Abstract
RATIONALE: Medullary thyroid carcinoma (MTC) is an aggressive subtype of thyroid cancer with frequent hematogenous metastasis. While its metastasis is usually observed in the lung, liver, or bone, it rarely migrates to the breast. PATIENT CONCERNS: Here we report 2 cases with a complaint of breast lump after initial treatment of MTC. DIAGNOSES: In both patients, the MTC characteristics of breast nodules were confirmed by pathologic analysis of biopsy specimens. INTERVENTIONS: The genetic mutations within the metastatic breast lesion were evaluated. Wide local excision was thus performed to 1st case, while no therapeutic intervention for another patient due to the wide-spread presence of the disease. OUTCOMES: No sign of relapse or metastasis was found in 1st case during a 14-month follow-up. For 2nd case, the breast nodule grew to 14 mm within 3 months before remaining stable for 10 months. LESSONS: MTC can be a very indolent disease despite its aggressiveness. Reoperation should be considered for patients with local recurrence or resectable distant metastasis of MTC. The findings for both cases supported serum calcitonin as an important marker for the evaluation of disease. Future studies are needed to advance our understanding of its molecular features and improve strategies for its diagnosis and treatment.