Clinicopathological characteristics of systemic mastocytosis in the intestine.

AIMS: Gastrointestinal (GI) involvement by systemic mastocytosis (SM) presents with non-specific symptoms, and pathological diagnosis can be difficult when a subtle mast cell infiltrate is present. The aim of this study was to characterize the clinicopathological features in diagnostically challenging cases.

METHODS AND RESULTS: Seven patients with GI biopsies showing an atypical mast cell infiltrate were identified, including three consultation cases in which mast cells were initially overlooked. Clinicopathological characteristics were evaluated. Biopsies showed involvement of the large bowel (n = 5), small bowel (n = 1) or both (n = 1) by a wide morphological spectrum of mast cells, including: bland spindle cells; small cells with irregular nuclei; and medium-sized monotonous cells with abundant pale cytoplasm. The patterns of mucosal involvement included: a polypoid mast cell aggregate (n = 1); a confluent subepithelial band of mast cells (n = 3); and multifocal aggregates of mast cells (n = 3). There were admixed eosinophils with a noticeable lack of plasma cells. Mast cells in all cases showed strong positive staining for CD117 and CD25. All patients fulfilled the World Health Organization (WHO) criteria for SM. On follow-up in four cases, none had progression of disease.

CONCLUSIONS: Atypical mast cell infiltrates in the intestine are often subtle and can easily be overlooked. Clues to diagnosis include lamina propria expansion by monotonous cells with pale cytoplasm, admixed eosinophils, and the absence of crypt distortion or significant plasmacytic infiltrate. Although most patients with GI involvement by SM are symptomatic, a subset remain asymptomatic, and the absence of clinical suspicion of mast cell disease adds to the difficulty in making a diagnosis of SM.