Case Report: Acute monocytic leukemia mimicking large-vessel vasculitis on ¹⁸F-FDG PET/CT

Systemic inflammatory manifestations caused by hematologic malignancies may be very similar to autoimmune diseases, making diagnosis difficult. Acute monocytic leukemia (AML-M5) is characterized by significant cytokine production and, in rare cases, can mimic large vessel vasculitis (LVV).

A 56-year-old female with a two-week history of recurrent fever accompanied by significantly elevated inflammatory markers. Laboratory tests showed elevated levels of erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), and ferritin. The infection and autoimmune assessment are not significant. ¹⁸F-FDG PET/CT showed increased metabolic activity along the aortic arch, descending aorta, and bilateral iliac arteries, which raised suspicion of LVV. Glucocorticoid treatment quickly alleviated fever and partially reduced inflammation index. However, follow-up showed marked leukocytosis, monocytosis, anemia and thrombocytopenia. Bone marrow examination confirmed acute monocytic leukemia. Cytogenetic analysis confirmed that t (11; 19) (q23; p13.1) is consistent with KMT2A rearrangement. Next generation sequencing detected KRAS p.G13D and ETV6 p.L341F mutations.

AML with monocyte differentiation may exhibit systemic inflammation and vascular FDG uptake, thus mimicking LVV. Understanding this presentation is crucial. For patients with atypical inflammatory features, early hematological assessment should be considered to avoid delayed diagnosis.