What happened, in summary

A man in his 70s had noticed a lump and an itchy rash under his left arm for 3 years. About 1 year before seeking specialist care, a second rash appeared under his right arm. He had no breast-cancer family history and initially had no pain or discharge.

Examination found a 4 cm mass beneath an 8 × 10 cm red, ulcerated patch in the left armpit, together with 2 smaller red patches on the right. These areas contained accessory breast tissue—breast tissue that develops outside the usual chest location. He underwent removal of the left accessory breast with an axillary lymph-node dissection and a wide excision on the right.

The left-sided tumor measured 2.5 cm and was grade 2 invasive ductal carcinoma. It also contained ductal carcinoma in situ and showed invasion into lymphatic vessels, skin and the overlying epidermis. None of 18 left axillary lymph nodes contained cancer. Testing showed that the tumor was estrogen receptor-negative, progesterone receptor-negative and HER2-positive by gene amplification, with a Ki-67 level of 70%.

The right-sided specimen showed Paget disease involving the skin of the accessory breast. Fourteen lymph nodes found in that specimen were also negative. This meant he had 2 synchronous cancers in accessory breast tissue on opposite sides: invasive ductal carcinoma on the left and Paget disease on the right.

After surgery, he received paclitaxel chemotherapy with trastuzumab targeted therapy, followed by radiation. Persistent armpit rashes or masses in accessory breast tissue can represent breast cancer, even in men, and both sides need careful assessment.