Multifocal Colonic Mastocytosis Mimicking Ischaemic Colitis: A Rare Cause of Lower Gastrointestinal Bleeding.

Introduction: Mastocytosis is a rare clonal disorder characterised by abnormal proliferation and accumulation of mast cells in one or more organs. Although gastrointestinal symptoms are common in systemic mastocytosis, direct mucosal infiltration by neoplastic mast cells is uncommon and remains poorly characterised. Isolated involvement of the colon is exceptionally rare and may present with non-specific clinical and endoscopic findings. Case description: We report the case of a 59-year-old male presenting with recurrent rectorrhagia without associated gastrointestinal symptoms or relevant medical history. Colonoscopic examination revealed mucosal changes suggestive of ischaemic colitis. Histopathological evaluation of colonic biopsies demonstrated dense aggregates of mast cells within the lamina propria and muscularis mucosae. The cells exhibited characteristic morphology with oval to spindle-shaped forms, abundant cytoplasm containing basophilic granules and centrally located hyperchromatic nuclei. Immunohistochemical analysis showed strong positivity for CD117 (KIT) and tryptase, while CD25 and CD138 were negative. No significant increase in eosinophils or lymphocytic infiltrates was observed in adjacent mucosa. These findings were consistent with multifocal colonic mastocytosis in the absence of evidence of systemic disease. Conclusion: Colonic mastocytosis is an exceedingly rare entity that may mimic other gastrointestinal disorders both clinically and endoscopically. Histological examination with immunohistochemical confirmation is essential for accurate diagnosis. Increased awareness of this rare presentation is important for appropriate evaluation and follow up, given the potential risk of progression to systemic mastocytosis. LEARNING POINTS: Gastrointestinal mastocytosis is rare and may present without systemic manifestations.Colonic involvement is exceptionally uncommon and may mimic ischaemic or inflammatory colitis on endoscopy.Histology with immunohistochemical staining (CD117 and tryptase) is essential for diagnosis.Long-term follow up is recommended to monitor for potential progression to systemic mastocytosis.