# Plexiform Fibromyxoma with MALAT1–GLI1 Fusion with Limited Myxoid Stroma, Aberrant KIT Expression, and Diffuse D2-40 Expression: A Case Report

## TL;DR

A rare case of gastric plexiform fibromyxoma with unique features like limited myxoid stroma and specific gene fusion is reported, aiding in distinguishing it from similar tumors.

## Contribution

This case report expands the known morphologic and immunophenotypic features of plexiform fibromyxoma.

## Key findings

- A gastric tumor case showed limited myxoid stroma and aberrant KIT expression, complicating diagnosis.
- Molecular analysis confirmed MALAT1–GLI1 fusion, supporting a diagnosis of plexiform fibromyxoma.
- Diffuse D2-40 expression was observed, suggesting potential diagnostic utility in similar cases.

## Abstract

Background and Clinical Significance: Plexiform fibromyxoma (PFM) is a rare benign gastric mesenchymal neoplasm characterized by multinodular plexiform growth of bland spindle cells in a myxoid or fibromyxoid stroma. We report a case of the cellular form of PFM with limited myxoid stroma and aberrant KIT expression, resulting in diagnostic difficulty by biopsy. Case Presentation: A 59-year-old woman presented with a slowly enlarging 15 mm gastric antral submucosal tumor. A resected specimen by laparoscopic and endoscopic cooperative surgery revealed spindle cell proliferation forming plexiform nodules with a myxoid background in limited areas. Positive immunoreactivity of a subset of spindle cells for KIT suggested a diagnosis of gastrointestinal stromal tumor (GIST), although DOG1 was negative. In addition, diffuse staining for CD10, smooth muscle actin, and D2-40 was confusing. MALAT1::GLI1 fusion was detected by next-generation sequencing analysis. Consequently, a diagnosis of PFM was established. Conclusions: This case expands the morphologic and immunophenotypic spectrum of PFM and indicates the possible diagnostic utility and biological significance of D2-40 expression. Although molecular confirmation of MALAT1::GLI1 fusion is definitive for the diagnosis of PFM, the findings of the present case may aid diagnosis in challenging cases that mimic GIST.