A Case of Thyroid Carcinoma Showing ThymusLike Differentiation With Breast Cancer Susceptibility Gene 2 Mutation: A Case Report and Literature Review

Carcinoma showing thymus-like differentiation (CASTLE) is a rare malignant tumor that originates from ectopic thymic or residual embryonic tissues. CASTLE is specified as a synonym for intrathyroidal thymic carcinoma. The patient is a 66-year-old male. Surgery was performed on the thyroid tumor with tracheal infiltration, and pathological examination revealed CASTLE. Multidisciplinary treatment, including chemoradiotherapy, was performed for recurrent tumors, and he has been alive for 90 months since the initial treatment. The cancer genome panel identified mutations in AT-rich interaction domain 1A (ARID1A) and breast cancer susceptibility gene 2 (BRCA2), but there were no available clinical trials or recommended drugs. BRCA2 may be involved in CASTLE. Herein, we review the literature and report the treatment method and gene mutation for recurrent metastatic cases of CASTLE, for which standard treatment has not been established.