Is the skin a sanctuary for breast cancer cells during treatment with anti-HER2 antibodies?

The occurrence of skin metastases is a common event in patients affected by advanced breast cancer, usually associated with systemic disease progression. Here we describe 2 cases of diffuse cutaneous metastases from HER2-overexpressing breast cancer occurring despite a dramatic response in liver and bone, respectively, during treatment with anti-HER2 antibodies Trastuzumab and Pertuzumab. We discuss the reasons for this discrepancy and suggest a possible implication of impaired immune response in the skin. Future research should provide strategies to overcome the induction of immune privilege in the skin in order to avoid discontinuation of effective treatments.