Transformation of myelodysplastic syndrome to acute myeloid leukemia with leukemic infiltration of the colon without visible change on endoscopy: A case report

Extramedullary leukemic involvement refers to the infiltration of leukemic cells into tissues or organs outside the bone marrow and peripheral blood and occurs in approximately 3-8% of adult patients with acute myeloid leukemia (AML). Gastrointestinal involvement is uncommon, particularly in cases of AML transformed from myelodysplastic syndrome (MDS). We report a rare case of leukemic colitis in an 85-year-old man with high-risk MDS who presented with persistent watery diarrhea. Initial laboratory findings showed leukocytosis (11.9 ×10 3 /μL with 7% peripheral blasts), thrombocytopenia (31 ×10 3 /μL), and elevated inflammatory markers (C-reactive protein 18.96 mg/dL). Abdominopelvic computed tomography demonstrated nonspecific colitis, while sigmoidoscopy performed on hospital day 1 revealed no remarkable mucosal abnormalities. Because diarrhea persisted despite supportive treatment, a full colonoscopy was performed on hospital day 7, which again showed no distinct mucosal lesions. Random biopsies were therefore obtained from multiple colonic segments despite the absence of visible abnormalities. Histopathological examination of the descending colon demonstrated leukemic infiltration of atypical myeloid cells in the lamina propria and submucosa with positive staining for CD15 and CD68. Subsequently, the peripheral blood blast count increased to 27% on hospital day 21, confirming transformation to overt AML. This case highlights the diagnostic challenge of endoscopically inapparent leukemic colitis and underscores the importance of random colonic biopsy in patients with hematologic malignancies presenting with unexplained gastrointestinal symptoms, even when endoscopic findings appear normal.