A case of HER2-positive male breast cancer with lung metastases showing a good response to trastuzumab and paclitaxel treatment.

We present a case of advanced HER2-positive male breast cancer, which showed a good response to a combined treatment of trastuzumab and paclitaxel. A 78-year-old man was diagnosed with invasive ductal carcinoma (T4d N3 M1, stage IV). He had advanced breast cancer consisting of multiple tumors with skin involvement and redness over the entire left chest region. A computed tomography (CT) scan of the chest revealed a metastatic tumor in the left lung. Histologically, both the primary breast cancer and the metastatic lung tumor were identified as invasive ductal carcinoma that was estrogen receptor-negative (ER)(-) and progesterone receptor-negative (PgR)(-), with a HER2 score of 3+ (IHC). The patient received a combination chemotherapy using trastuzumab and paclitaxel. Two months later, a follow-up chest CT scan showed that the left lung tumor had disappeared, suggesting a good response to trastuzumab and paclitaxel. During trastuzumab treatment, no severe adverse events above grade 3 were observed. This is the first reported case of advanced HER2-positive male breast cancer in which a good response to trastuzumab and paclitaxel was demonstrated at both primary breast cancer and metastatic sites.