Occult Breast Carcinoma Presenting with Extensive Bone Involvement in the Absence of an Identifiable Primary Breast Lesion: A Case Report.

Occult breast cancer (OBC) is a rare and diagnostically challenging subtype of breast carcinoma characterized by axillary or distant metastases without an identifiable primary breast lesion. We report an unusual presentation of OBC in a patient with diffuse osteolytic bone metastases and axillary lymphadenopathy in the absence of clinically palpable, mammographically, or sonographically detectable breast lesions. The diagnosis was established through bone marrow biopsy and immunohistochemical analysis. Review of the literature identified only four previously reported cases of OBC initially presenting with diffuse osseous metastases, three of which were hormone receptor-positive. This case highlights the importance of maintaining a high index of suspicion for OBC in patients with metastatic adenocarcinoma of unknown primary origin with diffuse skeletal involvement. Furthermore, it underscores the distinct biologic behavior of hormone receptor-positive breast cancer, which preferentially metastasizes to bone and may achieve durable disease control with endocrine-based therapy, allowing selected patients to avoid cytotoxic chemotherapy.