Room: AAPM ePoster Library
Purpose: The purpose of this study is to develop a simultaneous optimization method for energy-layer and spot-intensity considering the minimum monitor unit (MU) constraint for single field optimization (SFO) to reduce delivery time.
Methods: In the spot-scanning technique, the time it takes to change from one energy to another is a major factor in determining delivery time since scanning proton beams with the same energy can be fast. The number of energy layers at which optimized spots are sparsely arranged increases at low energy when creating SFO plans considering the minimum MU constraint, so to eliminate these layers and reduce delivery time, we developed an optimization method consisting of two steps. In step one, spot-intensity is optimized using the objective function consisting of a general dose constraint term under the minimum MU constraint, and spots less than the minimum are removed during optimization. In step two, the energy reduction term, which is defined based on the spot reduction ratio before and after the previous optimization step, is added to the objective function, and spot-intensity is optimized by using both terms under the minimum MU constraint. We applied our method to six datasets (two cases each of prostate, lung, and head and neck (H&N)) at Nagoya Proton Therapy Center and evaluated the number of energies and the delivery time when irradiating the optimized spot to solid phantom.
Results: The number of energies was reduced by 5.9–11.1% for prostate case, 18.2–22.9% for lung case, and 15.0–27.5% for H&N case. Moreover, the deterioration of the plan quality due to the reduced numbers of energies was negligibly small. The maximum reduction in delivery time was 27% in the H&N case.
Conclusion: Delivery time can be reduced without deteriorating the plan quality by using the developed method.
Funding Support, Disclosures, and Conflict of Interest: Shusuke Hirayama, Shinichiro Fujitaka, Toru Umekawa, Rintaro Fujimoto, Atsuhiro Ushikubo, and Yoshihiko Nagamine are employees of Hitachi Ltd., Tokyo, Japan