2470 - Investigating the Portability of an Universal PBS Beam Model in a Compact Proton Therapy Delivery System

X. Zhao¹, Y. Hao², J. Liu¹, S. M. Perkins², and T. Zhao²; ¹Washington University School of Medicine in St. Louis, St. Louis, MO, ²Washington University in St. Louis, St. Louis, MO

Purpose/Objective(s): Linear accelerator based machine matching is very common now a days to facilitate patient treatment. However, Due to complexity of particle therapy system, it is rare to match two independent cyclotrons dosimetrically to allow patient transfer between systems. In our routine clinical practice, weve observed only minor differences between the two proton therapy S250i cyclotrons and their associated beam lines, despite each single-room proton unit utilizing a unique beam model tailored to its distinctive beam characteristics. The purpose of this study is to investigate the feasibility of using an universal beam model for two proton machines. Materials/

Methods: A new universal pencil beam scanning (PBS) beam model was developed by evenly tuning the beam models of two different proton machines, averaging the visual source-axis distance, spot profiles and absolute dosimetry, i.e. Subsequently, a portability test was conducted by applying this universal PBS beam model to generate treatment plans for both proton machines. Patient-specific quality assurance (PSQA) plans of six different disease sites (Brain, Esophagus, Head and Neck, Abdomen, Chest wall and Lung) were generated by using this new PBS beam model. These plans were then delivered on both proton machines to access the clinical impact of beam model discrepancies.
Results: All the tests were conducted in normal clinical workflow through our clinical verify and record system. The universal beam model didn’t result in any noticeable deviation between the calculated and actual delivered doses in the PSQA. All reached a level more than 90% gamma passing rate with a 3mm/3% criterion, at both surface and fall-off depth. On the other hand, the absolute dose PSQA using ionization chambers were performed as well, and all passed within 3% difference tolerance.
Conclusion: A universal PBS beam model demonstrates feasibility both dosimetrically and clinically for multiple single-room proton therapy S250i units, with closely matched cyclotron and beam line components. Abstract 2470 – Table 1: QA results comparison between prior delivered clinical treatment and portability test of two proton machines

Treatment Site	Delivered Clinical Plan QA Results		Portability Test on Machine #1		Portability Test on Machine #2
	MatriXX QA (3mm/3%)	IC QA (< 3%)	MatriXX QA (3mm/3%)	IC QA (< 3%)	MatriXX QA (3mm/3%)	IC QA (< 3%)
Brain	92.50%	1.05%	91.40%	-2.92%	93.00%	-1.13%
Esophagus	97.50%	2.42%	91.50%	-1.99%	92.50%	-2.75%
Head & Neck	95.50%	1.41%	95.10%	1.23%	94.40%	0.52%
Abdomen	99.50%	-1.85%	93.50%	-1.74%	95.80%	-2.39%
Chest wall	96.30%	2.73%	98.00%	-2.21%	97.50%	-2.05%
Lung	91.00%	-1.85%	98.30%	-2.64%	94.50%	-2.30%