Metastatic Prostate Cancer Masquerading as Locally Advanced Breast Cancer.

We report a rare case of a 72-year-old man with prostatic adenocarcinoma metastasizing to gynecomastia breast tissue with ipsilateral axillary lymph node involvement. This presentation is notable for the absence of disseminated metastatic disease or prolonged androgen deprivation therapy. The clinical and radiological features initially suggested a primary breast carcinoma, highlighting the diagnostic challenge in such presentations. The patient had a history of high-risk prostate adenocarcinoma resected in 2020. A prostate-specific membrane antigen (PSMA) PET scan, performed for rising prostate-specific antigen (PSA) levels, demonstrated a new avid left breast lesion within longstanding bilateral gynecomastia tissue. The pattern of PSMA uptake, including involvement of ipsilateral axillary lymph nodes, further increased suspicion for a locally advanced primary breast carcinoma. Core biopsy of the breast lesion and fine-needle aspiration of the axillary lymph node were consistent with metastatic prostate adenocarcinoma, which was subsequently confirmed on surgical excision. This case adds to the limited literature describing prostate cancer metastasis to the breast and underscores the diagnostic challenges such presentations can create. It demonstrates how metastatic disease can closely mimic primary breast malignancy, reinforcing the importance of histopathological confirmation when imaging findings suggest a primary breast lesion.