Cystic mesothelioma, also known as benign multicystic peritoneal mesothelioma (BMPM), is a rare neoplasm and one of the five main histological types of peritoneal mesothelioma. Although it is usually considered benign histologically, it is increasingly accepted as a ‘borderline malignancy’ because the local recurrence rate is high. It is more common in middle-aged women and is generally asymptomatic. Considering that BMPM is often found in the pelvis, it is difficult to differentiate from other pelvic and abdominal lesions, such as cystic ovarian masses, especially mucinous cystadenoma–adenocarcinoma, pseudomyxoma peritonei, etc. Preoperative diagnosis is very difficult due to its rarity and lack of specific diagnostic imaging and clinical findings. Definitive diagnosis is made only by pathological evaluation. In this article, we aimed to present the imaging findings of a BMPM case in a woman who was diagnosed with mucinous ovarian neoplasm with pseudomyxoma peritonei preoperatively and underwent cytoreductive surgery with hyperthermic intraperitoneal chemotherapy.

An 88-year-old man was admitted to our hospital with a recurrent lower urinary tract infection. He had a history of open prostatectomy for benign prostatic hyperplasia 15 years ago and smoking. A mass arising within a bladder diverticula was suspected on the left lateral wall of the bladder on ultrasonography. Although no mass was observed in the bladder lumen on the cystoscopy, abdominal CT detected a left-sided pelvic soft tissue mass. A hypermetabolic mass was detected on 18F-FDG PET/CT performed because of suspicion of malignancy, and it was excised. Granuloma secondary to chronic vasitis was diagnosed histopathologically.