Solitary Fibrous Tumor of the Seminal Vesicle With Entrapped Benign Glands Mimicking a Mixed Epithelial and Stromal Tumor: A Case Report With Literature Review.

Solitary fibrous tumor (SFT) is a rare mesenchymal neoplasm known to occur in various soft tissue and visceral locations. Solitary fibrous tumor arising from the seminal vesicles is rare and reported in only 12 earlier patients. Herein, we describe a SFT arising from the left seminal vesicle in a 58-year-old male patient who underwent a robotic left seminal vesiculectomy. Histologically, the tumor resembled a mixed epithelial and stromal tumor due to secondary non-neoplastic proliferation of the benign seminal vesicle tissue admixed with SFT. The spindle cells of SFT had CD34+/STAT6+ immunophenotype, whereas the admixed benign seminal vesicle tissue was negative for both CD34 and STAT6 immunohistochemical stains. Next-generation sequencing confirmed the presence of NAB2::STAT6 fusion, thus confirming the diagnosis of SFT. Twenty months after surgery, the patient is on an annual follow-up with no local recurrence or metastasis. For either a biphasic or a spindle cell neoplasm of the seminal vesicles, SFT should be considered in the differential diagnosis.