# Case report of sigmoid colon cancer with synchronous peritoneal metastasis achieving a pathological complete response after mFOLFOX6 plus cetuximab

## TL;DR

A rare case of colon cancer with peritoneal spread achieved full recovery after chemotherapy with mFOLFOX6 and cetuximab.

## Contribution

Reports of pCR with peritoneal metastases are extremely rare, making this case a novel clinical observation.

## Key findings

- The patient achieved a pathological complete response after 12 cycles of mFOLFOX6 plus cetuximab.
- A peritoneal nodule resected after treatment showed no viable cancer cells.
- No recurrence was observed for 7 months following treatment completion.

## Abstract

Synchronous peritoneal metastasis occurs in about 5 % of colorectal cancer (CRC) cases, often with other distant metastases. Due to poor drug penetration, chemotherapy is typically ineffective, leading to poor prognosis. Here, we report a rare case of sigmoid colon cancer with synchronous peritoneal metastasis achieving pathological complete response (pCR) after systemic chemotherapy.

A 74-year-old man was diagnosed with acute generalized peritonitis from a perforated sigmoid colon cancer with two peritoneal metastases. One peritoneal lesion was left unresected, and sigmoid colectomy with partial jejunal resection was performed. Additionally, postoperative colonoscopy revealed a synchronous cancer in the cecum. Chemotherapy with mFOLFOX6 plus cetuximab was initiated. After 10 cycles, the patient developed strangulation ileus, requiring surgery, during which a peritoneal nodule was resected and showed no viable cancer cells, indicating pCR. Chemotherapy was resumed and completed to 12 cycles. Laparoscopic ileocecal resection for the cecal cancer also achieved pCR. A single peritoneal recurrence was surgically removed. No recurrence has been observed for 7 months.

Reports of pCR with peritoneal metastases are extremely rare. Although the mechanism of the remarkable response in this case remains unclear, given the rarity of pCR in CRC with peritoneal metastasis, further case accumulation and analysis are warranted.

This is a rare case of pCR to systemic chemotherapy for CRC with synchronous peritoneal metastasis.

•A rare case of synchronous peritoneal metastasis from colorectal cancer achieving pathological complete response after mFOLFOX6 plus cetuximab.•The case highlights remarkable chemotherapy efficacy even with poor drug penetration into the peritoneum.•This outcome is particularly noteworthy and represents an extremely rare clinical entity, contributing to the accumulation of knowledge in this field.

A rare case of synchronous peritoneal metastasis from colorectal cancer achieving pathological complete response after mFOLFOX6 plus cetuximab.

The case highlights remarkable chemotherapy efficacy even with poor drug penetration into the peritoneum.

This outcome is particularly noteworthy and represents an extremely rare clinical entity, contributing to the accumulation of knowledge in this field.