ESTRO 2024 - Abstract Book
S4216
Physics - Intra-fraction motion management and real-time adaptive radiotherapy
ESTRO 2024
1028
Digital Poster
Is volumetric repainting needed to minimize interplay induced dose effects in lung proton therapy?
Zubin Master, Wee Loon Ng, Hong Qi Tan, Andrew Wibawa, Calvin Wei Yang Koh, Kah Seng Lew, James Cheow Lei Lee, Sung Yong Park
National Cancer Centre Singapore, Division of Radiation Oncology, Singapore, Singapore
Purpose/Objective:
To evaluate dose perturbations due to interplay during the delivery of pencil beam proton therapy for lung tumors and to determine if additional volumetric repainting is required to minimize this dose perturbation.
Material/Methods:
Ten patients with lung tumors, previously treated with photon radiotherapy, were selected for this retrospective study. All cases had 4D CTs with tumor motion ranging from 0.6-1.6cm in the cranio-caudal direction and tumor volumes ranging from 11.6 - 450.7cc. Seven of the 10 patients had target volumes that were near the diaphragm. Proton therapy plans were generated for each case, using 3 beam angles and 4D robust optimization, which is robust optimization with the standard uncertainty parameters of CT calibration uncertainty (3.5%) and isocenter shifts (0.5cm), as well using the CT data of multiple breathing phases as additional scenarios to account for in the optimization. All cases were planned with a fractionation of 66Gy (BED) in 33 fractions and the plans were normalized so that 98% of the CTV was covered by 66Gy. For plan evaluation, in addition to the standard robust evaluation, the dose perturbation due to interplay between the proton beam delivery and the breathing motion was evaluated using an approximation of the proton machine’s beam delivery time structure and the patient’s breathing cycle. The parameters included in this interplay model were the proton beam dose rate, X and Y scanning magnet speeds, energy switching time, breathing cycle length and the CT data of all the breathing phases (see figure 1). The acceptance criteria used to evaluate the dose perturbation due to interplay was D95% > 95% isodose, D98% > 93%, D99% > 90% and D1% < 110% isodose. The OAR doses were also evaluated to ensure that no critical OAR dose parameters were beyond 5% of the planned dose. If a plan failed the interplay evaluation criteria, the plan was re-optimized with an additional volumetric repainting (every beam was delivered twice with half the dose) and the interplay was re-evaluated. The amount of volumetric repainting would be increased till the dose perturbation due to interplay was under the above-mentioned criterion.
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