2469 - Gating QA Measurement to Investigate the Impact of Volumetric Repainting on a Compact Proton Therapy Delivery System

X. Zhao¹, X. Chen², W. Smith¹, D. Yang³, A. Darafsheh⁴, T. Zhao⁴, T. Zhang⁴, and Y. Hao⁵; ¹Washington University School of Medicine in St. Louis, St. Louis, MO, ²Washington University in St Louis, Department of Biomedical Engineering, St Louis, MO, ³Duke University, Durham, NC, ⁴Washington University School of Medicine in St. Louis, Department of Radiation Oncology, St. Louis, MO, ⁵Washington University in St. Louis, St. Louis, MO

Purpose/Objective(s): Most of publishes talking about the benefits of volumetric repainting plan were focused on the proton system with energy selection. However, for this compact proton therapy delivery system, S250i proton unit, its energy layer selection is achieved by using the range shifter, which will increase the spot size to centimeter level at low energy. This work investigated the effect of volumetric repainting plan at different scenarios for this proton system. Materials/

Methods: Repainting allows the proton beam with different energy layers to be delivered more than once to achieve statistical averaging of motion effects. In this work, a respiratory motion platform was used to mimic the breathing motion of target by controlling the MatriXX ionization chamber detector arrays under different breathing pattern (regular, irregular, fast, slow etc). For every breathing pattern, we scanned the 4D CT and plan separately, and both exhale and inhale gating treatment delivery was performed in this work. Gamma analysis with 3% and 3 mm criteria was used to investigate the impact of repainting plan under different breathing pattern.
Results: 8 different scenarios with different breathing pattern and repainting number were investigated in this work. All of the measured dose distributions, either surface and fall off depth, have gamma passing rate >90% (at 3mm/3% criterion) within 3 mm lateral shift. For the 2D dose measurements at fall off depth, the repainting plans didn’t show significant improvement on the 2D dose distribution according to the MatriXX QA gamma analysis results. However, for the surface measurements, repainting plans have better gamma analysis results.
Conclusion: As a compact proton therapy delivery system, S250i proton unit uses range shifter to switch the energy layers of proton beam, which made the sigma/size of low energy layers’ spot to increase. Due to the utilization of range shifter on this proton machine, the low energy layer’s spot size is in centimeter level. Therefore, for the fall off depth MatriXX QA measurement, the large spot size is more forgivable for motion interplay effect during gated delivery. Abstract 2433 – Table 1: Gamma analysis results of 8 different scenarios with different breathing pattern and repainting number

Scenarios	Number of Painting	Breathing Motion Pattern	Surface MatriXX QA (3%/3mm)	Fall-off Depth MatriXX QA (3%/3mm)
#1	1	No Breathing Motion	97.20%	96.00%
#2	3	Sine wave	98.10%	93.60%
#3	5	Sine wave	97.80%	99.40%
#4	7	Sine wave	98.60%	95.90%
#5	3	Fast	97.80%	98.00%
#6	3	Slow	99.50%	96.50%
#7	3	Irregular	100.00%	98.80%
#8	15	Sine wave	NA	98.80%