# Pseudoendocrine Sarcoma: Unusual Manifestation in the Lower Leg

## TL;DR

A rare pseudoendocrine sarcoma was found in the lower leg of a 59-year-old man, showing unique features and a new location for this tumor type.

## Contribution

This is the first reported case of pseudoendocrine sarcoma occurring in the lower leg.

## Key findings

- The tumor exhibited neuroendocrine-like morphology but lacked typical neuroendocrine markers like chromogranin and synaptophysin.
- A CTNNB1 gene S37F mutation was identified, supporting the diagnosis of pseudoendocrine sarcoma.
- The tumor was positive for beta-catenin, CD56, and S100, aiding in differential diagnosis.

## Abstract

Pseudoendocrine sarcoma is a recently described rare soft tissue neoplasm. The tumor predominantly affects older adults, with the paravertebral soft tissue as a predilection site. We report on an unusual location of pseudoendocrine sarcoma in the right gastrocnemius muscle of a 59-year-old male patient. Histology showed a remarkable neuroendocrine-like morphology. Immunohistochemically, the tumor cells were negative for chromogranin, synaptophysin, and INSM-1 but positive for beta-catenin, CD56, and multifocally for S100. By molecular analysis, an S37F point mutation in exon 3 of the CTNNB1 gene was detected. This case, to our knowledge, illustrates a hitherto unreported location of a pseudoendocrine sarcoma in the lower leg. The relevant differential diagnoses are discussed with special regard to CD56 and S100.