Mastocytosis involving rectum: a case report and literature review

Mastocytosis is a rare heterogeneous group of diseases with multi-organ involvement. Gastrointestinal (GI) tract infiltration by neoplastic mast cell is uncommon in systemic mastocytosis. In our literature review, there are only 11 published mastocytosis reports including 42 cases of biopsy confirmed direct colon involvement. We report here a rare case of focal mastocytosis in the rectum without evidence of systematic mastocytosis, incidentally identified in a 66-year-old asymptomatic male. Endoscopically it appeared as a polyp-like lesion and histologically it showed aggregates of neoplastic mast cells infiltrating the rectal mucosa and superficial submucosa. The lesional mast cells showed typical mast cell morphology including oval and spindle shape, abundant cytoplasm, large dark blue nuclei and fine basophilic granules. They were also positive for CD117 and tryptase, and negative for CD138 and CD25. No increase of eosinophils or lymphocytes was identified in the areas adjacent to the mast cells. CD25 is a systematic mastocytosis marker, and is usually positive for a systemic mastocytosis, and negative in the cases of local mastocytosis. The immunohistochemical and morphological findings are suggestive of a mastocytosis with primarily GI origin, which could be a diagnostic challenge. Pathologists and clinicians need to have increased awareness of this entity. However, it is still possible that our patient had an indolent form of systemic mastocytosis involving the bone marrow and rectum. Our summary of reported cases shows that the patients with mastocytosis involving the colon were 50-years-old on average, more likely female (female:male = 25:5), and mostly presented with a polypoid or elevated lesion. Diarrhea, abdominal pain and weight loss were the most common complaints of these patients, although our patient was asymptomatic. In short, we provide early evidence that local mastocytosis may present as solitary involvement of rectum in an asymptomatic patient, and immunohistochemistry and histology features could confirm the diagnosis.