Microscopic description and IHC:

The hydrocele sack shows scattered large cells forming occasional “Indian filing” admixed with inflammatory cells with occasional prominent follicles. Epididymis with mature spermatids are present in the deeper layers.

Discussion:

Benign reactive mesothelial hyperplasia can be difficult to distinguish from diffuse malignant mesothelioma. Benign reactive mesothelial hyperplasia may have cytologic features ordinarily associated with malignancy, and diffuse malignant mesothelioma may be cytologically bland. Entrapment of benign reactive mesothelial cells within body cavity may mimic tissue invasion. In this case the hydrocele is formed by remnant peritoneum (tunica vaginalis) wrapped around the testicle and trapped during the process.

Features can be used to favor a diagnosis of malignancy over reactive/benign mesothelium include: 1)unequivocal invasion; 2)cellular nodules with stromal expansion; 3)atypical cells throughout the full thickness; 4) severe pleomorphism, atypical mitoses; 5)necrosis.

References:

Cagle, Philip T., and Andrew Churg. "Differential diagnosis of benign and malignant mesothelial proliferations on pleural biopsies." Archives of pathology & laboratory medicine 129.11 (2005): 1421.

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