Spindle Epithelial Tumor With Thymus-Like Differentiation (SETTLE) Misdiagnosed as Papillary Thyroid Carcinoma: A Case Report

Spindle epithelial tumor with thymus-like differentiation (SETTLE), a rare tumor of the thyroid gland, is difficult to diagnose irrespective of its unique morphology. It is usually misdiagnosed as synovial sarcoma, thymoma, teratoma, or other thyroid carcinomas. In the current case report, we detail a case of a 36-year-old male patient who presented with thyroid swelling that was initially misdiagnosed as papillary thyroid carcinoma instead of SETTLE. Based on fine needle aspiration, the tumor showed a variable pattern with features suggestive of follicular neoplasm in the right lobe and atypia of undetermined significance in the left lobe. Pathological examination showed multiple nodules on both the right and left lobes, with the largest nodule measuring 4.8 x 4.5 x 3 cm. On microscopic examination, a predominant papillary pattern was observed along with spindle cell areas. Immunohistochemistry revealed positive staining for thyroglobulin, CK, HMWCK, CD99, and BCL-2, which led to the diagnosis of SETTLE. The rare nature of the condition and the reduced awareness about it make this tumor a diagnostic challenge. This case report concludes that in case of any biphasic tumor with epithelial and spindle cells in the thyroid gland, it is important to consider the differential diagnosis of SETTLE. Immunohistochemistry is more useful for diagnosing SETTLE, and thus pathologists are encouraged to judiciously advise the patients for immunohistochemistry to establish accurate and efficient diagnosis.