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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.

Signet-ring cell/histiocytoid carcinoma (SRCHC) is a rare, aggressive neoplasm that often originates in the eyelid. We present a rare case of a 64-year-old male with SRCHC and papillary thyroid carcinoma (PTC) that underwent exome panel sequencing with next-generation sequencing (NGS). In addition, we reviewed reports of genetic mutations in SRCHC and compared them with our results. The imaging findings allowed us to recognize the differences in pathology between the left and right cervical nodes. For first-line treatment, an extended total maxillectomy with orbital exenteration and dissection of the left neck was performed. Two months later, total thyroidectomy and right neck dissection were performed. Two years after surgery, multiple bone metastases occurred. An exome panel sequence with NGS was used to determine the chemotherapy regimen. Notably, somatic mutations in cadherin 1 (CDH1), human epidermal growth factor receptor 2 (ERBB2), neurofibromin 1 (NF1), and tumor protein p53 (TP53) were detected. These mutations are rarely detected in PTC; therefore, cervical metastases are assumed to originate from SRCHC. To our knowledge, there have been no reports of simultaneous cancer of SRCHC and PTC. Somatic mutations in CDH1, ERBB2, NF1, and TP53 were detected in the exome panel sequence of the metastatic lymph nodes of SRCHC and correlated with previous reports of SRCHC.