Answer: Metastatic prostate adenocarcinoma

Histology: Within a testis that has a small rim of atrophic seminiferous tubules is a large mass composed of sheets of cells. In areas there is a papillary appearance to the tumor with necrosis between the fronds and necrotic cells lining the fronds. In areas of the tumor the cytology is relatively uniform consisting of large cells with abundant cytoplasm and round to oval nuclei with central prominent nucleoli. A minor component of the tumor consists of much more pleomorphic cells with marked variation in nuclear size and shape.

Discussion: The papillary structures seen in this case are artifactual. In between the “papillary fronds” is necrosis and the cells lining the “papillary” structures have necrotic cells and apoptosis on the surface. With necrosis away further from the blood supply it leaves residual viable cancer around the capillaries mimicking a papillary structure. True papillary fronds would not have intervening necrosis between fronds and would not have necrotic cells on the outer surface of the fronds. Embryonal carcinoma cells are much more pleomorphic with multiple irregular clumps of chromatin compared to the uniform population of round to oval nuclei with a single central nucleolus seen in this case. Embryonal carcinoma would also in areas show primitive luminal formation. Seminoma consists of loosely cohesive cells as opposed to the compact sheets of tumor in this case. Yolk sac tumors can on occasion show diffuse solid growth, which typically mimic seminoma. On one hand, yolk sac tumor cytology is more pleomorphic than the areas of the current case with uniform cytology, and on the other hand would not show the pleomorphism focally present in the current case. The uniform less cytologically atypical component of this tumor is typical of what one could see in metastatic prostate adenocarcinoma. What is unusual in this case is the more pleomorphic component that one typically does not associate with prostate cancer. However, there is the pleomorphic giant cell variant of prostate adenocarcinoma which is composed of Gleason score 9-10 usual acinar carcinoma with a minor component of bizarre pleomorphic tumor cells not usually seen in prostate cancer. Initially reported on only 6 cases, we are in the process of analyzing a series of 22 cases with a broader panel of immunohistochemistry. In order to prove the prostatic origin of the tumor, immunohistochemistry for NKX3.1 is much more sensitive than PSA or P501s (Prostein), where the latter two can give rise to false negative staining. Prostate cancer is the most common cancer to metastasize to the testis, typically in the setting of widespread metastatic disease.

References:

Pseudopapillary features in prostatic adenocarcinoma mimicking urothelial carcinoma: a diagnostic pitfall. Gordetsky J, Epstein JI. Am J Surg Pathol. 2014 Jul;38(7):941-5.

Pleomorphic giant cell adenocarcinoma of the prostate: report of 6 cases. Parwani AV, Herawi M, Epstein JI. Am J Surg Pathol. 2006 Oct;30(10):1254-9