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Inflammatory myofibroblastic tumor (IMT) is a rare mesenchymal neoplasm for which definitive radiotherapy in the retroperitoneum remains largely undefined, particularly when dose escalation must be balanced against the preservation of adjacent critical organs. A 59-year-old woman presented with a 10-day history of fever and abdominopelvic pain. Physical examination revealed deep tenderness in the right lower quadrant, and laboratory testing demonstrated leukocytosis. Imaging identified a 9.2 cm retroperitoneal mass invading the right ureter and inferior vena cava. Biopsy confirmed IMT, and capture-based next-generation sequencing revealed no class I actionable alterations. Given the unfavorable surgical anatomy and limited response to corticosteroids, curative-intent resection was precluded. The patient was treated with definitive dose-escalated volumetric modulated arc therapy (VMAT) employing a simultaneous integrated boost strategy (66 Gy/33 fractions to the boost volume and 50 Gy to the conventional planning target volume), with spatial sparing of adjacent organs at risk. Complete radiographic response was achieved at one month, without grade ≥2 treatment-related adverse events. At 19-month follow-up, the patient remained disease-free with stable renal function. This case demonstrates that dose-escalated VMAT with spatial organ avoidance can achieve durable complete remission in unresectable retroperitoneal IMT while preserving organ function, supporting its consideration as a curative-intent strategy in this challenging clinical setting.