Isolated Muscular NK/T Cell Lymphoma: A Rare Presentation of Post-Transplant Lymphoproliferative Disorders.

Posttransplant lymphoproliferative disorders (PTLDs) represent a spectrum of malignancies occurring in transplant recipients under immunosuppression, often linked with Epstein-Barr Virus infection. PTLD has a varied presentation, and isolated muscular involvement is infrequent. Here, we present the case of a 47-year-old female renal transplant recipient presenting with acute left knee joint swelling, initially suggestive of an infective or inflammatory etiology. Biopsy revealed high-grade non-Hodgkin lymphoma of natural killer/T-cell lymphoma. F-18 Fluorodeoxyglucose positron emission tomography-computed tomography scan revealed metabolically active soft tissue mass lesions isolated to thigh muscles. The patient was on a modified chemotherapy regimen tailored to accommodate renal function. This case underscores the necessity for heightened vigilance in diagnosing PTLD, particularly considering its atypical presentations.