Investigating beam matching for multi-room pencil beam scanning proton therapy

The purpose of this study was to investigate the proton beam matching for a multi-room ProteusPLUS pencil beam scanning (PBS) proton therapy system and quantify the agreement among three beam-matched treatment rooms (GTR1, GTR2, and GTR3). In-air spot size measurements were acquired using a 2D scintillation detector at various gantry angles. Range and absolute dose measurements were performed in water at gantry angle 0 degrees. Patient-specific quality assurance (QA) plans of four different disease sites (brain, mediastinum, sacrum, and prostate) and machine QA fields with uniform dose were delivered for various beam conditions. The results from GTR1 were considered as reference values. The average difference in spot sizes between GTR2 and GTR1 was - 0.3% +/- 2.2% (range, - 5.9 to 5.8%). For GTR3 vs. GTR1, the average difference in spot sizes was 0.6% +/- 1.7% (range, - 4.8 to 4.6%). The spot symmetry was found to be <= 4.4%. For proton range, the difference among three rooms was within +/- 0.5 mm. On average, the difference in absolute dose was - 0.1 +/- 0.7% (range, - 1.3 to 2.1%) for GTR2 vs. GTR1 and 0.7 +/- 0.6% (range, - 0.1 to 2.1%) for GTR3 vs. GTR1. The average gamma passing rate of patient-specific QA measurements (n = 29) was >= 98.6%. The average gamma passing rate of machine QA fields was 99.9%. In conclusion, proton beam matching was quantified for three beam-matched rooms of an IBA ProteusPLUS system with a PBS dedicated nozzle. It is feasible to match the spot size and absolute dose within +/- 5% and +/- 2%, respectively. Proton range can be matched within +/- 0.5 mm.