Epithelial proliferations in prostatic stromal tumors of uncertain malignant potential (STUMP)

Stromal tumors of uncertain malignant potential (STUMPs) are rare tumors characterized by an atypical, unique stromal proliferation of the prostate. Various stromal proliferations of STUMPs have been described; however, epithelial proliferations occurring within the STUMP have not been systematically described to date. We reviewed 89 cases of STUMP from our consultation service from 1990 to 2010. Nineteen cases without a glandular component were excluded. We next evaluated the glandular component of the remaining 70 cases of STUMP for glandular crowding and complexity, prostatic intraepithelial neoplasia (PIN), squamous metaplasia, urothelial metaplasia, basal cell hyperplasia, adenosis, and clear cell cribriform hyperplasia. In 58 cases (83%), the glandular component differed from glands on the same biopsy specimen uninvolved by STUMP. The most common abnormalities were glandular crowding in 35 of 70 (50%) and a very prominent basal cell layer in some glands in 32 of 70 (46%) cases. The next most frequent glandular variation from normal was prominent papillary infolding in 13 of 70 (19%) cases. Less-frequent epithelial changes within the STUMP were as follows: 10 of 70 (14%) showed cystically dilated glands; 7 of 10 (10%) had basal cell hyperplasia; 6 of 70 (9%) had urothelial metaplasia; 6 of 70 (9%) showed squamous metaplasia; 3 of 70 (4%) had cribriform hyperplasia; 3 of 70 (4%) had adenosis; and 1 case each showed high-grade PIN, low-grade PIN, and partial atrophy. The glandular component of STUMP was histologically normal in 12 (17%) cases. There was a tendency toward urothelial and squamous metaplasia in STUMPs with a phyllodes pattern, and a prominent basal cell layer in STUMPs with degenerative and cellular stroma. This is the first study to systematically describe the epithelial proliferations occurring in STUMP. This study suggests that, within STUMPs, there is epithelial-mesenchymal crosstalk, as has been described in benign prostate and in prostatic carcinogenesis. In unusual cases of STUMP, the epithelial proliferation may predominate to the extent that it can mask the diagnosis of STUMP.