INI1-Deficient Thyroid Carcinoma is an Aggressive Disease with Epithelioid and Rhabdoid Phenotype. A Case Report, Survey of INI1 Expression in Thyroid Lesions and Literature Review.

Integrase interactor 1 (INI1)-deficient carcinomas, recently described in several sites including the head and neck, are associated with basaloid or rhabdoid histology and aggressive behavior irrespective of origin. INI1-deficient thyroid carcinoma is extremely rare. We present here the phenotype and genotype of an INI1-deficient thyroid carcinoma and report on the INI1 protein expression in various thyroid lesions. Case report with clinicopathologic and molecular characterization and INI1 assessment in 184 thyroid lesions. A 67-year-old woman presented with globus sensation due to a large thyroid mass with extrathyroid extension, focal necrosis and cervical and mediastinal nodal involvement. Histologically, tumor cells had a solid, alveolar and pseudopapillary architecture in a myxoid stroma, exhibited monomorphic epithelioid and focal rhabdoid/plasmacytoid morphology and lacked glandular, squamous or follicular cell differentiation. Tumor cells were positive for AE1/AE3 and CK18 but negative for TTF1, thyroglobulin and PAX8. INI1 nuclear expression was absent. A frameshift SMARCB1/INI1 mutation was detected. In addition, TET2 and Notch1 mutations were present but alterations of BRAF, RET, PAX8/PPAR8 or RAS were not identified. Patient death occurred 14 months after diagnosis from post-therapeutic complications. None of the 184 benign and malignant thyroid lesions tested, including 12 poorly and undifferentiated thyroid carcinomas, were INI1-deficient. INI1-deficient thyroid carcinoma shares the phenotype, genotype and biology of other INI1-deficient tumors. Epithelioid and plasmacytoid/rhabdoid changes are most frequent whereas basaloid morphology is not reported, in contrast with sinonasal tumors. Poorly differentiated and undifferentiated thyroid tumors with epithelioid or rhabdoid morphology should be tested for INI1 protein expression to better characterize these aggressive neoplasms and identify patients eligible for targeted therapy.