What happened, in summary

A 59-year-old woman with a remote history of a dysplastic nevus developed a palpable 1.5 cm mass in the left breast and a fixed 5 cm mass in the left axilla. Diagnostic mammography and targeted ultrasound showed a 1.6 cm irregular breast mass and markedly enlarged axillary lymph nodes. Biopsy of the breast lesion confirmed invasive ductal carcinoma, no special type, Nottingham grade 2, with a triple-negative profile: estrogen receptor-negative, progesterone receptor-negative, and HER2-negative. Biopsy of the axillary node showed metastatic carcinoma consistent with a breast primary. CT and bone scans did not show distant metastasis. She received neoadjuvant systemic therapy with 5 months of dose-dense doxorubicin and cyclophosphamide followed by paclitaxel. In the sixth month, additional chemotherapy was given with pembrolizumab, followed by pembrolizumab 4 mg/kg every 6 weeks for 3 months. After treatment, examination found no palpable breast or axillary mass, and MRI showed major improvement. She underwent left partial mastectomy with seed localization and axillary lymph node dissection; 18 lymph nodes were removed. The breast specimen showed treatment-related fibrosis without residual tumor, confirming a complete pathologic response in the breast. Four axillary lymph nodes contained melanocytic proliferations that initially raised concern for melanoma or unusual treated carcinoma. Additional immunohistochemistry and dermatopathology review showed S100 and SOX10 positivity, PRAME negativity, retained p16, very low Ki-67, and absence of epithelial markers. Combined with her history and lack of melanoma on clinical or radiologic evaluation, the nodal findings were diagnosed as benign nodal nevi, not metastatic melanoma or recurrent breast cancer.