[A Case of Breast Cancer Associated with Dermatitis That Was Difficult to Differentiate from Dermatomyositis].

A 46-year-old woman visited our hospital with a chief complaint of a mass in the right breast. Breast ultrasonography revealed a hypoechoic area with an indistinct border on the right breast, and right axillary lymph node swelling. A core needle biopsy revealed invasive ductal carcinoma, and the diagnosis was right breast cancer, cT2N2M0, Stage III A, HER2-enriched type. We administered 4 courses of FEC followed by weekly paclitaxel plus trastuzumab. After the treatment, she had eruption and erythema on the face, precordium and forearm. Though dermatomyositis associated cancer was suspected, a definite diagnosis was not made. However, skin symptoms were improved significantly after mastectomy, suggesting that she had dermatomyositis. For the skin symptom during breast cancer treatment, the examination should be made with the possibility of the adverse effect of chemotherapy and dermatomyositis associated cancer.