Thoracic Spinal Metastasis With Hip Flexion Failure and Psoas Muscle Atrophy Successfully Improved With Radiotherapy: A Case Report.

When a malignant tumor infiltrates the psoas muscle, it is termed malignant psoas syndrome (MPS). We are reporting this case because the malignancy led to atrophy of the psoas muscle, and the clinical course differed from the typical presentation of MPS. A 72-year-old Japanese female with advanced sigmoid colon cancer and multiple metastases had been undergoing systemic chemotherapy for four years. She complained of severe back pain on a numeric rating scale (NRS) of 4-5, left groin pain, and hip flexion weakness. Although she could stand up, she started experiencing difficulties while walking and became reliant on a wheelchair. At the time of referral to our department, her performance status was 2. On examination, she was capable of hip adduction and abduction, and flexion was impossible on the left side and possible on the right side. Imaging revealed metastases to the 11th and 12th thoracic vertebrae, extending to the upper portion of the first lumbar vertebra, leading to atrophy of the left psoas major muscle and impairment of hip flexion. She received palliative radiation therapy (RT) of 30 Gy in 10 fractions over a period of 2 weeks. Following RT, she had grade 1 skin inflammation but no severe complications. Two weeks after RT, her pain improved (NRS 0-1) and she regained hip flexion. When hip flexion failure occurs in patients with malignant tumors, it is important to recognize that it may be caused by a tumor located near the lower thoracic or upper lumbar spine, even if the psoas muscle itself is not directly infiltrated by the tumor.