A middle-aged patient with perineal pain and frequent urination had a periprostatic mass detected via Transrectal Ultrasound (TRUS). The serum prostatic surface antigen (PSA) level was within normal range. Multiparametric MRI (mp-MRI) revealed a mildly enlarged prostate. No abnormal focal or diffuse T2W hypointense lesions were detected in the prostate. A well-defined periprostatic mass measuring 27 x 18.4 x 22.9 mm, exhibiting T1W/T2W hypointense signals, was identified. The mass closely abutted prostatic apex, anteroinferior capsule, anterior rectal wall, and left puborectalis sling, without invasion or infiltration. DWI indicated no diffusion restriction. The dynamic contrast-enhanced MRI (DCE-MRI) showed slow continuous enhancement within the mass, potentially excluding malignancy. Additionally, absence of T2W hypointense lesions in transition-zone ruled out stromal nodules of BPH. Furthermore, absence of such lesions in the transition/ and or peripheral-zone, along with an intact prostatic-capsule, excluded prostatic malignancy with extracapsular extension. The absence of T2W hyperintense lesions in peripheral-zone of prostate excluded STUMP. The lack of irregular margins, invasion, or infiltration into surrounding structures, along with absence of diffusion restriction in DWI, and the lack of early hyperintensity in DCE-MRI collectively rules out the likelihood of various malignant mesenchymal tumors. TRUS-guided biopsy for histopathological examination (HPE) supported by immunohistochemistry (IHC), conclusively identified the mass as a periprostatic Leiomyoma with Bizarre Nuclei (LBN). LBN is extremely rare and carries a potential for malignant transformation. In conclusion, mp-MRI stands as a valuable modality for characterizing a periprostatic mass, facilitating differentiation between benign and malignant lesions. However, a definitive diagnosis requires HPE and IHC.

Keywords:Multiparametric MRI; Prostatic Mesenchymal Tumors; Periprostatic Mass; Leiomyoma with Bizarre Nuclei