Recurrent primary cutaneous mucinous carcinoma with neuroendocrine differentiation: case report and review of the literature.

We report the case of a 60-year-old woman presenting with primary cutaneous mucinous carcinoma (PCMC) with neuroendocrine differentiation, revealed by neuroendocrine tumor lymph node metastasis 7 years before identification of the skin tumor. Only five cases of PCMC with neuroendocrine differentiation have been reported to date. The frequency of this neuroendocrine component may be underestimated, as it can require immunohistochemistry for identification, rather than being obvious on initial histopathologic examination. In the case presented here, the prominent neuroendocrine component displayed the morphological features of a well-differentiated neuroendocrine tumor with expression of common neuroendocrine markers, strong expression of estrogen and progesterone receptors and low Ki67 proliferation index (5%). This case shows that not all primary cutaneous neuroendocrine carcinomas are Merkel cell carcinomas (MCCs). In addition to rare primary cutaneous carcinoid tumors, the diagnosis of PCMC with neuroendocrine differentiation must be considered, particularly when confronted by a mucinous tumor or lymph node metastases of neuroendocrine carcinoma of unknown origin. On the basis of this case, identification of a neuroendocrine component in a PCMC might be an adverse prognostic indicator of recurrence or of lymph node metastasis and should support wider excision margins of the tumor.