Recurrent Metaplastic Breast Cancer with Subtype Converted from Triple-Negative to HER2-Positive: A Case Report and Literature Review

Background: Metaplastic breast cancer (MBC) is an extremely rare malignant breast disease that has rarely been reported. The molecular subtype of MBC is mostly triple-negative, with a high recurrence rate and a worse prognosis. Due to its low HR- and HER2positive rate, reports on endocrine and targeted therapy are very limited.Case report: We report a case of infrequent triple-negative MBC, which, although at an early stage, quickly developed multiple recurrent lesions in the chest wall. The tumor relapsed repeatedly after comprehensive treatment, including surgery, chemotherapy and radiotherapy. However, pathological results after the third surgery suggested that the molecular subtype had changed from triplenegative to HER2-positive. The previous comprehensive treatment had not been able to effectively control the disease, but the patient achieved a long progression-free survival time through chemotherapy and trastuzumab targeted therapy after the subtype change. To date, there has been no recurrence for over eight years.Conclusion: Among repeatedly relapsed MBC patients, further investigation should be taken into consideration. As in the case presented in our study, it is possible that the HER2 status can convert from negative to overexpression. Moreover, for HER2-positive MBC patients, anti-HER2 therapy is recommended. The decision-making process requires multidisciplinary involvement.