Correct: B

Histology: The tissue shows hyperplastic prostatic glands surrounded by hypercellular stroma with scattered pleomorphic cells with degenerative nuclear atypia.

Discussion: Stromal tumors of the specialized prostatic stroma are classified as STUMP and prostatic stromal sarcoma. STUMPS present at any age with a median age of 58 years. STUMP can be detected as an incidental finding in prostate needle biopsy performed for elevated PSA or due to obstructive urinary symptoms or rectal fullness in cases of large tumors (up to 19 cm in size). There are five histological patterns of STUMPs. The most common is the hypercellular stroma with scattered large pleomorphic stromal cells with degenerative nuclear atypia growing in between benign glands. The glands entrapped within the lesion frequently show hyperplasia and other changes including clear basal cell hyperplasia, clear cell cribriform hyperplasia, urothelial metaplasia, or PIN. The glandular complexity may mask the underlying stromal neoplasm, and in these cases STUMP may be undiagnosed. A second histologic pattern that is much less frequent than the previous is the phyllodes subtype, characterized by leaf-like stromal projections lined by benign prostatic epithelium. A third pattern is with sheets of myxoid stroma containing bland stromal cells usually without glands. The fourth pattern is hypercellular stroma with eosinophilic cytoplasm but without the classic degenerative nuclear atypia. The last pattern is very subtle and characterized by a cellular stroma with epithelioid or round stromal cells. Similar to the myxoid and the hypercellular types, the nuclei are bland and lack significant atypia, making the diagnosis more challenging. Mitotic figures or necrosis are typically absent and when these features are present, prostatic stromal sarcoma should be considered.
While the name of this tumor suggests that the malignant potential is “uncertain,” once completely excised and if the specimen does not show a prostatic stromal sarcoma, the tumor will have a benign behavior. However, low-grade or high-grade prostatic stromal sarcomas are identified in approximately 10% of the cases or may develop over time if unresected, where the term the uncertain malignant potential applies on needle biopsy. Tumors that compress the rectum often show a peritumoral fibrous reaction that should not be interpreted as rectal invasion. STUMPs are positive for CD34, and for progesterone receptor, and variable expression of smooth muscle markers including SMA or desmin. The differential diagnoses of STUMPs include stromal nodule of BPH, stromal sarcoma, solitary fibrous tumor, inflammatory myofibroblastic tumor, gastrointestinal stromal tumor (GIST), leiomyosarcoma, and synovial sarcoma. The myxoid pattern of STUMP is the subtype that more closely resembles a stromal nodule of BPH given the lack of associated glands or significant nuclear atypia. The presence of thick-walled blood vessels is a feature of stromal nodule of BPH and not of STUMP. Immunohistochemistry is usually not helpful in the differential diagnosis of STUMP and stromal nodule of BPH or prostatic stromal sarcoma. The other differential diagnoses are discussed below.

Reference:
1. Leong JY, Chandrasekar T, Sebastiano C, Rshaidat H, Steward JE, Trabulsi EJ. Prostatic Stromal Tumors of Uncertain Malignant Potential. Urology. 2019.
2. Nagar M, Epstein JI. Epithelial proliferations in prostatic stromal tumors of uncertain malignant potential (STUMP). Am J Surg Pathol. 2011;35(6):898-903.
3. Sadimin ET, Epstein JI. Round cell pattern of prostatic stromal tumor of uncertain malignant potential: a subtle newly recognized variant. Hum Pathol. 2016;52:68-73.
4. Herawi M, Epstein JI. Specialized stromal tumors of the prostate: a clinicopathologic study of 50 cases. Am J Surg Pathol. 2006;30(6):694-704.