Metastatic dermatosis in breast carcinoma on adjuvant trastuzumab: Is skin a sanctuary site in human epidermal growth factor receptor-2-amplified disease?

Cutaneous metastasis (CM) from internal malignancies is commonly seen. Sometimes, skin metastases can be the first sign of advanced cancer or an indicator of cancer recurrence. Cases of breast cancer with cutaneous progression after or during trastuzumab therapy have been described in the past, frequently associated with systemic disease progression. However, CM during adjuvant trastuzumab therapy is very rare. It has been hypothesized that cancer cells located in the skin survive and take proliferative advantage by virtue of an immune-tolerance mechanism that hampers trastuzumab-mediated antibody-dependent cell-mediated cytotoxicity. We describe a case of human epidermal growth factor receptor-2-overexpressing breast cancer presenting with diffuse CM during adjuvant trastuzumab therapy.