Answer: Metastatic prostatic adenocarcinoma

Histology: The excisional biopsy specimen focal areas of fibrous tissue largely replaced by nests and sheets of uniform cells with clear to eosinophilic cytoplasm, uniform nuclei with punctate nucleoli and scattered mitotic figures. No necrosis is evident. There is no definite glandular differentiation, however in some areas there is a vaguely cribriform architecture. Immunostains for D2-40, WT1, calretinin, TTF1, napsin and p40 are all negative. Additional immunostains are performed, and the lesional cells are positive for PSA, P501S and NKX3.1.

Discussion: The clinical suspicion was malignant mesothelioma in an elderly male with occupational asbestos exposure and a new presentation of lung collapse with bulky pleural masses. The histologic features are in keeping with an epithelioid mesothelioma, or involvement by a non-small cell lung carcinoma, but all of the mesothelial (D2-40, WT1, calretinin), lung adenocarcinoma (TTF1), and squamous cell carcinoma (p40) markers are negative. Upon further review, the patient had a history of prostatic adenocarcinoma treated with radiation therapy. The morphologic features of the tumor are nonspecific but are compatible with high grade prostatic adenocarcinoma, and indeed the immunoprofile supports the diagnosis of metastatic high grade prostatic adenocarcinoma (PSA+, P501S+, NKX3.1+). This case illustrates the importance of clinical history, as well as the importance of keeping prostatic adenocarcinoma in the differential of any metastatic or new lesion in a male patient, particularly in elderly male patients (just as breast carcinoma should be in the differential diagnosis of carcinomas of unknown origin in female patients).

Reference(s):
1. Gurel B, Ali TZ, Montgomery EA, et al. NKX3.1 as a marker of prostatic origin in metastatic tumors. Am J Surg Pathol. 2010 Aug;34(8):1097-105.