Room: ePoster Forums
Purpose: The quality of spot scanning proton therapy treatments relies on the accuracy of proton beam position and the stability of proton beam size. It is common to measure a small sample of spot positions and beam energies during periodic quality assurance, but this practice can miss important deviations in beam performance. We present an efficient method for measuring spot position and size accuracy for all beam energies and across the entire treatment field.
Methods: A commercial planar scintillator detector (Logos XRV-4000) was modified in house to improve its positional measurement accuracy. The detector was aligned at isocenter, and proton spots were delivered in a grid with 5 cm inter-spot spacing spanning the 30x40 cm2 maximum field size for each of 97 clinical proton energies, for a total of 6,111 spot positions per measurement. Each measured proton spot was fit with a two-dimensional Gaussian function to determine its position and size. The spot position deviations were evaluated, and the measured spot sizes were compared with the Monte Carlo data used to commission the treatment planning system. Measurements were performed monthly on four matched gantries for 18 months.
Results: Spot position and size measurements required 15 minutes of setup time and 5 minutes of beam delivery time per gantry. The spot position measurement accuracy was within 0.49mm. The mean spot position difference from planned values was 0.58mm (Ïƒ=0.31mm), and the spot position reproducibility was 0.24 mm (Ïƒ=0.17mm). The mean difference between the measured and calculated spot sizes was 0.05mm (Ïƒ=0.04mm).
Conclusion: A planar scintillator detector can measure proton spot position accuracy at isocenter across the entire field size for all clinical proton energies in a reasonable time period. This provides an independent test of beam delivery positional accuracy and enables evaluation of the stability of spot position and size over time.
Not Applicable / None Entered.