Small‐cell neuroendocrine carcinoma of the breast

We read the recent report of three cases of small‐cell neuroendocrine carcinoma (SCNC) of the breast with interest.¹ We do agree with the authors that the vexing question of whether this is a primary breast neoplasm or a secondary deposit from other sites, most commonly from the bronchus, has to be considered. We would like to share our experience with two cases that we dealt with in the Department of Histopathology, East Lancashire Hospitals NHS Trust, Burnley, UK, and, in particular, the application of two more immunological markers, neuronal adhesion molecule (CD56) and thyroid transcription factor 1 (TTF1) to those already used by the authors. One of the patients was an elderly woman with a large mass in the breast. Histologically, the mass was a SCNC, with positive immunohistochemistry for CD56 and TTF1. No in situ component was observed. Clinical and imaging studies showed that this was a metastasis from a primary lung tumour. The second patient was a middle aged woman who presented with a breast mass of 5 cm. A core biopsy specimen showed SCNC, again positive for CD56 and TTF1. Clinical and imaging studies showed that this was an isolated lesion in the breast, and it was therefore diagnosed as a primary breast tumour. After neoadjuvant chemotherapy, a mastectomy was performed. It showed a 3 cm tumour composed predominantly of SCNC, but with a central focus of invasive grade III carcinoma of no specific test, which was negative for CD56 and TTF1. Interestingly, the in situ component was of SCNC type, showing positive staining for CD56 and TTF1 (fig 1). We re‐emphasise that TTF1 cannot be used to discriminate between a primary breast SCNC and a secondary deposit from a primary in the bronchus, as extrapulmonary SCNC can often be positive for TTF1.²,³

Figure 1 In situ carcinoma showing positive staining for thyroid transcription factor 1.

In summary, we recommend the addition of CD56 to the other immunological markers for SCNC and advise caution in interpreting TTF1 immunoreactivity in extrapulmonary SCNC.