Abstract
A 65-year-old man presented with painful swelling and thickening of both lower limbs, initially responding to oral steroid treatment. However, the patient later developed similar symptoms, accompanied by general malaise. No abnormalities were detected upon fluorine-18 fluorodeoxyglucose positron emission tomography/computed tomography. Magnetic resonance imaging (MRI) showed extensive fascial edema in the thigh and gluteal regions, mimicking fasciitis. Concurrently, a bone marrow biopsy revealed intravascular large B-cell lymphoma (IVLBCL), a rare lymphoma with a generally poor prognosis. This case highlights the significance of fasciitis-like MRI findings in diagnosing IVLBCL, underscoring the need for integrating imaging and histological evaluations in atypical presentations.