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Impact of Isocenter Configuration and Flattening Filter Free Delivery on VMAT Planning for Synchronous Bilateral
Jiyong Zhang1, Danli Hong2, Yong Gan1
1Department of Radiation Oncology, Cancer Hospital of Shantou University Medical College, Shantou, Guangdong, 515000, People's Republic of China.
Purpose:
To evaluate the relative impact of isocenter configuration and flattening filter free (FFF) beam delivery on dosimetric performance, radiobiological outcomes, and treatment efficiency in volumetric modulated arc therapy (VMAT) for synchronous bilateral breast cancer (SBBC).
Methods:
Twenty-two treatment plans were generated for patients with SBBC using four VMAT configurations, including single-isocenter and dual-isocenter strategies with either flattened (FF) or FFF beams. Target coverage and plan quality were assessed using standard dose volume parameters. Doses to organs at risk (OARs), including the whole lung, heart, left anterior descending artery (LAD), aorta, liver, spinal cord, and skin, were evaluated. Treatment times and total monitor units (MUs) were recorded. Radiobiological outcomes were estimated using tumor control probability (TCP) and normal tissue complication probability (NTCP) models. Statistical comparisons were performed between planning strategies and beam delivery modes.
Results:
Dual-isocenter VMAT consistently achieved superior target coverage, dose homogeneity, and sparing of thoracic OARs compared with single-isocenter VMAT. In particular, dual-isocenter planning reduced the mean heart dose by approximately 35% (11.83 ± 1.41 Gy vs 7.69 ± 1.11 Gy) and decreased heart V10Gy by more than 50% (41.19 ± 8.14% vs 20.45 ± 6.27%) compared with conventional single-isocenter VMAT. These improvements translated into more favorable TCP and NTCP estimates, indicating that plan quality was primarily influenced by isocenter configuration. Within the same isocenter strategy, FFF beam delivery produced comparable target coverage, OAR doses, and radiobiological outcomes relative to FF beams. Although FFF plans required higher MUs, they did not significantly reduce treatment times despite the availability of higher dose rates.
Conclusion:
Isocenter configuration had a greater impact on dosimetric and radiobiological outcomes than beam flattening mode. Dual-isocenter VMAT improved target coverage and OAR sparing, while FFF delivery offered limited benefit. Single-isocenter techniques may improve efficiency and simplicity, highlighting the tradeoff between plan quality and treatment efficiency.
