Defining Tumor-Induced Iliacus Syndrome: A Case Report on Radiotherapy and Gait Recovery.

Tumor invasion of the iliacus muscle is rare, and its clinical implications remain unclear. While malignant psoas syndrome (MPS) is characterized by severe pain and hip flexion contracture due to psoas muscle involvement, isolated iliacus muscle infiltration presents differently. In this report, we propose tumor-induced iliacus syndrome as a distinct clinical entity characterized by gait initiation impairment and slowed walking speed rather than fixed contracture or complete hip flexion failure. A woman in her sixties was diagnosed with stage IVB ovarian endometrioid carcinoma after routine lung cancer screening revealed abnormal diaphragmatic shadows. Imaging studies identified peritoneal nodules, a left ovarian mass, and a lytic bone lesion in the right iliac bone. A biopsy confirmed ovarian endometrioid carcinoma. She underwent chemotherapy with carboplatin and paclitaxel but developed sudden gait impairment without neurological abnormalities on brain and spinal imaging. Despite being able to stand and walk slowly with assistance, she experienced delays initiating each step, requiring a conscious effort to move her leg forward. Follow-up imaging showed iliacus muscle thickening and infiltration adjacent to the enlarging iliac bone lesion. Given her progressive gait impairment and worsening pain, chemotherapy was discontinued, and palliative radiotherapy was initiated. She received a total dose of 40 Gy in 15 fractions using 10 MV and 6 MV X-ray beams, targeting the iliac bone lesion and iliacus muscle involvement. Initially, she required a wheelchair due to difficulty initiating movement. After radiotherapy, her ambulatory function improved, allowing her to walk independently with mild residual gait disturbances. This case introduces tumor-induced iliacus syndrome as a newly recognized clinical entity distinct from MPS. Unlike MPS, which presents with severe pain and hip flexion contracture, iliacus muscle invasion predominantly affects gait initiation and walking speed. Recognizing this syndrome may improve diagnostic accuracy and optimize treatment strategies. Further research and case accumulation are needed to define its clinical significance and establish appropriate management approaches.