Spindle Epithelial Tumor With Thymus-Like Elements: A Case Report.

Spindle epithelial tumor with thymus-like elements (SETTLE) presents as a neck swelling in children and adolescents. It is a rare malignant neoplasm derived from ectopic thymus or branchial pouch derivatives. It has a propensity for late hematogenous metastasis. Morphologically, it is a biphasic tumor composed of fascicles of spindle cells and epithelial components in the tubulo-papillary pattern. Due to its rarity, pathologists often misdiagnose it. Synovial sarcoma is its main morphologic mimic, but SETTLE lacks t(X;18) translocation. Surgery is the mainstay of treatment. Radiotherapy can be offered for local recurrence and chemotherapy for distant metastases. We report a case of gradually increasing left thyroid swelling in a boy from Pakistan. His thyroid function tests were normal. Cytology showed a spindle cell lesion, Bethesda category 5. The patient underwent a total thyroidectomy. The left thyroid lobe showed a partially encapsulated, tan-white, firm, homogenous nodule. Histology showed an encapsulated, focally infiltrating tumor, composed of spindle and epithelial cells in lobules with mild atypia. Based on immunohistochemistry, medullary carcinoma and carcinoma with thymus-like elements (CASTLE) were ruled out. Molecular analysis for t(X;18) (p11.2; q11.2) ruled out synovial sarcoma. Therefore, the final diagnosis was SETTLE. The patient died at 42 months of follow-up due to local recurrence and lung metastases. SETTLE is a rare childhood thyroid neoplasm with late metastatic potential. Early diagnosis, surgical resection, and surveillance can ensure disease-free survival. Diagnosed cases should be reported to help formulate effective chemotherapy and radiotherapy regimens for the treatment of recurrence or metastasis.