Abstract
Metaplastic Breast Cancer (MBC) is a rare group of tumors often presenting as triple-negative. MBC accounts for less than 1% of all breast cancers with the spindle cell variant comprising less than 0.5%. While rare, spindle cell carcinoma is the commonest subtype in the western world. It has a more aggressive biological behavior with increased risk of recurrence and death due to disease compared to triple negative breast cancers. There is no treatment guideline for management of MBC due to the rarity of the disease. Instead, treatment is theorized based off success with other types of aggressive breast and metaplastic cancers of different tissue. We present the first known case report of male spindle cell carcinoma of the breast treated with KEYNOTE-522 regimen. Therapy included a first phase with pembrolizumab (dose of 200 mg) every 3 weeks plus paclitaxel and carboplatin and second phase, with four cycles of pembrolizumab with doxorubicin-cyclophosphamide.