Round cell pattern of prostatic stromal tumor of uncertain malignant potential: a subtle newly recognized variant.

Prostatic stromal tumor of uncertain malignant potential (STUMP) is a distinct entity which includes several different patterns. Four patterns of STUMP have been described including stroma with (1) degenerative atypia, (2) hypercellular spindle cells, (3) myxoid spindle cells, and (4) phyllodes-like pattern. The current study identified a novel round cell pattern. We searched our database from 1999 to 2015 and identified 7 patients with round cell pattern out of a total number of 98 patients with STUMP. All 7 cases showed mildly increased stromal cellularity with rounded nuclei, diagnosed on core biopsies in 5 cases, transurethral resection in 1 case, and radical prostatectomy in 1 case. Some degree of glandular displacement was observed in 4 cases. In 2 of the cases, STUMP was not recognized histologically by the referring pathologists and was initially diagnosed as benign prostatic hyperplasia. As has been described with other patterns of STUMP, several cases showed associated epithelial proliferations that in some instances masked the neoplastic stromal process. The round cell pattern of STUMP is a new deceptively subtle pattern that may not be recognized as a neoplasm and may be misdiagnosed as benign prostatic hyperplasia. Although there was no direct evidence in our study that the round cell pattern of STUMP has the same behavior as other variants of STUMPs, increased recognition of this entity will hopefully lead to additional studies to further understand its malignant potential.