Answer: Metastatic prostatic adenocarcinoma

Histology: This lesion has a predominant cribriform and nested architecture, and the nuclei are round and fairly uniform, which suggests the possibility of neuroendocrine neoplasm. However, there are prominent nucleoli. While the architecture is nested like a low grade neuroendocrine neoplasm, the lesion has a high mitotic rate which would be more consistent with a high grade neuroendocrine neoplasm, and hence there is discordance. Given the prominent nucleoli, a history of prostate cancer was sought, and confirmed clinically. The neoplastic cells demonstrated cytoplasmic labeling for PSA and P501S, and nuclear labeling for NKX3.1, supporting the diagnosis.

Discussion: Prostatic and mammary carcinomas often have a nested appearance which can mimic a neuroendocrine neoplasm. When one encounters what appears to be a neuroendocrine neoplasm that doesn’t fit into an accepted category, one should consider the possibility of a metastasis from prostate or breast which can mimic a neuroendocrine neoplasm. Both prostatic and mammary carcinomas can show expression of neuroendocrine markers, which furthers the potential for confusion.