Experiences of trastuzumab plus paclitaxel combination therapy in metastatic human epidermal growth factor receptor 2‐positive extramammary Paget’s disease: Four cases and a review

Tumor cells in extramammary Paget's disease sometimes overexpress human epidermal growth factor receptor 2 (HER2). Several case reports indicated successful response to HER2 inhibitor in patients with HER2-positive metastatic extramammary Paget's disease. However, these were single-case reports, and most cases were evaluated only by immunohistochemistry and treated with HER2 inhibitor monotherapy. Here, we report cases of HER2-positive metastatic extramammary Paget's disease identified by both immunohistochemistry and in situ hybridization, and the patients were treated with HER2 inhibitor (trastuzumab) and paclitaxel combination chemotherapy. Partial response was observed in one case. The case was positive on both immunohistochemistry (3+) and in situ hybridization (HER2/chromosome 17 centromere, ≥2.0). Our observations suggest that HER2 should be checked in patients with advanced and/or metastatic extramammary Paget's disease, and that therapy with HER2 blockers should be considered as an option for treatment of HER2-positive extramammary Paget's disease, especially in cases positive for both HER2 gene amplification and overexpression.