A Case of Dermatitis Caused by Metronidazole Gel That Needed to Be Differentiated from Breast Cancer Skin Metastasis

Seventy years old woman noticed a mass in her right breast before 3 years. Since she had ulcer bleeding, she visited our hospital. In physical findings, a hemorrhagic about 8 cm mass with an ulcer was found in the upper right breast. Breast ultrasonography revealed a large tumor of approximately 8 cm in the right A area, and needle biopsy revealed invasive ductal carcinoma(ER positive, PgR positive, HER2 positive, Ki-67 low expression). Right axillary lymph node metastasis was confirmed, but no clear distant metastasis was observed. Pretreatment diagnosis was right breast cancer, cT4bN1M0, Stage ⅢB, Luminal HER. Chemotherapy was started with pertuzumab, trastuzumab, and docetaxel, and the tumor was reduced after 6 cycles. Due to side effects, the drug was changed to a molecular targeted drug only and the treatment was continued. However, redness was observed in the entire right breast, and breast cancer skin metastasis was suspected. Since the dermatitis caused by metronidazole gel was also distinguished, the redness was improved when the application was stopped. When confirmed by a patch test, a reaction to metronidazole gel was observed, leading to the diagnosis of dermatitis caused by metronidazole gel.