Breast metastasis as the first clinical manifestation of ileal neuroendocrine tumor. A challenging diagnosis with relevant clinical implications.

Ileal neuroendocrine tumors are slow-growing grade 1 or, more rarely, grade 2 neuroendocrine tumors which, however, are frequently metastatic to regional lymph nodes and the liver. A few cases of ileal neuroendocrine tumors that are metastatic to the breast have also been reported in the medical literature. The knowledge of this uncommon clinical presentation is of great importance because it needs to be differentiated from primary breast carcinomas with neuroendocrine features, which represent completely different entities with a different therapeutic approach. The diagnosis of a breast metastasis from an ileal neuroendocrine tumor and its distinction from a well-differentiated primary neuroendocrine tumor of the breast is a challenging task for clinicians and pathologists. This workup is particularly difficult when the breast lesion is the first sign of malignancy. In the present paper, we describe the clinicopathological features of an ileal neuroendocrine tumor first presenting with a breast metastasis in a 50-year-old woman and we discuss the key diagnostic features for the differential diagnosis with primary well-differentiated neuroendocrine tumor of the breast. Moreover, we have reviewed the medical literature to give the reader a comprehensive overview on this topic.