Abstract: This article reports a case of medullary thyroid carcinoma (MTC) discovered during postoperative follow-up of a patient with triple-negative breast cancer (TNBC).The patient was diagnosed with TNBC (cT2N1M0,stage Ⅱb) in March 2019.After six cycles of neoadjuvant chemotherapy (TAC),the patient underwent "left breast cancer breast-conserving radical mastectomy" and completed radiotherapy (50 Gy/2.0 Gy,in 25 fractions) and adjuvant intensification chemotherapy.In March 2023,the patient presented with a right cervical mass,which was assessed as benign on imaging but ultimately diagnosed as MTC.This case is rare,suggesting that for breast cancer patients with thyroid nodules,the possibility of malignancies such as medullary carcinoma should be vigilant.

Key words: multiple primary carcinoma, triple negative breast cancer, medullary thyroid carcinoma, case report