Currently there are no clear guidelines regarding the management of multiple primary malignancies when they occur simultaneously. Furthermore, there have been no cases to date describing axillary breast cancer recurrence in combination with metastatic axillary squamous cell carcinoma (SCC) of unknown primary origin. Both of these cancers are rare and associated with poor outcomes. Treatment for both SCC and breast cancer axillary metastasis includes a combination of axillary lymph node dissection, radiation, and chemotherapy. We describe here a woman with prior hormone-receptor-positive breast cancer who completed a 5-year course of aromatase inhibitor therapy and had a prior mastectomy with negative sentinel lymph nodes, who then presented with a rapidly growing axillary mass. Pathological analysis demonstrated recurrent breast cancer and metastatic SCC.

Keywords: Breast cancer; Squamous cell carcinoma; Axillary recurrence; Metastasis; Metachronous cancer; Synchronous cancer; Targeted therapy