Intrathyroidal thymic carcinoma comprising squamous cell and small cell carcinoma components: A case report

Intrathyroidal thymic carcinoma (ITC) is a rare malignant epithelial tumour of thymic origin occurring within the thyroid. Histologically, it resembles thymic carcinoma, with squamous cell carcinoma being the most common subtype, and immunohistochemical staining typically exhibits features consistent with thymic neoplasms.

We report the case of a 68-year-old woman who presented with a left-sided neck mass of one year's duration. And the neck lump had been gradually enlarging over the course of a year, reaching the size of a goose egg within six months. Thyroid ultrasound revealed a normally sized thyroid gland. A 3.9 cm × 3.4 cm × 2.7 cm hypoechoic lesion with irregular echogenicity was observed outside the capsule of the lower pole of the left lobe. The mass exhibited regular morphology, well-defined margins, and close adherence to the thyroid's lower pole. Microscopic examination revealed two distinct tumour cell populations: Squamous cell carcinoma and small cell carcinoma. Immunohisto-chemical staining demonstrated divergent differentiation - one population was positive for cluster of differentiation 5 and cluster of differentiation 117, supporting thymic origin, while the other exhibited neuroendocrine differentiation with synaptophysin and chromogranin A positivity. At eight months postoperatively, the patient remained recurrence-free on chemotherapy.

Based on these findings, the patient was diagnosed as ITC with both squamous cell and small cell carcinoma components. To date, nearly 100 cases of ITC have been reported in the literature. However, no prior reports of ITC exhibiting both squamous cell and small cell carcinoma components. This case report provides information on the microscopic morphological features of ITC with both squamous cell and small cell carcinoma components, which can help pathologists to expands the understanding of the pathological spectrum of the disease.