B. Y. Yi, M. J. MacFalane, W. Yao, and M. Zakhary; Department of Radiation Oncology, University of Maryland School of Medicine, Baltimore, MD
Purpose/Objective(s): Spot size (SS) consistency in the proton beam treatment using pencil beam scanning (PBS) is important to deliver the same treatment quality as the plan. AAPM TG-224 recommend the tolerance to be within 10% and also a few studies report the delivered treatments start to deviate from planning if SS changes >10%. SS is often measured in-air or at the surface since it is not practically possible to measure at the treatment depths. No reports have been suggested what is the tolerance of the surface SS (SSS) variation. This report seeks the clinical background of the SSS tolerance from a series of plan comparisons and phantom simulations. Materials/
Methods: Eight cases of 4 anatomic sites (2 brain, 2 Lungs, 2 Abdomen and 2 Pelvis) who have been treated under PBS proton beams are chosen. Five plans were generated for each plan, which have different SSs (±5, ±10, ±15 %) but the same plan parameters. A 2-cm range shifter (RS) with different skin-to-RS distances was used to simulate SS variations. A set of phantom studies were performed to determine the relation between the variation of SST and that of SSS. Results: Due to the geometrical limitations, SS variations were able to be simulated up to ±15%. Clinical Target Volume (CTV) coverage degraded when SS increases while high dose increases when SS decrease. V95% degraded up to 1.5% when SS variation is <+10% for all tested cases and it starts to be >1.5% when SS variation =+10% for some of the cases. When SS decreases, D1 increase up to 1.5% when SS decreases < 10% for all tested cases and it starts to be >1.5% when SS decreases = 10% for some of the cases. No significant changes were observed in the doses of the organs at risk when SS variation is <10%. SST also changes linearly to SSS variation; the ration of the SST variation in % to the SSS variation in % is 1.003. The coefficient of determination of the line demonstrating the %variation of SSS and that of SST for all of the energies shows >0.95. This suggests that the same amount of % deviation of SSS from the baseline will be transferred to SST. Conclusion: The series of planning simulation when SS changes shows that the variation of SS should be maintained <10% at the treatment depth to achieve the intended dose distributions. Phantom simulation suggests that the variation of SSS should be maintained <10% to achieve <10% at treatment depths, which lines with AAPM TG-224 recommendation. This study provides the tolerance of the SSS variation by analyzing the clinical needs and the phantom plans. References 1. Arjomandy B, Taylor P, Ainsley C, Safai S, Sahoo N, Pankuch M, Farr JB, Yong Park S, Klein E, Flanz J, Yorke ED, Followill D, Kase Y. AAPM task group 224: Comprehensive proton therapy machine quality assurance. Med Phys. 2019 Aug;46(8):e678-e705. 2. Kraan AC, Depauw N, Clasie B, Madden T, Kooy HM. Impact of spot size variations on dose in scanned proton beam therapy. Phys Med. 2019 Jan;57:58-64
