Solitary fibrous tumors involving the prostate and periprostatic region A multi-institutional cohort of 33 cases with literature review

Background: Solitary fibrous tumor (SFT) is a mesenchymal neoplasm characterized by NAB2::STAT6 fusion and nuclear STAT6 expression. Primary SFT of the prostate is rare, and its biological behavior remains incompletely defined. We present a multi-institutional cohort and comprehensive literature review to better characterize clinicopathologic features, management, and outcomes. Design: We analyzed 33 patients with prostatic SFTs from multiple institutions, integrating clinical, radiologic, histopathologic, immunohistochemical, and follow-up data. A systematic review of published cases was performed, and findings were contextualized using established risk models. Results: Patients ranged from 30 to 93 years (mean, 59; median, 61). Tumor sizes ranged from 0.6 to 19.5 cm (mean, 6.8; median, 5.75). Presenting symptoms included urinary obstruction, pelvic fullness, elevated PSA, or incidental detection. In two patients, the tumor caused urinary obstruction requiring transurethral resection. Location adjacent to the seminal vesicles was noted in several patients with one growing as a large mass into the pelvis. In several patients, the SFT was an incidental finding on needle biopsy performed for elevated PSA. Histologically, all tumors demonstrated classic SFT morphology with a spindle cell proliferation and patternless architecture. Tumor borders varied, with both well-circumscribed and infiltrative patterns observed. Cellularity ranged from low to high, and nuclear atypia was most often mild to moderate. Mitotic activity was generally low (<5 mitoses/10 HPF in most cases), although two tumors showed elevated indices (12 and 15/10 HPF). Necrosis was identified in 5 tumors and was focal in 3. STAT6 was positive in 27/27 tested cases (100%), and CD34 was positive in all cases with available results (31/31, 100%). Management was primarily surgical. Only three tumors in this cohort developed metastatic disease, limiting the ability to confirm or refute the applicability of the Demicco risk stratification model. Metastatic cases occurred in relatively older patients, two of the three were >5 cm with focal necrosis, and two had increased mitotic activity (>4/10 HPF), while the third was 4.5 cm without necrosis but showed a high mitotic count. No overtly high-grade morphology or consistent morphologic predictors of metastasis were identified in this series. Follow-up was available in 16 patients (10-171 months; mean, 38; median, 29). Aside from the three patients who developed metastatic disease, all others were alive and without recurrence or metastasis at last contact. Conclusions: This is among the largest reported cohorts of prostatic SFTs. These tumors, though rare in the prostate, appear to display similar morphologic and immunophenotypic features to SFTs in other anatomic sites. While generally indolent, rare cases may metastasize. Accurate diagnosis relies on a combination of morphologic evaluation and STAT6 IHC. This study expands the clinicopathologic spectrum of SFTs and emphasizes the need for long-term follow-up due to the potential for late recurrence or metastasis.