Case Report: Interpectoral lymph node metastasis as an indicator of occult breast cancer: a case of double primary malignancy.

Occult breast cancer (OBC) is a rare clinical entity, characterized by the presence of metastatic adenocarcinoma in the axillary lymph nodes without an identifiable primary tumor in the breast. We present an unprecedented case of synchronous OBC and primary lung cancer, which initially manifested as an isolated interpectoral (Rotter's) lymph node metastasis. The patient underwent neoadjuvant chemotherapy, followed by wide local excision with sentinel lymph node biopsy, as well as a subsequent wedge resection of the lung primary. The patient declined further recommended adjuvant radiation therapy and genetic testing. Twelve months later, a solitary brain metastasis developed. Histopathological examination confirmed its breast origin, and a craniotomy achieved complete resection. At over 56 months of follow-up, the patient remains recurrence-free. Metastasis to Rotter's lymph node can be the sole inaugural presentation of OBC. Pathological distinction between synchronous primaries is paramount for accurate diagnosis and management. Importantly, multidisciplinary management must address metachronous metastases in the context of treatment non-adherence.