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Background Bone metastases cause significant pain and functional limitation. Conventional external beam radiotherapy (EBRT) provides effective symptom relief, but local progression remains frequent. Stereotactic body radiotherapy (SBRT) offers improved local control but is often resource-intensive and associated with higher vertebral compression fracture (VCF) rates. Integrating a simultaneous gross tumour volume (GTV) boost within a conventional EBRT regimen may provide a feasible and safe alternative. Methods This is a prospective, multicentre, multinational, single-arm study enrolling 100 adults with painful bone metastases from solid tumours. Eligible patients receive 20 Gy in 5 fractions with a 5 Gy “stereotactic-lite” GTV boost (total 25 Gy) or 30 Gy in 10 fractions with a 6 Gy boost (total 36 Gy), delivered using intensity modulated radiotherapy or volumetric modulated arc therapy. The primary endpoints are feasibility (commencement of radiotherapy within 10 working days of computed tomography simulation in at least 80% of patients) and safety (incidence of Common Terminology Criteria for Adverse Events version 5.0 grade ≥ 2 acute toxicity within 3 months). Secondary endpoints include pain response, radiation site-specific progression-free survival, rates of VCF and long bone fracture, skeletal-related events, quality of life changes via EORTC QLQ-C30 and BM22, and overall survival. Discussion This protocol evaluates a hybrid EBRT approach with a simultaneous integrated boost as a practical strategy to enhance local tumour control and symptom relief without delaying palliation. If feasible and safe, this approach may bridge the gap between conventional EBRT and SBRT. Trial registration Australian and New Zealand Clinical Trial Registry (ACTRN12625000615482).

Systematic and random errors in radiation dose delivery necessitate the use of planning target volume (PTV) margins to ensure adequate clinical target volume (CTV) treatment. Advances in magnetic resonance-guided radiation therapy (MRgRT) have enabled improved imaging with possible margin reduction; however, the optimal PTV margins remain uncertain. This study aimed to evaluate the adaptive radiotherapy component of intra-fractional prostate movement in MRgRT for prostate cancer (PCa) patients and determine appropriate PTV margins. This study retrospectively analyzed 18 PCa patients treated using a 1.5 T MR-Linac. The initial fusion MR and verification MR scans were registered offline to assess prostate displacement between the two scans in the anterior-posterior (AP), left-right (LR) and superior-inferior (SI) directions. Random and systematic errors were calculated, and the PTV margins were determined using the Van Herk formula. The average time between MR scans was 22 min (range 9-54 min) compared to an average beam-on time of 6 min (range 2-11 min). Mean and standard deviation of translational displacement was 1.2 ± 0.9 mm in the AP, 0.6 ± 0.5 mm in the LR, and 1.1 ± 0.8 mm in the SI directions. The calculated PTV margin was 3.2 mm in AP, 1.7 mm in LR, and 3.2 mm in SI directions. There was an observed trend of increased prostate motion with increased treatment duration. MRgRT facilitates PTV margin reduction for PCa; however, our findings suggest that increased on-couch time may be associated with greater prostate motion. Future studies with larger patient cohorts and real-time motion monitoring are recommended to optimise margin strategies.