Metastases from Neuroendocrine Tumors to the Breast Are More Common than Previously Thought. A Diagnostic Pitfall?

Background Metastases from neuroendocrine tumors (NETs) to the breast have been described as a rare phenomenon. Presentation, imaging results, and cytopathologic findings of these tumours may closely mimic those of a mammary carcinoma. Methods This study was a retrospective review of 661 patients with metastatic NETs, of whom 280 were females, treated at Uppsala University Hospital, Uppsala, Sweden. Patients with pathological breast lesions were identified. Histopathological slides from available NET breast lesions were analyzed for mammary carcinoma and neuroendocrine markers. Results We have identified 20 female patients with NET metastases to the breast, 11/235 with small intestinal NETs, 8/55 with lung NETs, and 1/6 with thymic NETs. There were no male patients with NET metastatic to the breast. Four patients had their breast lesion initially diagnosed as mammary carcinoma. Retrospectively, these lesions showed negative staining for mammary carcinoma markers. Conclusions Metastases to the breast from neuroendocrine tumors may be more common than previously thought. Patients with a lesion to the breast and symptoms typical for NET may benefit from additional histopathological investigation, because NET metastases and mammary carcinoma have different immunohistochemical profiles.