Mixed Epithelial Stromal Tumour of the Seminal Vesicle Presenting with Recurrent Acute Urinary Retention: A Case Report.

Background: Mixed epithelial stromal tumour (MEST) is a rare biphasic neoplasm, most commonly arising in the kidney. Seminal vesicle involvement is exceedingly uncommon, and its clinical and imaging characteristics remain poorly defined. We present a rare case with detailed clinical and urological evaluation, multiparametric MRI characterisation, and radiologic-pathologic correlation. Case presentation: A 34-year-old man presented with recurrent acute urinary retention requiring repeated catheterization and, subsequently, an indwelling urinary catheter. Urological evaluation, including ultrasonography and cystoscopy, demonstrated marked mass effect on the bladder and prostatic urethra without an intravesical lesion. Contrast-enhanced CT and MRI revealed a large retrovesical mass arising from the right seminal vesicle, measuring approximately 11.5 × 9 × 10.5 cm. MRI demonstrated a predominantly multiloculated cystic lesion containing haemorrhagic or proteinaceous material, with an enhancing papillary solid component showing mild diffusion restriction and progressive enhancement. The lesion produced marked mass effect on the bladder, prostate, and rectum, without local invasion. Robotic-assisted resection of the right seminal vesicle was performed. Histopathology demonstrated a biphasic epithelial and stromal proliferation without atypia, necrosis, or increased mitotic activity, consistent with MEST. Conclusions: This case highlights the value of integrating clinical, urological, radiological, and pathological findings in the evaluation of a large complex retrovesical mass. Multiparametric MRI provided detailed characterisation of the solid-cystic architecture, with imaging features correlating with the epithelial and stromal components identified histologically. Recognition of this pattern may raise suspicion for MEST and help guide differential diagnosis and surgical planning, although definitive diagnosis requires histopathological and immunohistochemical evaluation.