# Pathological Complete Response in a Rare Case of Rectal Small Cell Neuroendocrine Carcinoma Following Neoadjuvant Radiochemotherapy: A Case Report

## TL;DR

A rare case of aggressive rectal cancer showed complete response to radiochemotherapy before surgery, suggesting potential for improved treatment outcomes.

## Contribution

This case report presents a rare pathological complete response to neoadjuvant radiochemotherapy in a high-grade rectal neuroendocrine carcinoma.

## Key findings

- The patient achieved a pathological complete response after neoadjuvant radiochemotherapy.
- Immunohistochemistry was critical for diagnosing the poorly differentiated tumor.
- The case supports the potential of neoadjuvant therapy in improving surgical outcomes for this rare cancer.

## Abstract

Rectal neuroendocrine neoplasms are rare, aggressive tumors comprising a small fraction of rectal malignancies but with increasing incidence due to improved endoscopic detection. Small cell neuroendocrine carcinoma, a high‐grade variant, poses significant diagnostic and therapeutic challenges due to its rapid progression and poor prognosis. We report a case of a 32‐year‐old male presenting with per‐rectal bleeding and pain for 6 months. Imaging revealed an anorectal mass with locoregional lymphadenopathy. Histopathology confirmed poorly differentiated small cell neuroendocrine carcinoma (Grade 3) via immunohistochemistry, with a high Ki‐67 index and neuroendocrine marker expression. The patient received neoadjuvant radiotherapy (59.4 Gray in 30 fractions over 6 weeks) with concurrent chemotherapy etoposide and cisplatin, followed by abdominoperineal resection. Postoperative histology showed a pathological complete response. This case underscores the importance of considering rectal neuroendocrine neoplasms in patients with persistent rectal symptoms and highlights the role of immunohistochemistry in diagnosis. Despite limited literature, our case supports the potential efficacy of neoadjuvant radiochemotherapy in tumor downstaging and improving surgical outcomes in poorly differentiated rectal neuroendocrine carcinoma. Early diagnosis, protocol‐based neoadjuvant radiochemotherapy, and appropriate surgical intervention can lead to favorable outcomes in high‐grade rectal neuroendocrine neoplasms. This case reinforces the need for awareness, guideline‐based management, and long‐term surveillance to optimize prognosis in these rare but aggressive tumors.