Occult Primary Neuroendocrine Tumor Metastasis to the Breast Detected on Screening Mammogram

Metastatic tumors are rare in the breast. Well-differentiated neuroendocrine tumors (WDNETs) are slow-growing neoplasms that arise from neuroendocrine cells, particularly in the gastrointestinal tract and bronchial tree. Metastatic WDNET to the breast is a rare entity. We present a case report of ileal WDNET metastatic to the breast which was initially identified as a small mass in the patient's left breast on screening mammography. Targeted ultrasound identified a suspicious mass, and ultrasound-guided percutaneous core biopsy was performed. Pathology revealed metastatic WDNET. Breast magnetic resonance imaging (MRI) was then performed and demonstrated left axillary Level 2 lymphadenopathy, and liver lesions were suspicious for metastasis. The patient underwent abdominal computed tomography (CT) to evaluate for distant metastatic disease. A spiculated mass was found near the ileocecal valve, suggestive of primary ileal WDNET. In addition, CT identified multiple liver lesions, most compatible with metastasis. Indium 111 OctreoScan confirmed radiotracer uptake in the ileum consistent with primary neuroendocrine tumor. In this report, we review the imaging characteristics of metastatic WDNET to the breast by different imaging modalities including mammogram, ultrasound, and breast MRI.