# Synchronous Hepatic Neuroendocrine Carcinoma and Rectal Adenocarcinoma With Pontine Metastasis: A Diagnostic Dilemma

## TL;DR

This case study describes a rare and complex situation where a patient had two different cancers that presented together, causing a diagnostic challenge.

## Contribution

The novelty lies in highlighting the diagnostic difficulties of synchronous neuroendocrine and adenocarcinoma tumors with metastasis.

## Key findings

- The patient had a hepatic neuroendocrine carcinoma and a rectal adenocarcinoma with distinct immunohistochemical profiles.
- The case presented a diagnostic dilemma between multiple primaries, mixed tumors, or metastasis.
- Molecular profiling was not performed, leaving the primary origin of the neuroendocrine carcinoma uncertain.

## Abstract

We present a complex case of a 53-year-old male with stage IV large-cell neuroendocrine carcinoma (NEC) of unconfirmed primary origin, manifesting with a pontine brain mass, multiple hepatic lesions, lymphadenopathy, and a synchronous rectal adenocarcinoma. Imaging and pathology revealed distinct morphologic and immunohistochemical profiles: hepatic biopsy confirmed large-cell NEC positive for synaptophysin and CD56, whereas rectal biopsy showed adenocarcinoma with intestinal markers and no neuroendocrine differentiation. Differential diagnoses included two independent primary malignancies, mixed adenoneuroendocrine carcinoma (MANEC), or colorectal adenocarcinoma with neuroendocrine differentiation in the liver. Although molecular profiling or clonal comparison was considered to clarify the primary origin, it was not performed due to clinical constraints. This report underscores the diagnostic challenges in distinguishing synchronous multiple primaries from metastatic or mixed tumors, with significant implications for staging and treatment.