# Metastatic Merkel Cell Carcinoma Incidentally Detected on PSMA PET/CT in a Patient With Metastatic Prostate Cancer

## TL;DR

A rare case of Merkel cell carcinoma was found incidentally during a PSMA PET/CT scan for prostate cancer, highlighting the need to consider other cancers when interpreting PSMA uptake.

## Contribution

First reported case of PSMA-avid Merkel cell carcinoma detected incidentally in a patient with prostate cancer.

## Key findings

- PSMA PET/CT incidentally detected a PSMA-avid Merkel cell carcinoma lymph node metastasis.
- Diagnostic biopsy confirmed the presence of metastatic Merkel cell carcinoma.
- The case emphasizes the importance of considering nonprostatic malignancies in PSMA imaging interpretation.

## Abstract

Positron emission tomography (PET) radiotracers targeting prostate-specific membrane antigen (PSMA) are now widely used in the evaluation of prostate cancer. However, PSMA activity has also been described in several nonprostatic malignancies, where PSMA is primarily expressed in tumor neovasculature. Here, we describe to the best of our knowledge the first case of a PSMA-avid Merkel cell carcinoma (MCC) inguinal lymph node metastasis, detected incidentally in an 80-year-old man with advanced metastatic prostate adenocarcinoma. Clinical history and disease distribution prompted the need for a diagnostic biopsy, confirming PSMA-avid metastatic MCC. This case highlights the importance of recognizing nonprostatic causes of PSMA uptake, as synchronous malignancies can alter diagnostic interpretation and treatment planning.