A case of renal diffuse large B-cell lymphoma concurrent with disseminated muscle involvement.

BACKGROUND: Renal involvement in the diffuse large B-cell lymphoma (DLBCL) is rare and usually occurs as part of systemic disease. Muscle infiltration is even less common and can mimic inflammatory or obstructive conditions. CASE PRESENTATION: We present a 58-year-old male with right flank pain, myalgia, oliguria, and weight loss. Initial evaluation suggested obstructive nephropathy due to ureteral stones, but renal function did not improve after stone removal. Urine cytology revealed atypical lymphoid cells positive for PAX-5. 18F-Flourodeoxyglucose Positron Emission Tomography- Magnetic Resonance Imaging (18F-FDG-PET-MRI) showed pathological 18F-FDG uptake in the kidneys, multiple muscles, and lymph nodes. Renal biopsy confirmed DLBCL. The patient was treated with daratumumab, rituximab, etoposide, cyclophosphamide, doxorubicin, vincristine, and prednisone, achieving hematologic remission, but remained dialysis-dependent. CONCLUSION: This case highlights an unusual presentation of secondary renal DLBCL with diffuse muscle involvement, mimicking obstructive acute kidney injury (AKI). The disease should be considered in atypical AKI presentations. Despite systemic treatment response, renal prognosis may be poor in advanced disease.