Capicua transcriptional repressor (CIC)-rearranged sarcoma (CRS) is a rare and highly aggressive malignant soft tissue small round cell sarcoma. Most CRS tumors occur in the soft tissues of the limbs and trunk and cases in the thyroid gland are extremely rare, with no extant reports in an adult thyroid. This report aims to familiarize clinicians and pathologists with CRS in the thyroid gland, thereby helping them identify new cases, and provide pathological evidence for comprehensive diagnosis and treatment. A 30-year-old woman presented with an asymptomatic right thyroid mass of 5 months. Neck ultrasonography revealed a hypoechoic hypervascular nodule with a maximum diameter of 5 cm. The noncalcified nodule was classified as Ti-RADS 4a. Besides thyroid dysfunction, her thyroglobulin and anti-thyroid peroxidase antibody concentrations were 35.83 IU/mL and >
1,000.00 IU/mL, respectively. CRS was diagnosed based on the histomorphology, immunohistochemistry, and molecular genetics results. The patient opted for further treatment at Zhejiang Cancer Hospital. Ten months after surgical removal, positron emission tomography/computed tomography revealed tumor metastasis to the cervical lymph nodes and lungs. In February 2023, the patient underwent postoperative adjuvant radiotherapy for the tumor bed and cervical lymph nodes and received doses of PGTVtb 6,000 cGy/30F/DT and PTV 5,400 cGy/30F/DT. The patient last visited the clinic in May 2024 due to fever, fatigue, diarrhea, and other symptoms. CRS in solid organs is extremely rare
however, when the morphology shows multinodular, small, round cells with mild-to-moderate atypia, vacuolated nuclei, and prominent nucleoli, it should be considered as a potential differential diagnosis.