Metastatic Neuroendocrine Tumour in the Breast: a Potential Mimic of In-situ and Invasive Mammary Carcinoma

Aims: The aim of this study was to review the clinicopathological characteristics of neuroendocrine tumours (NETs) metastasizing to the breast, in order to identify features that could be useful in distinguishing these metastatic lesions from primary breast neoplasms.

Methods And Results: Eighteen metastatic NETs in the breast were identified from two large hospitals over a 15-year period. Eleven (62%) tumours originated in the gastrointestinal tract, 5 (28%) originated in the lung, and the other two were of indeterminate origin. Eight (44%) cases were initially misdiagnosed as primary mammary carcinomas. In retrospect, all metastatic tumours exhibited architectural and cytological features that would suggest neuroendocrine differentiation. Immunohistochemistry can further aid in the distinction between metastatic neuroendocrine and primary mammary carcinoma. All 11 tumours from the gastrointestinal tract expressed CDX-2, 3 (60%) of five tumours from the lung expressed thyroid transcription factor-1, and only 2 (11%) of 18 showed weak oestrogen receptor positivity. Additionally, unlike primary carcinomas, the majority (82%) of metastatic NETs were negative for cytokeratin 7, and all were negative for gross cystic disease fluid protein 15 and mammoglobin.

Conclusions: There is a high propensity for metastatic NETs to mimic primary breast carcinomas. Careful attention to cytological and architectural features can help to identify cases that require further immunophenotypic workup with a panel of tissue-specific antibodies. However, clinical history is paramount for optimal diagnosis.