Sclerosing Mucoepidermoid Carcinoma With Eosinophilia: A Diagnostic Challenge

Sclerosing mucoepidermoid carcinoma with eosinophilia (SMECE) is a very rare thyroid cancer with fewer than 100 reported cases in the literature.1 It is associated with Hashimoto's thyroiditis and differs from salivary gland-type mucoepidermoid carcinomas in the thyroid and larynx.2 Its rarity makes it a diagnostic challenge. We present a case of SMECE in a patient initially diagnosed with squamous cell carcinoma (SCC). The Penn State University Institutional Review Board determined that review and approval were not required (STUDY #00024879). A 62-year-old female with no significant past medical history presented with an incidentally found left thyroid mass that had been present for 2 months (Figure 1A). The initial biopsy was suggestive of SCC. The patient underwent direct laryngoscopy with biopsies obtained from different regions, and tracheal biopsy was positive for keratinizing SCC. This led to the impression that the SCC arose from the trachea and invaded the thyroid, stage T4N0M0 (IVA). The patient underwent concurrent chemoradiation but had persistent 18F-fluorodeoxyglucose avidity on positron emission tomography-computed tomography within the thyroid a few months later. The patient underwent total thyroidectomy, left central neck dissection, and partial tracheal resection. Surgical pathology reported focal SCC of the left paratracheal mass and chronic lymphocytic thyroiditis and fibrosis with invasion by SCC of the thyroid. Six years later, she developed left vocal fold paralysis. Imaging showed a soft tissue mass surrounding the cervical trachea, adhering to the cervical trachea and esophagus (Figure 1B). Fine needle aspiration showed highly atypical cells suspicious for recurrent SCC. She preferred to undergo systemic therapy but, unfortunately, had significant toxicity, including nephrotoxicity. A total laryngectomy with tracheal resection, cricopharyngotomy, and partial esophagectomy was performed. Histologic sections showed infiltrative carcinoma composed of epidermoid/squamous cells and mucous cells arranged in infiltrative sheets, strands, and pseudoangiomatous patterns. The neoplastic cells had enlarged nuclei and prominent nucleoli. Tumor cells showed focal intracytoplasmic and extravasated mucin on hematoxylin and eosin morphology and a mucicarmine stain. Dense sclerosis with lymphoplasmacytic and abundant eosinophilic infiltrate was observed in the background (Figure 2). SMECE of the thyroid is a very rare malignancy with a strong female predominance (16:1) that typically affects patients ages 50 to 70.2 Due to its rarity, its clinical course is poorly understood but may be more aggressive than previously thought.3, 4 SMECE is a primary thyroid carcinoma that contains epidermoid/squamoid and mucous cells in a background of stromal sclerosis and an inflammatory infiltrate containing eosinophils. It should be differentiated from solid cell nests with squamous metaplasia, papillary thyroid carcinoma with squamous metaplasia, medullary thyroid carcinoma, conventional (salivary gland type) mucoepidermoid carcinoma, primary SCC of the thyroid (now considered anaplastic thyroid carcinoma), and, similar to this case, SCC of a nearby organ such as the larynx with invasion or metastasis to the thyroid. Extrathyroidal, lymph node or distant metastasis may be present.1 This case highlights the challenges of diagnosing SMECE. The majority of the tumor was initially within the thyroid, but it was presumed to be a primary tracheal malignancy extending into the thyroid, as primary SCC of the thyroid is extremely rare. Although present in the previously excised specimen, the mucinous component was scant, while the predominant tumor showed squamous morphology. However, after combining the patient's findings (thyroid mass, biphasic, ie, squamous and mucinous tumor morphology, prominent eosinophils, sclerotic background, and associated Hashimoto's thyroiditis), SMECE is more in line with this patient's findings. Chan et al first described the distinct histopathology of this malignancy. It is often associated with chronic lymphocytic thyroiditis, particularly the fibrosing variant of Hashimoto thyroiditis.2 This carcinoma lacks immunophenotypic expression of thyroid follicular epithelial cells (thyroglobulin, PAX8, TTF-1). It is included in the category of tumors of uncertain histogenesis in the WHO classification of thyroid tumors. However, the histogenesis of this carcinoma subtype is considered to be of solid cell nest origin.5 It may resemble but is considered unrelated to the conventional (salivary gland type) mucoepidermoid carcinoma and lacks the associated characteristic MAML2 rearrangement.1 Awareness and a high index of suspicion for this entity can help reduce misdiagnosis. Bao Y. Sciscent, writing—original draft, visualization, data collection and interpretation; Hanel W. Eberly, writing—review and editing, data collection and interpretation; Sanica Bhele, conceptualization, visualization, writing—review and editing; Neerav Goyal, supervision, conceptualization, visualization, writing—review and editing. None. None.