cats were euthanized after diagnosis, and thus no post-diagnostic clinical information was available. Although FIV and FeLV infection status was not determined for most of our cases, these viral infections have not been associated with primary lymphomas of the nervous system. ²⁰ The clinical diagnosis of primary nervous system lymphoma is often challenging, particularly when diagnostic imaging is not available for a thorough evaluation and detection of tumors in other organ systems.^8,12 MRI was performed on 2 cats in our study, revealing a tumor in the spinal cord (case 3) and muscle atrophy in the pelvic limb associated with tumor infiltration in the spinal cord and sciatic nerves (case 9).

In cats, primary CNS lymphoma occurs mainly in the spinal cord. ¹⁰ In our study, 6 of 10 cases had spinal cord involvement. Tumors occurred exclusively in the spinal cord in 2 cases and in the spinal cord and brain or spinal nerves in 4 other cases. Spinal cord lymphomas are typically characterized as epidural masses involving the extradural adipose tissue of the thoracic and lumbosacral segments, ¹⁶ but intradural or intraparenchymal lymphomas can also occur, especially in cases of spinal cord involvement secondary to multisystemic lymphoma. ¹¹ The reason for these differences in the neurolocalization of neoplastic cells remains undetermined. However, epidural spinal lymphomas are thought to arise from epidural extramedullary hematopoietic tissues surrounding the spinal cord, which could explain the typical epidural location of most lymphomas with no extraneural involvement. ¹¹ Tumors occurred in the thoracolumbar and sacral spinal cord segments in 4 of our 6 cases, and were restricted to the epidural space in 3 cases.

Primary CNS lymphomas affecting the brain can occur as distinct neuroparenchymal masses or as widespread infiltrations in the neuroparenchyma and along the leptomeninges.^2,13,18,19 Widespread lymphomas, particularly those distributed diffusely along the leptomeninges (leptomeningeal lymphomatosis), can mimic inflammatory disease and make diagnosis difficult.^2,13 In the 5 cats with telencephalic involvement in our study, 2 had widespread leptomeningeal involvement (1 also extending throughout the spinal cord and spinal nerves), and 3 had distinct parenchymal masses in the right cerebellar peduncle, frontal lobes, and frontal and parietal lobes, similar to cases described elsewhere. ⁸ Unlike spinal cord lymphomas, there appears to be no difference in the distribution of tumors in the brain in cases of primary CNS lymphoma and those occurring associated with widespread lymphoma. ⁸

Infiltration by neoplastic lymphocytes in the brain and spinal cord parenchyma occurred in both the gray and white matter. Perivascular infiltration was observed in over half of our cases and occurred in leptomeningeal and parenchymal vessels, with only one case with perivascular infiltration around choroid plexus vessels. These findings have been described in CNS lymphomas.^5,8

Lymphomas of the PNS have been reported uncommonly in cats.^3,6,9 Tumors can occur concomitantly with CNS lymphoma, as observed in our cases, or exclusively in the PNS. ⁸ Affected cats can develop muscle atrophy as a result of denervation,^3,6 as well as grossly visible swelling or masses involving the affected nerves.^6,9 Muscle atrophy of the pelvic limbs (case 10) and swelling of affected nerves (case 8) were reported in our cases. Widespread infiltration of neoplastic lymphocytes within nerve roots or nerves with or without CNS involvement is referred to as diffuse neurolymphomatosis, and has been described in rare occasions.^3,9 The 4 cases with nerve involvement in our case series had widespread infiltration of multiple nerves by neoplastic lymphocytes, and all cases had CNS involvement.^3,9

In humans, primary lymphomas of the nervous system and mainly diffuse large B-cell lymphomas occur predominantly in the telencephalon and less often in the cerebellum, brainstem, spinal cord, and nerves. ⁷ Although molecules such as LFA1/ICAM-1, neural cell adhesion molecule, L-selectin, N-cadherins, and CD44 have been implicated in the homing, spread, and adherence of neoplastic lymphocytes within the CNS and PNS in human medicine, no such investigations have been conducted in veterinary medicine, to our knowledge, and the mechanisms of lymphoma tumorigenesis in cats remain elusive. ¹

In our study, 50% of the cases were T-cell lymphomas and 50% were B-cell lymphomas, similar to other reports.^2,4,13 Interestingly, 3 of the B-cell lymphomas were T-cell–rich, large B-cell lymphomas, a subtype that has not been found commonly in the nervous system of cats. ⁵ The main differential diagnosis for CNS and PNS lymphoma should include lymphocytic inflammation, especially in cases of leptomeningeal lymphomatosis,^2,13 other round-cell neoplasms, ²⁰ or CNS neoplasms such as oligodendroglioma. ¹⁴ As demonstrated by our case set, IHC using lymphocytic markers and Olig2 (if lymphocytic markers are negative), as described in the literature, should suffice for diagnostic confirmation of lymphoma and for ruling out oligodendroglioma, a glial neoplasm that can be morphologically similar to lymphoma. ¹⁴