Squamous Differentiation in the Thyroid: Metaplasia, Neoplasia, or Bystander?

Abstract

Background.

Squamous differentiation within the thyroid is seen in a variety of settings. Squamous epithelium is non-native to the thyroid, and its debated origins span reactive metaplasia and developmental/embryologic remnants. Despite a lack of clarity as to its evolution, squamous epithelium may be associated with both neoplastic and non-neoplastic processes.

Methods.

Thyroid pathology reports spanning a 30-year period were reviewed for terms indicating squamous features. Associated diagnostic and clinical information was collated.

Results.

Four hundred and twenty seven of 17,452 (2.4%) thyroid surgical pathology cases during this period utilized terminology indicating squamous differentiation including 243 malignant (58%) and 178 benign (42%) diagnoses. There were 111 (26%) primary thyroid malignancies with squamous differentiation, 116 (28%) malignancies of non-thyroid origin including local extension from nearby cancers, and 16 (4%) malignancies of uncertain primary. Most benign lesions were non-neoplastic (84%). The minor subset representing benign neoplasia was interpreted as secondary reactive changes.

Conclusion.

While squamous differentiation is seen routinely in the thyroid, it is most commonly reported in malignancy. For primary thyroid malignancies reported to demonstrate a squamous component, biologically aggressive tumors were overrepresented. Available evidence suggests that multiple pathways may contribute to the presence of squamous epithelium in the thyroid including metaplasia of mature follicular cells, development from established embryonic remnants, or inception in putative, incompletely characterized stem-like cells. Our retrospective review presents an institutional landscape from which further investigation into the frequency and unique histologic and molecular context of intrathyroidal squamous differentiation as a driver or terminal event in thyroid pathophysiology.

Introduction

Squamous differentiation within the thyroid is seen in a wide spectrum of diseases, from benign and reactive conditions such as chronic lymphocytic thyroiditis or post-fine-needle aspiration (FNA) biopsy changes to biologically aggressive malignancies like anaplastic thyroid carcinoma (ATC), including the squamous cell carcinoma (SCC) subtype.¹ Squamous epithelium is not native to the thyroid, and its debated origins span reactive metaplasia to remnants of thyroglossal duct and branchial pouch.^2,3

Historically, some authors have distinguished several distinct types of cells considered “squamous cells” in the thyroid: basaloid cells, epidermoid cells, and true squamous cells.² Basaloid cells are small and dark, associated with solid cell nests (ultimobranchial body remnants) and are frequently found in a lateral distribution within the gland, attesting to their origin.³ Epidermoid cells are described as squamoid cells without keratinization or intercellular bridges which are reminiscent of the intermediate tumor cells in mucoepidermoid carcinoma. True squamous cells demonstrate keratinization and/or intercellular bridges.

Thyroglossal duct remnants occur along the midline of the neck and the thyroid isthmus as a result of embryonic migration with the fetal pharynx. While this phenomenon likely accounts for midline thyroglossal duct cysts and some proportion of midline squamous epithelium, it does not account for squamous lesions occurring throughout the gland, in sites of inflammation and reactive processes, or as a component of neoplastic processes. Squamous metaplasia adjacent to these processes, and sometimes with transitional metaplastic forms between squamous cells and follicular epithelial cells, argues that reactive and neoplastic changes are also a source of squamous cells in the thyroid.² Indeed, 20% to 40% of papillary thyroid carcinomas (PTC) have squamous differentiation.⁴

Pathologists are not infrequently confronted with the presence of squamous differentiation in the thyroid and must infer its significance or lack thereof in the absence of robust, definitive data. The purpose of this study is to examine and characterize the range of entities in which one finds squamous differentiation reported in the thyroid to approach a better understanding of this process and to perform an integrative review of relevant literature regarding what is known and yet to be elucidated regarding this phenomenon. We additionally aim to illustrate key benign and malignant squamous entities within the thyroid.

Materials and Methods

After Institutional Review Board (IRB) approval at Massachusetts General Hospital (2011P000013), the pathology database was queried for thyroid specimen reports with squamous differentiation indicated in the diagnosis over a 30-year period. Relevant pathology reports were selected for review, and data extracted included demographic information, diagnosis, prior and subsequent pathology reports/diagnoses, and molecular testing, if available. Variables related to the clinical course were derived from the electronic medical record.

Results

The presence of squamous epithelium in the thyroid was reflected in pathology reports by a range of terminologies including the search term “squamous.” The following phrases indicated pathological findings including a squamous component in this study sample: “squamous cell carcinoma,” “carcinoma, squamous type,” “carcinoma, consistent with squamous cell,” “poorly differentiated carcinoma, probably poorly differentiated squamous cell carcinoma,” “squamous metaplasia,” “squamous metaplastic epithelium,” “with squamous component,” “with squamous differentiation,” “with squamous features,” “squamous epithelium,” “squamous epithelial lining,” “squamous-lined,” “squamous cell nests,” and “squamous morules.” Pathology reports utilizing any of the above terminologies to describe lesions within the thyroid were selected for inclusion in this study (n = 421 cases), representing 2.4% of all thyroid surgical pathology specimens diagnosed during the study period (n = 17,452 cases).

The series included 421 patients: 214 females, 207 males (M:F ratio = 0.97), mean age 53 years (SD = 16.7, range: 1-95 years). In males, almost half of all cases were SCC (47%, n = 97). More than half of the series of squamous lesions of the thyroid were malignant (58%, n = 243 of 421). Among malignancies, secondary malignant neoplasms (48%, n = 116) were similar in frequency to primary thyroid neoplasms (46%, n = 111). In 16 cases of SCC, the primary site was not determined. Most of the secondary neoplasms were head and neck SCC, most commonly of laryngeal origin. Secondary involvement was more often by direct extension (63%, n = 73) compared to metastasis (24%, n = 28). A minority of cases were undetermined (13%, n = 15). Nearly all the secondary tumors represented SCC; there was one adenosquamous carcinoma, the larynx, and one prostatic adenocarcinoma with squamous differentiation (Table 1).

Table 1.

Malignancies Showing Squamous Differentiation in the Thyroid.

		n (%)
Primary (n = 111)	Papillary carcinoma	84 (34%)
	Unspecified	32 (13%)
	Diffuse sclerosing	20 (8%)
	Classic/conventional	19 (8%)
	Follicular variant	7 (3%)
	With high-grade features	1 (<1%)
	Tall cell	1 (<1%)
	Columnar cell	1 (<1%)
	Oncocytic	1 (<1%)
	Anaplastic thyroid carcinoma	21 (9%)
	Follicular carcinoma	3 (<1%)
	Poorly differentiated carcinoma	2 (<1%)
	Mucoepidermoid carcinoma	1 (<1%)
	Medullary carcinoma	1 (<1%)
	Intrathyroid thymic carcinoma	1 (<1%)
Secondary (n = 116)
	Larynx	73 (30%)
	Pharynx	13 (5%)
	Tongue	10 (4%)
	Trachea	8 (3%)
	Esophagus	5 (2%)
	Lung	3 (<1%)
	Tonsil	1 (<1%)
	Lip	1 (<1%)
	Vulva	1 (<1%)
	Prostate	1 (<1%)
Uncertain (n = 16)	Squamous cell carcinoma	16 (8%)
Total		243

PTC accounted for the majority (n = 82; 74%) of the primary thyroid malignancies. ATC was second in frequency in the series (n = 21) (Table 1). Approximately half of the ATCs (9 of 21) and poorly differentiated thyroid carcinomas (1 of 2) in the series were noted to arise in association with PTC as was the single case of mucoepidermoid carcinoma. Figure 1 summarizes the variety of these malignant primary thyroid tumors, and Figure 2 illustrates representative examples.

Figure 1.

Primary malignant thyroid neoplasms with squamous features, with papillary thyroid carcinoma (PTC) being the most common (n = 82; 74%). Among PTC with a specified subtype, diffuse sclerosing variant and classical type were the most common.

Abbreviations: Car, carcinoma; Diff, differentiated.

Figure 2.

Malignant thyroid neoplasms with squamous differentiation. (A) Papillary thyroid carcinoma (PTC) with foci of squamous metaplasia and fibrosis (upper right) corresponding to prior biopsy site tract. (B-C) Diffuse sclerosing variant of PTC characteristically displays foci of squamous metaplasia. (D) Classical type PTC with regions of squamous metaplasia is seen in association with cystic change. (E-F) By immunohistochemistry, the foci of squamous differentiation are reactive for p40 (E) and keratin 5 (F). (G) Another classical type PTC shows an abrupt transition to the squamous metaplastic component (bottom). (H) Mucoepidermoid carcinoma with its characteristic squamoid tumor cells and scattered admixed mucocytes. (I) Anaplastic thyroid carcinoma (ATC), squamous cell subtype, with pleomorphic tumor cells and mitotic activity.

Pathologic staging information was obtainable in 84 (76%) of the primary thyroid carcinomas as follows: T1a (n = 13), T1b (n = 19), T2 (n = 10), T3 (n = 27), T4a (n = 7), and T4b (n = 8) by AJCC (seventh edition) staging. Three cases were recurrent, and 24 were multifocal. Lymph nodes were sampled in 54 cases, with nodal staging as N0 (n = 14), N1a (n = 21), and N1b (n = 19). Follow-up was available for 73 (65%) of the primary thyroid carcinomas (mean = 128 months, median = 36 months, range: 2-455). Forty-two patients were alive and disease free, 20 patients were alive with disease, 7 patients died of disease (5 ATC, 2 PTC), and 4 patients died of unrelated causes.

Molecular information was available in 4 cases, including three thyroid primaries and one metastasis, each in a patient with clinically aggressive disease. BRAF V600E mutations were detected in one case of ATC (epithelial subtype) and one case of poorly differentiated thyroid carcinoma, each co-occurring with PTC. A KRAS mutation (G12V in exon 2) was identified in one case of ATC (SCC subtype) and its subsequent lung metastases. Lastly, HPV type 16 was detected in a metastatic tonsillar SCC.

A spectrum of benign entities was described as exhibiting squamous differentiation, including reactive, degenerative, inflammatory, developmental, and neoplastic conditions (Table 2). Thyroid cysts were the commonest diagnosis (32%, n = 57) followed by chronic lymphocytic thyroiditis (25%, n = 45). Nonspecific reactive changes (12%, n = 22) were described in the setting of fibrosis and hemorrhage. Neoplasia (follicular adenoma) accounted for only a minority of these cases (16%, n = 28), and in all of these cases, the squamous metaplasia was interpreted as a reactive secondary change, associated with hemorrhage, fibrosis, cystic change, and/or calcification, frequently in a post-FNA biopsy setting. Three cases listed as atypia not otherwise specified comprised diagnostic challenges precluding a definitive diagnosis but were favored to represent benign changes. Figure 3 summarizes the variety of these benign entities. Figure 4 highlights representative cases.

Table 2.

Nonmalignant Entities Showing Squamous Differentiation in the Thyroid.

	n (%)
Cyst	57 (32%)
Thyroglossal	42 (24%)
Degenerative/NOS	8 (4%)
Branchial cleft	7 (4%)
Chronic lymphocytic thyroiditis	45 (25%)
Follicular adenoma	28 (16%)
Reactive changes	22 (12%)
Multinodular goiter	17 (10%)
Adenomatous nodule	4 (2%)
Atypia NOS	3 (2%)
Diffuse goiter	1 (0.6%)
Granulomatous thyroiditis	1 (0.6%)
Total	178

Figure 3.

Benign diagnoses of the thyroid demonstrating squamous features, with cystic lesions being the most common (32%, n = 57).

Abbreviation: NOS, not otherwise specified.

Figure 4.

Benign thyroid processes with squamous differentiation. (A) At scanning magnification, a thyroglossal duct cyst shows multiple dilated cysts surrounded by a background of fibrosis and reactive changes. (B) This thyroglossal duct cyst lining is composed of true squamous cells with intercellular bridges. (C) At scanning magnification, long-standing chronic lymphocytic (Hashimoto) thyroiditis shows multiple cysts surrounded by chronic inflammation and atrophic changes. (D-F) The nests of proliferative cells within chronic lymphocytic thyroiditis show intermediate-type squamoid cells with scant cytoplasm in the upper left and periphery of nests, and well-developed squamous cells with abundant cytoplasm centrally (D). By immunohistochemistry, both cell populations are positive for p63 (E) while the peripheral component stains more strongly with keratin 5 (F). (G) Foci of squamous metaplasia showing intercellular bridges and early cyst formation in chronic lymphocytic thyroiditis. (H) Prominent reactive, squamoid-appearing solid cell nests present in non-neoplastic, uninflamed thyroid tissue. (I) A focus of squamous metaplasia (circle) is identified among cholesterol clefts, multinucleated giant cells, fibrosis, and chronic inflammation within a prior biopsy site tract.

Discussion

Pathological conditions of the thyroid, both neoplastic and non-neoplastic, may manifest with variable squamous differentiation. In this reporting-based retrospective series, the presence of squamous differentiation is most frequently documented in cases of malignancy (58%, n = 243). Among primary thyroid tumors, approximately 20% are the biologically more aggressive subtypes of thyroid carcinomas (ATC, poorly differentiated thyroid carcinoma, and medullary thyroid carcinoma). In comparison, the usual prevalence of these more aggressive subtypes is collectively less than 10% of all primary thyroid malignancies.^5,6 These findings suggest that prominent squamous features, when reported, are associated with more aggressive thyroid tumor types. Additionally, this principle extends to the usually indolent PTC. In this case series, more aggressive subtypes of PTC were disproportionately represented including diffuse sclerosing variant (n = 20, 24%), tall cell variant (n = 1, 1%), and columnar cell variant (n = 1, 1%).^7,8 Squamous metaplasia is a characteristic feature of the diffuse sclerosing variant,⁹ in keeping with its over-representation in this case series.

Within the broader literature, squamous differentiation has been associated with aggressive tumor biology in the thyroid, most notably the SCC subtype of ATC.^10,11 Whether squamous features are independently predictive of poorer prognosis has yet to be discerned, as squamous differentiation is commonly seen in the context of other features predictive of aggressive behavior such as large size, invasion/metastases at presentation, and certain inherently more aggressive subtypes of PTC such as diffuse sclerosing variant, as noted above.¹² Indeed, recent data from a large cohort of ATC indicate that squamous features are not independently prognostic, as those with a pure SCC phenotype demonstrate the same dire prognosis as other ATC.¹³

The majority of the nonmalignant diagnoses in this series were non-neoplastic cysts and chronic lymphocytic thyroiditis. Benign neoplasia (follicular adenomas) occasionally showed a squamous component, but these were uniformly interpreted as superimposed reactive changes rather than inherent tumor characteristics. Preceding FNA biopsy was typical, and secondary changes were frequently attributed to the prior procedure even though the defined molecular mechanisms for these changes have never been reported.

The overall diversity of primary thyroid diagnoses exhibiting squamous differentiation suggests similarly complex etiologies. Summarizing the available literature, a trio of principal pathways emerge and include: (1) development from established embryonic remnants (ultimobranchial bodies/solid cell nests or thyroglossal duct); (2) metaplasia from mature neoplastic or non-neoplastic follicular cells; and (3) variable putative reservoirs (including p63-positive stem-like cells).^2,4,14-17 Development from established embryonic remnants, as in thyroglossal duct cysts, is the most well-established pathway, accounting for most thyroid cysts.² Similarly, ultimobranchial bodies/solid cell nests are the presumptive origin for sclerosing mucoepidermoid carcinoma with eosinophilia.¹⁷ Squamous metaplasia is the favored explanation for thyroiditis, primary thyroid mucoepidermoid carcinoma, and ATC.^4,14 Bai et al also invoke this process to explain squamous differentiation within a follicular adenoma, describing a follicular-patterned adenoma with squamous differentiation and a transitional intermediate cell population with mixed morphology and immunohistochemical properties.¹⁶ The third possibility, that of a reservoir of p63-positive stem-like cells, is less well understood. Burstein et al propose that p63-positive embryonal remnants with stem cell-like properties may form the basis for squamous cells in the thyroid via squamoid maturation, follicular epithelial differentiation, immune reaction, or oncogenic transformation.¹⁵ They observed a stem cell-like partitioning in solid cell nests and PTC wherein one cellular component was p63-positive and the other negative, and, although there is some controversy regarding the specificity of this data,^18,19 the data and its implications are relevant to consider.

The metaplasia to dysplasia to neoplasia pathway has been well established in other organ systems.²⁰ It is controversial in the thyroid, where extensive squamous metaplasia, such as that often associated with long-standing chronic lymphocytic thyroiditis, does not appear to predispose to squamous thyroid malignancies such as the SCC subtype of ATC^21,22; however, squamous metaplasia has been linked to PTC in multiple meta-analyses.^23,24 A related concept, “follicular epithelial dysplasia” has been proposed in the thyroid; putative dysplastic foci show morphologic features evoking PTC that demonstrate a PTC-like immunophenotype and occur in the setting of chronic lymphocytic thyroiditis.²⁵ However, this concept has not been reproduced within the literature, and formal studies investigating putative mechanisms or molecular pathogenesis are lacking. Further, the concept of premalignant lesions in the thyroid, in general, is marked with controversy, including the idea that follicular thyroid adenomas and the noninvasive follicular neoplasms with papillary-like nuclear features are precursor lesions.²⁶

Molecular and genetic studies of malignant entities with squamous differentiation in the thyroid also support a heterogeneous mix of oncogenic pathways for these entities. While historically primary SCC of the thyroid was considered a separate entity from ATC,²⁷ recent studies substantiate its classification as a morphologic subtype of ATC, a change endorsed in the upcoming fifth edition of the World Health Organization (WHO) Endocrine Tumor classification. In addition to similar clinical features and aggressive biology, thyroid carcinomas with a pure SCC phenotype are often associated with a prior or concurrent diagnosis of differentiated thyroid carcinoma and frequently harbor BRAF V600E mutations like other ATC.¹³ While cases of classical PTC that show transformation to the SCC subtype of ATC harbor BRAF mutations,^28,29 the diffuse sclerosing variant of PTC, with its characteristic squamoid features, has reduced rates of alterations in BRAF. This variant more frequently demonstrates RET-translocations, particularly in young patients.^30,31 Cribriform morular carcinoma, with its typical squamous morules, frequently demonstrates AFP mutations³² and is now classified as a distinct entity from PTC (rather than a PTC variant) given its distinct morphologic, immunophenotypic, and molecular features.³³ Non-PTC associated entities with squamous differentiation include primary thyroid mucoepidermoid carcinoma, which in at least one case demonstrated the classic CRTC1/MAML2 translocation associated with its salivary gland counterpart,^34,35 and sclerosing mucoepidermoid carcinoma with eosinophilia which has a notable lack of recurrent genomic alterations, including no BRAF, MAML2, or common driver mutations identified to date.^14,17

Regardless of etiology, this study supports that squamous epithelium is a feature in a diverse set of thyroid diagnoses, including neoplastic, non-neoplastic, benign, and malignant entities, with the observation that when squamous features are noted in thyroid malignancies, they tend to be associated with more aggressive phenotypes. Limitations in this study include the use of retrospective pathology reports where reporting bias is expected to capture significantly fewer non-neoplastic and/or benign entities whose detailed features may be overlooked, or—if noted—are less frequently explicitly characterized. Therefore, our methodology of specifically reviewing cases that mention squamous features likely markedly underrepresents the non-neoplastic and benign neoplasias in which these features may be considered routine and/or would not warrant mention in the report. Indeed, given how common the benign and reactive thyroid diagnoses are and the seemingly routine or unremarkable nature of some forms of squamous differentiation, it is quite plausible that benign lesions more commonly demonstrate squamous differentiation than malignant ones within the thyroid. The inherent selection bias in this study precludes the ability to infer the actual prevalence of squamous differentiation within benign and malignant thyroid entities. Additionally, there was inevitable variability in report content and style, with nonuniform working definitions of concepts such as squamous and squamoid. However, despite these limitations, our findings and summary of the literature set the stage for future studies which would outline diagnostic criteria for squamous lesions in the thyroid and systematically review consecutive thyroid specimens for these features to establish true prevalence, including assessment of morphologic types of squamous cells such as basaloid cells, epidermoid cells, and true squamous cells as well as the morphologic degree of differentiation and cytologic atypia of this component. Single-cell analysis in heterogeneous lesions might also be able to distinguish unique molecular characteristics of squamous cells within both purportedly metaplastic squamous cells and their neoplastic and/or malignant counterparts. These studies would be costly to perform in the thyroid and would await rationale, likely based upon therapeutic options from other organ systems manifesting more frequent lethal squamous neoplasia. Studies of this type would be key to identifying the driver/bystander role of squamous differentiation in thyroid neoplasia.

Conclusion

We have described over 400 thyroid lesions with squamous features associated with a diversity of diagnoses. Our findings are notable for a higher than expected proportion of diagnoses with poor prognosis and aggressive biological behavior, though our case selection method effectively enriched for malignant diagnoses. Further investigation of the molecular features of isolated squamous cells, in both neoplastic and non-neoplastic contexts, should provide necessary insight into the role of squamous cells in the thyroid as a consequence or instigator of neoplasia.