Case

Patient with diarrhea.  What is your diagnosis?

Answer:

These are colonic biopsies from a patient with presumed systemic mastocytosis. The diagnosis is probably best made as a clinicopathologic correlation. Note that the sheets of mast cells express CD117/KIT and CD25. There is really no role for mast cells counts in histologically normal samples. Systemic mastocytosis differs from mast cell activation syndrome. Two helpful references appear below.

1: Johncilla M, Jessurun J, Brown I, Hornick JL, Bellizzi AM, Shia J, Yantiss RK. Are Enterocolic Mucosal Mast Cell Aggregates Clinically Relevant in Patients Without Suspected or Established Systemic Mastocytosis? Am J Surg Pathol. 2018 Oct;42(10):1390-1395. PMID: 30001235.

2: Doyle LA, Sepehr GJ, Hamilton MJ, Akin C, Castells MC, Hornick JL. A clinicopathologic study of 24 cases of systemic mastocytosis involving the gastrointestinal tract and assessment of mucosal mast cell density in irritable bowel syndrome and asymptomatic patients. Am J Surg Pathol. 2014 Jun;38(6):832-43. PMID: 24618605; PMCID:

PMC4086834.