Answer: Benign phyllodes tumor

Histology: The prostatic chips are involved with glands showing cystic dilatation. Many of the glands appear somewhat slit-like due to compression by prostatic stroma. In some areas, the stroma protrudes into the cystic space with a polypoid growth. The epithelium lining the cystically dilated prostate glands is benign with an obvious basal cell layer present in many areas. The stroma appears either unremarkable or in other areas more myxoid. The overall stromal cellularity is either normal or hypercellular and lacks cytologic atypia or appreciable mitotic activity.

Discussion: Prostatic cystadenoma could have some of the features seen in this case. However, prostatic cystadenomas are typically extra-prostatic or attached to the posterior prostate by a thin pedicle. Rare intraprostatic cystadenomas occur but can only be diagnosed if there is a localized nodule in an otherwise normal gland. BPH can have focal fibroadenomatous foci yet not to the extent seen in this case. The findings in this case are typical of a benign phyllodes pattern. Prostatic stromal tumors arising from the specialized prostatic stroma are rare and distinct tumors with diverse histological patterns. Stromal tumors of the prostates are classified into stromal tumors of uncertain malignant potential (STUMPs) and stromal sarcomas. Microscopically, four patterns of STUMP have been described and include: hypercellular stroma with scattered atypical, but degenerative appearing cells admixed with benign prostatic glands, which is the most common pattern; (2) hypercellular stroma consisting of bland fusiform stromal cells with eosinophilic cytoplasm admixed with benign glands, most likely pattern to be overlooked as cellular BPH; (3) leaf-like hypocellular fibrous stroma covered by benign appearing prostatic epithelium similar in morphology to a benign phyllodes tumor of the breast; and (4) myxoid stroma containing bland stromal cells and often lacking admixed glands, most commonly raised in the differential of the much more common myxoid BPH stromal nodules. Cases can exhibit a mixture of the above patterns. The benign, phyllodes pattern of STUMP may be associated with a variety of benign epithelial proliferations, including basal cell hyperplasia, adenosis, and sclerosing adenosis. Although STUMPs are generally considered to represent a benign neoplastic stromal process, a subset of STUMPs has been associated with stromal sarcoma on concurrent biopsy material or has demonstrated stromal sarcoma on repeat biopsy, suggesting a malignant progression in at least some cases. There appears to be no correlation between the pattern of STUMP and association with stromal sarcoma. As most STUMPs are confined to the prostate and rarely progress to sarcoma, STUMPs are in general associated with a good prognosis. Although many STUMPs may behave in an indolent fashion, their unpredictability in a minority of cases and the lack of correlation between different histological patterns of STUMPs and sarcomatous dedifferentiation, warrant close follow-up and consideration of definitive resection in younger individuals. Factors to consider in deciding whether to proceed with definitive resection for STUMPs diagnosed on biopsy include patient age and treatment preference, presence and size of the lesion on rectal exam or imaging studies, and extent of the lesion on tissue sampling. Expectant management with close clinical follow-up could be considered in an older individual with a limited lesion on biopsy where there is no lesion identified on digital rectal exam or on imaging studies.