Cytologic Features of Aggressive Variants of Follicular-Derived Thyroid Carcinoma

Abstract

Certain carcinomas of the thyroid gland behave aggressively resulting in increased patient morbidity and poor patient prognosis. The diagnosis of these aggressive thyroid cancer subtypes is sometimes challenging and subject to increased interobserver variability. This review deals with the cytological features of such tumors including aggressive variants of papillary thyroid carcinoma, poorly differentiated thyroid carcinoma, and anaplastic thyroid carcinoma. These malignancies fall into 2 groups based on their cytomorphology: those that exhibit distinct microscopic features (eg, nuclear findings typical of classical papillary thyroid carcinoma or marked anaplasia) and those that present with more subtle cytologic features (eg, nuclear pseudostratification, “soap bubble” nuclei, supranuclear or subnuclear cytoplasmic vacuoles, rosette-like structures, hobnail cells). We review the literature regarding these aggressive thyroid cancers and highlight important phenotypic characteristics that can be useful for their diagnosis based on fine needle aspiration.

INTRODUCTION

The vast majority of patients with differentiated thyroid carcinomas have an excellent prognosis after treatment.^1–13 They have an overall 5-year survival that approximates 98%.^1–4 However, those patients that present with more aggressive variants of thyroid carcinoma have a less favorable prognosis and frequently present with tumors that are larger, have a higher histological grade, and exhibit extracapsular spread (eg, to the lymph node) and even distant metastases.^10–12 In many cases, these aggressive variants of thyroid carcinoma will require more aggressive or innovative forms of clinical management. The aggressive variants of papillary thyroid carcinoma (PTC) include tall cell, columnar cell, solid, hobnail, diffuse sclerosing, and diffuse follicular variant. Other carcinomas of the thyroid gland with an aggressive behavior include poorly differentiated thyroid carcinoma (PDTC), anaplastic thyroid carcinoma (ATC), and medullary thyroid carcinoma (MTC).

In the 1st edition (2010) of The Bethesda System for Reporting Thyroid Cytology, the authors described the architectural and cytological features of PTC (Fig. 1) along with some of its variants.¹ The PTC variants were discussed separately, highlighting the morphologic heterogeneity in this family of tumors. It was acknowledged that some of these PTC variants are associated with more aggressive behaviors relative to the more indolent conventional (classical) PTC.^14–21 Since the 1st edition was published, several new variants of thyroid carcinoma have been described. The revised 2nd edition included a more detailed definition of the criteria for classifying some of the PTC variants² and expanded the description of PTC variants to include solid, diffuse sclerosing, cribriform, and hobnail variants to improve their cytological identification.^22–26 The 2nd edition also enriched the description of PDTC and ATC.² The Bethesda System has been shown to be very effective in predicting PTC, and different authors including a large series from Gupta et al¹⁵ and Miftari et al¹⁶ concluded that fine needle aspirates (FNAs) could provide clues to the diagnosis of these aggressive PTC variants. Herein, we review the literature regarding these aggressive thyroid cancers and emphasize notable phenotypic characteristics that can help with their cytologic diagnosis.

Figure 1.

Classical morphological features of conventional papillary thyroid carcinoma characterized by papillary structures and cells with atypical and pleomorphic nuclei with intranuclear pseudo-inclusions. (A and B) Direct smears for Papanicolaou test (original magnification ×200) and (C) liquid-based cytology (original magnification ×400).

AGGRESSIVE VARIANTS OF PTC

By definition, variants of PTC have at least some of the essential nuclear features of PTC (eg, nuclear grooves, intranuclear inclusions) but a different architectural pattern, cellular features, and/or background (Table 1).^22–26 Table 2 summarizes the different immunophenotypes, while Table 3 summarizes the molecular findings in these aggressive tumors. Some of these variants occur more frequently in elderly patients. They typically manifest with aggressive findings at the time of diagnosis (eg, extrathyroidal extension, metastases) as well as a poorer prognosis compared with classical PTC.¹⁹ Interestingly, the incidence of these aggressive variants has increased by 120% to 150% in recent decades.^{5,19,20,22,26} This increased incidence may be attributed to improved detection of thyroid malignancies in general and greater accuracy in the diagnosis of some variants. The new World Health Organization classification of thyroid tumors, which includes the aggressive variants, incorporates the genetic-molecular characterization of these neoplasms.²⁷

TABLE 1.

Morphological Features of Aggressive Thyroid Carcinomas

Feature	TCV	CCV	SVPTC	DSVPTC	Hobnail	Diffuse FVPTC	PDTC	ATC
Architecture	Papillary clusters with single cells	Papillary clusters with nuclear stratification	Syncytial-type fragments, microfollicular/trabecular structures, with single cells	3D ball-like clusters with cohesive groups of cells	Papillary/Micropapillary structures	Follicular structures	Solid, trabecular, and insular pattern	Single cells and variably sized groups
Background	Scant	Scant	Scant	Lymphocytes	Scant	Scant	Scant	Tumor diathesis
Colloid	Scant/absent	Scant/absent	Scant/absent	Scant/absent	Absent	Scant/absent	Scant/absent	Scant/absent
Psammoma bodies	Few	Few/absent	Absent	Abundant	Few/absent	Absent/few	Absent	Absent
Necrotic component	Absent	Absent	Absent	Absent	Absent	Absent	Present	Present
Mitoses	Common	Rare	Rare	Rare	Rare	Rare	Common	Common
Nuclear shape	Elongated with typical PTC nuclear features	Hyperchromatic, elongated and stratified/pseudostratified	PTC nuclear features	PTC nuclear features	Apical placed, cuboidal or oval	Pleomorphic with PTC nuclear features	Round to oval	Markedly atypical with multinucleation
Nuclear chromatin	Granular	Stippled	Clearing	Clearing	Clearing	Clearing or granular	Granular	Clumping
Nuclear grooves	Present	Occasional	Occasional	Occasional	Occasional	Occasional	Absent	Absent
Nuclear pseudo-inclusions	Numerous or soap-bubble appearance	Occasional	Occasional	Occasional	Numerous or soap-bubble appearance	Occasional	Absent	Absent
Cytoplasm	Eosinophilic with oncocytic-like features	Supranuclear or subnuclear vacuoles	Scant	Dense with large vacuoles	Abundant eosinophilic	Scant	Scant and oncocytic-like	Scant

Abbreviations: 3D, 3-dimensional; ATC, anaplastic thyroid carcinoma; CCV, columnar cell variant; DSVPTC, diffuse sclerosing variant of papillary thyroid carcinoma; FVPTC, follicular variant of papillary thyroid carcinoma; PDTC, poorly differentiated thyroid carcinoma; PTC, papillary thyroid carcinoma; SVPTC, solid variant of papillary thyroid carcinoma; TCV, tall cell variant.

TABLE 2.

Immunohistochemical Features of Aggressive Thyroid Carcinomas

Immunostain	TCV	CCV	SVPTC	DSVPTC	Hobnail	Diffuse FVPTC	PDTC	ATC
Thyroglobulin	++	++	++	++	++	++	+	−
TTF1	+	+	+	+	+	+	+	−
HBME-1	+	+	+	+	+	+	−	−
Galectin-3	+	+	+	+	+	+	−	−
CK19	+	+	+	+	+	+	+/−	−
P53	+/−	+/−	+/−	+/−	+	+/−	+	+
PAX-8	+	+	+/−	+	+	+	+	+/−
CAM 5.2	+	+	+	+	+	+	+	+/−
CDX2	−	++	−	−	−	−	−	−

Abbreviations: ATC, anaplastic thyroid carcinoma; CCV, columnar cell variant; DSVPTC, diffuse sclerosing variant of papillary thyroid carcinoma; FVPTC, follicular variant of papillary thyroid carcinoma; PDTC, poorly differentiated thyroid carcinoma; SVPTC, solid variant of papillary thyroid carcinoma; TCV, tall cell variant; +, positive; −, negative.

TABLE 3.

Molecular Features of Aggressive Thyroid Carcinomas

Molecular Finding	TCV	CCV	SVPTC	DSVPTC	Hobnail	Diffuse FVPTC	PDTC	ATC
BRAFV600E	++	++	+	−	++	++	+/−a	+/−a
Other BRAF mutations	−	−	+	−	+	+	+/−a	−
RAS	−	−	−	−	−	+	+/−a (NRAS)	+/−a
Ret/PTC	+	−	++	++	+	−	+/−a	−
PAX8/PPAR	−	−	−	−	−	+/−	+/−	−
P53	+	−	−	+	+	−	+	+
TERT mutations	+	−	−	−	−	−	+	+/−a
ALK fusion	−	−	−	−	−	−	−	+

Abbreviations: ATC, anaplastic thyroid carcinoma; CCV, columnar cell variant; DSVPTC, diffuse sclerosing variant of papillary thyroid carcinoma; FVPTC, follicular variant of papillary thyroid carcinoma; PDTC, poorly differentiated thyroid carcinoma; SVPTC, solid variant of papillary thyroid carcinoma; TCV, tall cell variant; +, mutated; −, wild-type; +/−, possibly mutated or nonmutated.

^aCombination of different genetic alterations.

Tall Cell Variant

The tall cell variant of PTC (TCV), first described by Hawn and Hazard in 1976,¹³ is a rare subtype of PTC that accounts for 5% to 11% of all PTC variants. The description of this variant is based on a carcinoma with a papillary growth pattern comprised of tumor cells that are 2 to 3 times as tall as they are wide.^28–40 TCV is the most common aggressive variant of PTC, most often diagnosed in elderly patients.¹⁹ Although Leung et al⁴¹ suggested that the TCV could not be reliably recognized in FNA specimens, Guan et al⁴² and Das et al²¹ showed that TCV was correctly recognized in 30% to 100% of their cytological samples.

The cytological features of TCV include papillary clusters or discohesive single cells with a characteristic elongated-to-tall shape (2 to 3 times longer than wide), and nuclei that exhibit the typical nuclear changes of PTC (nuclear enlargement, grooves, chromatin clearing, and pseudo-inclusions) (Fig. 2). These cells have distinct cell borders and a moderate amount of eosinophilic cytoplasm. Typically, fine needle aspirates include “tail-like cells” and “tadpole cells” and occasionally exhibit a rosette-like microfollicular pattern (Fig. 2D). FNA samples of the TCV tend to have fewer psammoma bodies than conventional PTC. Nuclei often contain inclusions with a soap-bubble appearance and nuclear chromatin that is granular. Solomon et al⁴³ found that nuclei with a soap-bubble appearance are rarely seen in conventional PTC. Pleomorphism, cellular atypia, and mitotic figures are also more common in the TCV than in conventional PTC. In addition, the presence of intraepithelial neutrophils without associated necrosis may be linked with the TCV. Several studies have described the cytologic features of TCV,^21,40–43 and Lee et al⁴⁰ demonstrated that the TCV was commonly recognized in their series of Korean cases. Suzuki et al²³ and Lee et al⁴⁰ reported that tall cell features might be more readily appreciated in liquid based preparations than conventional smears.

Figure 2.

(A-D) The tall cell variant (TCV) of papillary thyroid carcinoma (PTC) is characterized by elongated cells that are 2 to 3 times as tall as they are wide. There is a moderate amount of eosinophilic cytoplasm, and nuclei have conventional features of PTC, including frequent nuclear pseudo-inclusions. Panel D shows detail of a TCV microfollicle with a rosette-like pattern (C: direct smears for Diff-Quick and A, B, and D: Papanicolaou test, original magnification ×400).

There are challenges, however, to recognizing the TCV by FNAs, since the cytologic features of this variant may be subtle and confused with those of other thyroid neoplasms.³⁴ TCV differs from a Hürthle cell neoplasm because the tumor cells display characteristic nuclear features of PTC. In addition, the TCV can be differentiated cytologically from the oncocytic variant of PTC based on the elongated rather than polygonal cell shape and prominence of intranuclear pseudo-inclusions.

Using immunocytochemistry, the TCV stains for thyroglobulin, PAX-8, pancytokeratin, TTF-1, mesothelium-associated antibody HBME-1, and galectin-3 (Table 2). Urano et al²⁸ reported that TCV displays a high Ki-67 (MIB-1) labeling index indicating a higher proliferation activity than conventional PTC. At the molecular level, a BRAF^V600E mutation is present in up to 80% of TCV cases, TP53 mutation in 60% of cases, and TERT promoter mutations in 30% of tumors (Table 3).¹²

Columnar Cell Variant

The columnar cell variant (CCV) of PTC was first described by Evans⁴⁴ as a rare variant with more aggressive behavior compared with classical PTC. CCV is one of the least common variants of PTC and is seen more frequently in male patients with a median age of 55 years. Patients with the CCV present with a high frequency of lymph node and distant metastases.^44–53

Although infrequently described, the cytological features of CCV include columnar cells with hyper-chromatic, oval, and pseudostratified nuclei with supranuclear or subnuclear cytoplasmic vacuoles.^44–53 Nuclei have dark stippled chromatin and indistinct nucleoli. Nuclear pseudo-inclusions and nuclear grooves are rare. Despite the absence of the aforementioned nuclear features, it is important to be aware that the CCV may be overlooked, as their nuclei are often darker than those of conventional PTC. In addition, Sen et al⁴⁶ reported the occurrence of rosette-like structures in cytologic samples. In histopathology sections, tumor cells are arranged in papillary structures with pseudostratification of the columnar cells forming a tram track appearance. Therefore, cell block evaluation may be helpful to reveal this supportive architectural feature.

CCV can often be differentiated from TCV based on the fact that CCV cells have greater height, more striking nuclear hyperchromasia, nuclear pseudostratification, and lack oxyphilic changes (Fig. 3). CCV may mimic a metastasis from a colorectal or endometrial primary. Indeed, the description “cancerization of follicles” was used by Afrogheh et al⁴⁸ to describe malignant endometrial adenocarcinoma admixed with oncocytic adenomatous epithelium. However, unlike colorectal cancer, CCV typically lacks a necrotic component. Furthermore, CCV is thyroglobulin- and TTF-1–positive. However, PAX-8 may be positive in both CCV and gynecological carcinomas. CDX2 may be positive in up to 50% of CCV, but it is not expressed in other benign or malignant thyroid lesions.⁵³ BRAF^V600E is found in one-third of CCV cases.⁵⁴

Figure 3.

(A-C) The columnar variant of papillary thyroid carcinoma (PTC). The cytoplasm might be bipolar and wispy, and cigar-shaped nuclei have some features of PTC (direct smears for Papanicolaou test, original magnification ×400).

Solid Variant

According to the World Health Organization, the solid variant of PTC (SVPTC) refers to a papillary carcinoma with a solid growth pattern present in all or nearly all of the tumor.²⁷ SVPTC is rare, representing only 3% of PTC variants,^55–60 and it has been documented largely in children. There is a high prevalence (30%) of this variant reported in survivors of the Chernobyl accident.⁵⁷ However, SVPTC may also be seen in children without radiation exposure.⁶¹ SVPTC is associated with a higher frequency of distant metastases and has a less favorable prognosis compared with classic PTC.^58–61 Nonetheless, Chang et al,⁵⁶ in their series of 14 cases, indicated that SVPTC might not be as aggressive as previously thought.

Due to the lack of specific cytomorphological criteria, unless there is a clinical history of radiation exposure, a cytological diagnosis of SVPTC is unlikely. The majority of cases tend to be classified as “suspicious for malignancy” or “positive for malignancy favoring PTC” due to the presence of nuclear features suggesting PTC. Cytologically, specimens are variably cellular and generally lack colloid. The neoplastic cells are characterized by cohesive, syncytial-type, 3-dimensional fragments, microfollicular or trabecular arrangements, and even single cells (Fig. 4). True papillary structures with fibrovascular cores and psammoma bodies are typically absent.² The nuclei show classical nuclear changes of PTC, although they may be rounder and slightly darker than those of conventional PTC.

Figure 4.

The solid variant of papillary thyroid carcinoma (PTC) consists of cohesive, solid arrangements of cells with nuclear features of PTC (direct smear for Papanicolaou test, original magnification ×400).

Ohashi et al⁶⁰ studied 26 FNA cases of SVPTC that they stratified according to the amount of solid component present. They concluded that the presence of cohesive solid nests, a trabecular pattern, overlapping, enlarged nuclei, nuclear pleomorphism, distinct nucleoli, and the absence of colloid and psammoma bodies were noted more often than in conventional PTC. Moreover, they did not find any significant difference in the cytological features based on the percentage of solid component within these tumors. Higuchi et al⁵⁹ studied 18 FNA cases of histologically confirmed SVPTC and observed solid (77.8%) and small papillary clusters (72.2%) in the majority of cases, with no colloid and nuclear features consistent with conventional PTC. Giorgadze et al⁶² described 13 cases of pure SVPTC and noted that given its heterogeneous morphology with different patterns (including cohesive, syncytial, and microfollicles/trabecular), SVPTC can be misdiagnosed as medullary, FVPTC, or PDTC.⁶²

FNAs of SVPTC with a predominant microfollicular pattern may be especially difficult to distinguish from other follicular-patterned lesions, such as FVPTC. Given the solid architectural pattern, SVPTC morphologically overlaps with PDTC. In contrast to PDTC, however, SVPTC has more characteristic PTC nuclei, has a lower nuclear/cytoplasmic (N/C) ratio, and lacks mitoses and necrosis. While cases of SVPTC with a single-cell pattern could suggest a diagnosis of MTC, SVPTC can be distinguished by their PTC nuclear features. In contrast to MTC, SVPTC is positive for thyroglobulin and negative for calcitonin. SVPTC is also positive for TTF-1, PAX-8, pancytokeratin, HBME-1 and galectin-3. Molecular analysis of SVPTC associated with the Chernobyl accident demonstrated RET/PTC rearrangements and deletions in the BRAF gene.⁵⁷ Prasad et al⁶³ described a series of 28 pediatric thyroid carcinomas in which 28% were classified as SVPTC and associated with a high prevalence of NTRK1/NTRK3 gene fusions and BRAF^V600E mutation.

Diffuse Sclerosing Variant

The first description of the diffuse sclerosing variant (DSV) of PTC was provided by Vickery et al in 1985.⁶⁴ DSV is a rare PTC variant representing only 2% of PTC. It occurs most commonly in children and young adults. DSV is usually characterized by bilateral involvement of the thyroid gland and accordingly may mimic goiter or thyroiditis.^65–76 Occasionally, DSV can be associated with antithyroglobulin and antimicrosomal antibody expression. Radiologically, DSV can have a “snowstorm appearance” due to the abundance of psamomma bodies. Several studies have demonstrated that DSV is indeed an aggressive variant of PTC, with lymph node metastases occurring in nearly 100% of cases, as well as frequent distant metastases.^65–76 However, despite its aggressive behavior, the prognosis of DSV is often comparable to conventional PTC, possibly due to the young age of affected patients.

FNA samples of DSV are moderately to highly cellular with scant or absent colloid. Typical cytological features include neoplastic cells organized in 3-dimensional ball-like clusters or cohesive groups of cells intermingled with inflammatory cells.² Prototypical large papillary clusters of cells may be present in these samples. The neoplastic cells are round to polygonal and have dense cytoplasm with well-defined cellular borders and large cytoplasmic vacuoles (Fig. 5). Hobnail cells may be present. Tumor nuclei show less chromatin clearing, fewer nuclear grooves, and less pseudo-inclusions than classic PTC (Fig. 5). Additionally, squamous metaplasia, numerous lymphocytes, and psammoma bodies may be present. Lee et al⁶⁶ emphasized that through the combination of typical imaging findings and careful cytological examination, DSV can often be diagnosed preoperatively, potentially facilitating more aggressive surgical treatment.

Figure 5.

(A-D) Fine-needle aspiration of the diffuse sclerosing variant of papillary thyroid carcinoma (PTC). The aspirate shows cohesive groups of epithelial cells, some with squamoid features, and one showing a rosette-like structure. Nuclei show classical features of PTC, including an intranuclear pseudo-inclusion, grooves, and pallor (direct smears for Surepath, Papanicolaou test, original magnification ×1000).

The cytologic differential diagnosis of DSV includes chronic lymphocytic thyroiditis and lymphoma. The presence of a prominent atypical cellular component with nuclear features of PTC distinguishes DSV from thyroiditis. Flow cytometry should be used when lymphoma is a concern. As confirmed in a different series, DSV is positive for thyroglobulin, TTF-1, and CK19, but it exhibits different expression patterns of epithelial membrane antigen, galectin-3, cell adhesion molecules, p53 and p63 when compared with conventional PTC.^68,72,76

Sheu et al⁷⁶ found no BRAF^V600E mutation in their series of DSV, while all of their cases had a RET/PTC rearrangement. The presence of RET/PTC rearrangement is more commonly found in tumors seen in children and young adults, especially in those associated with radiation exposure. The detection of the RET/PTC rearrangement may also explain the susceptibility of DSV to radioactive iodine therapy.

Hobnail Variant

The first description of the hobnail variant of PTC was reported by Asioli et al in 2014.^77,78 This tumor was defined as an aggressive variant of PTC in which more than 30% of the tumor cells show hobnail cytomorphologic features, characterized by nuclei located in the apex or middle of the cytoplasm giving these cells their typical “hobnail” appearance⁷⁸ (Fig. 6). Similar appearing hobnail cells may also be seen in serous papillary carcinoma of the ovary, serous papillary carcinoma of the peritoneum as well as breast, lung, kidney, and bladder carcinomas.^77–81 It was suggested that this tumor may be related to the oncocytic variant of PTC, mostly due to the oncocytic features of the tumor cell cytoplasm and the evidence of mitochondria by electron microscopy, but their biologic behavior is often more aggressive.

Figure 6.

(A and B) The hobnail variant of papillary thyroid carcinoma has an unusual cytomorphology that includes tumor cells with an eccentrically placed nucleus imparting a hobnail-like appearance to the cell (A: direct smears for Papanicolaou test and B: ThinPrep, original magnification ×400).

The cytological diagnosis of the hobnail variant is difficult to make, not only due to its rarity but also because of the paucity of published series documenting any helpful cytomorphologic characteristics. Cytological samples have been reported to show papillary and micropapillary growth patterns without a fibrovascular core. Specimens contain cuboidal, oval, or teardrop-shaped cells with moderate amounts of eosinophilic cytoplasm. Nuclei are apically placed with a bulge that leads to a hobnail pattern, while still exhibiting nuclear features of PTC.² Comet-like cells and cells showing multiple intranuclear inclusions resulting in a soap bubble nuclear appearance may be detected.^79–81 Lee et al⁷⁷ studied 10 cases with both conventional and liquid-based cytology and noted that in liquid-based cytology samples, the papillary and micropapillary structures were less frequent than in conventional smears.

The most important differential diagnosis of the hobnail variant of PTC includes TCV, which lacks the hobnail morphology and has more elongated cells. It is noteworthy, however, that combinations of PTC variants exhibiting a hobnail component are not uncommon. The hobnail variant should be distinguished from metastatic lesions such as ovary, breast, or lung with a hobnail and/or micropapillary growth pattern. The immunocytochemical profile of the hobnail variant of PTC shows positivity for thyroglobulin, HBME-1, cyclin D1, and p53 in up to 50% of cases as reported by Lee et al. BRAF^V600E mutation has been found in up to 80% of cases.⁷⁷

Diffuse Follicular Variant

The diffuse follicular variant of PTC (diffuse FVPTC) is a rare and aggressive form of PTC.^82–84 This entity is more frequent in young female patients. It is defined by a diffuse neoplastic involvement of the thyroid gland characterized by neoplastic pseudo-areas with a follicular-patterned tumor displaying nuclear features of PTC. As the name implies, the cytological features are essentially those of FVPTC, so on FNA it is indistinguishable from FVPTC (Fig. 7). According to Gupta et al,⁸³ diffuse FVPTC appears to present and behave biologically in a more aggressive fashion than either FVPTC or conventional PTC. This tumor also has a higher rate of central nodal metastasis and BRAF^V600E mutations in comparison with encapsulated FVPTC and invasive FVPTC. Results seen with ancillary tests have shown overlap with classical PTC and FVPTC (Tables 2 and 3).

Figure 7.

The diffuse follicular variant of papillary thyroid cancer consists of loose groups of microfollicles comprised of small cells with pale, grooved nuclei (direct smear for Papanicolaou test, original magnification ×400).

POORLY DIFFERENTIATED THYROID CARCINOMA

Poorly differentiated thyroid carcinoma (PDTC) was introduced as a distinct thyroid entity by Granner et al in 1963.⁸⁵ In 1984, Carcangiu et al⁸⁶ defined these tumors as a follicular carcinoma with trabecular, solid, and insular growth patterns. PDTC accounts for a small proportion of all thyroid carcinomas, ranging from 0.3% in Asian countries to 1.8% in the United States. Early publications reported that the morphology, biology, and clinical behavior of PDTC was somewhere between well-differentiated and anaplastic carcinoma. PDTC is frequently widely invasive, with perithyroidal infiltration seen in 60% to 70% of cases at presentation.^86–99

In 2006, the Turin diagnostic criteria for PDTC were established based on histological specimens. Contributors noted the absence of nuclear features of PTC in these insular, solid, or trabecular tumors and at least 1 of the following 3 criteria: convoluted dedifferentiated nuclei, ≥3 mitotic figures per 10HPF, or tumor necrosis.⁸⁷ Cytologically, the diagnosis of PDTC is difficult given their cytomorphologic overlap with other follicular neoplasms²; however, approximately 30% of cases can be suggested on FNA.^89,90 Aspirates of PDTC show high cellularity with the presence of cell clusters, very marked crowding, and also isolated cells (Fig. 8). Colloid is scant to absent. There is often a uniform population of follicular cells with scant cytoplasm. PDTC cells have a high N/C ratio with variable nuclear atypia and occasional mitotic figures.

Figure 8.

(A and B) Fine-needle aspirates of poorly differentiated thyroid carcinoma containing groups of follicular cells with a high nuclear/cytoplasmic ratio with marked crowding. In some cases, the presence of peripheral vessels highlights the insular arrangement of the cellular clusters (direct smears for Papanicolaou test, original magnification ×200).

The largest published series include those by Bongiovanni et al,⁸⁹ with 40 PDTC cases, and Kane and Sharma,⁹⁰ with 44 PDTC cases. A cytological diagnosis of PDTC was made in 5% and 14.5% of these published series, respectively.^89–97 The remaining cases were diagnosed mostly as “suspicious for a follicular neoplasm” or as “carcinoma (either papillary carcinoma, follicular variant of papillary carcinoma, or not otherwise specified).” The most important clues to the FNA diagnosis were marked cellular crowding, an insular/trabecular growth pattern, isolated cells, and a high N/C ratio.^100–107 The presence of mitoses or necrosis should prompt consideration of a PDTC. More recently, an oxyphilic (Hürthle cell) variant of PDTC characterized by a prominent Hürthle cell component in a solid or trabecular arrangement was described.^95,99

PDTC cells typically show granular chromatin in contrast to the fine, evenly distributed chromatin of PTC or the so-called “salt and pepper” chromatin of MTC. In some cases with a prominent single plasmacytoid cell pattern, the PDTC may mimic a lymphoproliferative disorder. Immuno-cytochemically, PDTC is positive for thyroglobulin, TTF-1, PAX-8, and negative for calcitonin. Thyroglobulin expression in PDTC is sometimes characterized by peculiar dot-like paranuclear expression.¹⁰⁷ Other immune-cytochemical markers suggestive of PDTC include p53 that is often positive and an increased Ki-67 level (≥15%). PDTC can resemble a metastasis to the thyroid gland from an extrathyroidal primary tumor, but the positive immunoreactivity of PDTC for thyroglobulin, TTF-1 and PAX-8 helps to exclude a metastasis.

The molecular profile of PDTC is not specific. Rearrangements in RET/PTC and PAX8/PPAR have been reported in 10% and 5% of cases, respectively, and NRAS mutation in codon 61 has been found in 23.5% of PDTCs.¹⁰⁶ A subset of aggressive, radioiodine refractory PDTC (usually in association with BRAF^V600E mutation) have been reported to have AKT1 mutations. Clonal TERT promoter mutations are another finding in a subset of PDTC, and they are also found in some anaplastic thyroid carcinomas.^106–112

ANAPLASTIC THYROID CARCINOMA

Anaplastic thyroid carcinoma (ATC) is recognized as one of the most aggressive primary thyroid malignancies.^108–112 ATC accounts for less than 5% of thyroid cancers, with a peak incidence in the 6th to 7th decades.^108–122 The majority of cases are fatal, often between 6 months and 1 year after presentation, with a median 1-year survival rate of only 10% to 20%. The typical clinical appearance is that of an older patient with a rapidly enlarging firm and fixed infiltrative anterior neck mass.^122–126 These tumors may be associated with pain, dyspnea, hoarseness, dysphagia, and vocal cord paralysis. Up to 50% of patients have distant metastases to the lung, bone, and brain.

ATC is defined as a high-grade pleomorphic and epithelial-derived malignancy with epithelial and spindle features.^112–118 Cytological samples show variable cellularity, but most FNA cases are moderately or highly cellular (Fig. 9). The cyto-morphological features of ATC are those of a high-grade malignancy with epithelioid and/or spindle-shaped cells, as well as some tumor cells with multinucleate giant, plasmacytoid, and/or rhabdoid cell shapes present in a background of tumor diathesis (Fig. 9). Tumor cells show enlarged and pleomorphic nuclei with coarse chromatin clumping, parachromatin clearing, and prominent irregular nucleoli (Fig. 9). Typical and atypical mitotic figures, a necrotic background, and extensive inflammation may be present. While the giant cell variant is a well-recognized morphologic pattern, the presence of osteoclast-like giant cells is less common than other cell patterns.

Figure 9.

(A-C) Fine-needle aspirates of anaplastic thyroid carcinoma consisting of markedly atypical cells with pleomorphic and bizarre shapes, as well as hyperchromatic nuclei, frequent mitoses, and background tumor diathesis (A: direct smears for Diff-Quick and B-C: Papanicolaou test, original magnification ×400).

Some cases of ATC may coexist with well-differentiated and/or poorly differentiated thyroid carcinoma, most often PTC (especially the TCV), but sometimes follicular carcinoma, Hürthle cell carcinoma, insular carcinoma, and other types of PDTC.^106,108 Consequently, on occasion several components can be observed in an aspirate. If ATC arises from a well-differentiated carcinoma, FNA smears may also show cells with features more typical of papillary carcinoma or a follicular neoplasm. The presence of only necrotic material and an inflammatory background—especially in elderly patients with a rapidly growing neck mass—should raise concern for ATC.

It is important to differentiate ATC from other malignant entities such as PDTC, MTC, melanoma, lymphoma, sarcoma (especially for those ATCs with predominant spindle features), and metastatic tumors.^108–134 ATC is usually negative for TTF-1 and thyroglobulin. PAX-8 is retained in over 50% of cases, especially those with epithelioid morphology, and cytokeratin positivity suggests the epithelial nature of ATC. P53, beta-catenin, and cyclin-D1 are overexpressed in a significant proportion of ATC cases. A subset of ATC, especially those with spindled cytomorphology, are negative for epithelial markers. One of the most frequent genetic alterations is a mutation in TP53 (from 30% to 70% of cases) while mutations in BRAF^V600E, RAS genes (NRAS, HRAS, KRAS), PIK3CA, PTEN, and ALK have been reported in approximately 20% of ATCs.^{108–112,135}

CONCLUSION

PDTC, ATC, and certain variants of PTC are important to recognize by FNA because they are associated with a more aggressive biologic behavior and have important implications for patient prognosis and clinical management. For example, ATC is often considered an oncologic emergency, especially if there is an invasion of the airway. Patients afflicted with these cancers may undergo a more extensive clinical diagnostic workup. Given that extrathyroidal invasion is highly likely, patients may require preoperative evaluation of vocal cord function and endoscopy to delineate the extent of invasion. They may also need staging scans to exclude metastases. A cytological diagnosis of PDTC, ATC, or certain variants of PTC can trigger more extensive surgery (eg, tumor debulking, tracheostomy, modified radical neck dissection), and in certain cases—such as those of ATC patients—they may receive adjuvant chemoradiotherapy, including immunotherapy.¹³⁵ Moreover, these patients also require closer surveillance. Individuals with advanced disease and multiple metastases may need symptom palliation. According to the literature, aggressive variants of PTC are responsible for the majority of recurrences, increased morbidity, and shortened disease-free survival.¹³⁶ Their recognition warrants a more aggressive surgical strategy and minimization of treatment-related sequelae. Thus, correctly diagnosing these tumors is of crucial importance in the presurgical phase of evaluating thyroid lesions. Nonetheless, in some cases it may not be possible to diagnose a specific PTC subtype by FNA. However, even a note describing the possibility of an aggressive PTC variant can still be useful. In addition to cytomorphologic and clinical features, ancillary markers and molecular profiling can assist in a subset of cases.^137–140 Familiarity with the cytologic features of PDTC and several of the aggressive variants of PTC is also important to avoid their misdiagnosis as other types of primary or secondary thyroid tumors.