Dosimetric Evaluation and Patient-Specific Quality Assurance of Multi- Isocenter VMAT Plans for Pelvic Target Volumes on Halcyon 4.0

Abstract Background and purpose Large pelvic targets may exceed the available field dimensions of ring-gantry linear accelerators, requiring multi-isocenter volumetric modulated arc therapy (VMAT). This study compared plan quality and measurement-based patient-specific quality assurance (PSQA) among different isocenter configurations for pelvic and extended pelvic-femoral targets treated on Halcyon 4.0. Materials and methods Twenty patients were retrospectively analyzed. For PTV-Pelvis, single-isocenter (1 ISO) and two-lateral-isocenter (2Lat ISO) plans were compared, whereas PTV-Extended was evaluated using 2Lat ISO and two-longitudinal-isocenter (2Long ISO) plans. All plans prescribed 30 Gy in 10 fractions and were calculated in Eclipse using Acuros XB. Plan quality was assessed using dosimetric indices and monitor units. PSQA was performed with ArcCHECK using 3%/3 mm global gamma analysis with a 10% dose threshold. Results For PTV-Pelvis, 2Lat ISO improved HI (0.318 vs. 0.348), COVI (0.982 vs. 0.975), and Dmax (109.94% vs. 111.53%) compared with 1 ISO (adjusted p < 0.05). For PTV-Extended, 2Lat ISO improved HI (0.321 vs. 0.375), COVI (0.971 vs. 0.935), D95% (96.43% vs. 94.82%), D98% (95.09% vs. 93.44%), and EI (4.18% vs. 11.28%) compared with 2Long ISO, with all differences remaining significant after correction. Gamma passing rates were 98.65–99.16%. Conclusion Lateral multi-isocenter VMAT improved target coverage and dose homogeneity, particularly for extended pelvic-femoral targets, while maintaining high PSQA passing rates.