# Rare confounder: benign multicystic peritoneal mesothelioma in a patient with mucinous colon adenocarcinoma

## TL;DR

A rare case of benign multicystic peritoneal mesothelioma was diagnosed in a patient with colon cancer, highlighting the challenge of distinguishing it from cancerous spread.

## Contribution

This case highlights the diagnostic challenge of distinguishing BMPM from mucinous carcinomatosis in cancer patients.

## Key findings

- Frozen section analysis can be misleading in differentiating BMPM from metastatic disease.
- Final histologic analysis is crucial for accurate diagnosis before proceeding with treatment.
- BMPM can coexist with unrelated malignancies and may be incidentally discovered.

## Abstract

Benign multicystic peritoneal mesothelioma (BMPM) is a rare condition, in which patients have multiple cystic lesions of the peritoneum. BMPM can mimic mucinous carcinomatosis and can thus create a diagnostic dilemma. We present the case of a 76-year-old woman who was referred for management of ascending colon adenocarcinoma and was noted to have several nonspecific cystic lesions in the abdomen and pelvis on preoperative computed tomography and diagnostic laparoscopy. Frozen section analysis suggested the lesions contained ‘mucin’. Due to concern for metastases, right colectomy was aborted. Final histologic analysis of the laparoscopic biopsies revealed mesothelial cysts, consistent with BMPM, unrelated to her colon adenocarcinoma. Laparoscopic right colectomy was performed 2 weeks later. BMPM can create diagnostic and therapeutic uncertainty in patients with known visceral malignancies when discovered incidentally. Frozen section analysis may not be accurate in differentiating the two, and final histologic confirmation should be sought prior to definitive treatment.