Malignant Hip Flexion Failure Syndrome: An Oncologic Disease Compared to Malignant Psoas Syndrome.

Malignant psoas syndrome (MPS) causes painful hip immobilization when a malignant tumor reaches the psoas muscle. However, there exists a different condition in which a malignant tumor invades the psoas muscle, leading to hip flexion failure without painful hip immobilization. This study aimed to define malignant hip flexion failure syndrome (MHFFS) as tumors located in the upper lumbar region or at the lesser trochanter of the femur, near the origin or termination of the psoas muscle, and to compare its prevalence, characteristics, and outcomes with those of classical MPS. We analyzed 291 patients who received palliative radiotherapy (RT) in the lumbar, pelvic, and lower leg regions from 2013 to 2023. The prevalence of MPS and MHFFS, pathological features, distinctive clinical presentations, treatment modalities, and treatment outcomes have been summarized. We also defined the 'Clinical sign reported by Ishikawa and Teramura (IT sign)' to describe the characteristic action of lifting the affected lower leg with both hands in MHFFS cases and assessed its clinical significance. Among the 291 patients, 6 (2.1%) had MHFFS and 11 (3.8%) had MPS. MHFFS resulted from metastatic tumors in the 11th and 12th thoracic vertebrae, as well as the 1st and 2nd lumbar vertebrae or the lesser trochanter of the femur, and it was characterized by hip and groin pain along with hip flexion dysfunction. All cases showed a positive IT sign. The response to RT varied, with symptomatic improvement observed in 50% of the patients. MPS is characterized by tumor invasion of the psoas muscle, causing severe lumbosacral nerve pain. Strong opioids were used for pain management in all patients, and epidural anesthesia was required in some patients. The median survival time of patients with MPS and MHFFS was 13.2 months. MPS required opioids more potently than MHFFS, but MHFFS responded relatively well to early RT.