Supplementary Material for: Mast Cell Sarcoma of the Small Intestine Diagnosed by Ascitic Fluid Cytology: A Case Report with Molecular and Immunophenotypic Correlation

Introduction: Mast cell sarcoma (MCS) is a rare and aggressive neoplasm with limited cytologic description, particularly in effusion specimens. Case Presentation: A 55-year-old man presented with a small intestinal tumor and peritoneal dissemination. Ascitic fluid cytology revealed numerous large atypical cells with round to horseshoe-shaped nuclei and abundant cytoplasm containing metachromatic granules. Some cells displayed hand mirror–like morphology. Immunocytochemistry showed CD117 and tryptase positivity, and a KIT D816Y mutation was identified. The diagnosis of MCS was later confirmed by biopsy. Conclusion: This is the first reported case of MCS diagnosed by effusion cytology. Recognition of hand mirror–like morphology and metachromatic granules, combined with immunocytochemistry and molecular analysis, may support early diagnosis.