Treatment Outcomes of Radiotherapy for Malignant Psoas Syndrome: A Single-Center Retrospective Study.

Introduction: The efficacy of radiotherapy for symptomatic relief of malignant psoas syndrome (MPS) remains unknown because there are limited publications with high level evidence, including analyses with sufficient number of cases, clinical trials, and systematic reviews about radiotherapy for MPS. We aimed to investigate the characteristics of and symptom relief rates in patients treated with radiotherapy for MPS in palliative intent. Methods: In this single-center retrospective study, we analyzed data of 22 consecutive patients treated with radiotherapy for MPS at our insti-tution in Japan between 2012 and 2022. We recorded patient charac-teristics, including primary site, invasion pattern, recognition of MPS by the attending physician, radiation regimen, biological effective dose with alpha/beta = 10 Gy (BED10), and adverse events. Since no objective evaluation index for palliative radiotherapy for non-bone metastases has been established, we modified and used an International Consen-sus on Palliative Radiotherapy Endpoint, which was originally used for bone metastases, to evaluate symptom relief in the present retro-spective study. "Response" was defined as symptom relief described in medical records or the use of analgesic medications reduced by >= 25% within 3 months post-initiation of radiotherapy. Results: Genitourinary organs (41%) were the most common primary-tumor sites. MPS was caused by metastasis in the iliopsoas muscle in 14 patients (64%) and by direct invasion of the primary tumor in eight patients (36%). Since the optimal radiation dose for MPS has not been established, the radiation dose varied from low dose, which are used in palliative radiotherapy for painful bone metastases, to high dose with conventional fraction using 1.8 to 2 Gy per fraction, with a median BED10 of 48 Gy (range, 10.6-79.2 Gy). Fifteen patients (68%) achieved a response. No acute nor late adverse events of grade 2 or higher, according to Common Terminology Criteria for Adverse Events version 5.0, were reported during the observation period. Conclusion: Radiotherapy for symptomatic MPS might be an effective treatment option with a high response rate (68%) and minimal adverse events. Since the present study is a retrospective study with small number of cases, a prospective study with a larger sample size is required.