Evaluation of a prototype array for daily quality assurance in spot scanning proton therapy

Background

Quality assurance (QA) on a daily basis is an essential task in radiotherapy. In pencil beam scanning proton therapy (PBS), there is a lack of available practical QA devices for routine daily QA in comparison to conventional radiotherapy.

Purpose

The aim was to characterize and evaluate a prototype for the task of daily QA routine for PBS with parameters recommended by the AAPM TG 224, that is, the dose output constancy, the spot position and the distal range verification. Furthermore, a time efficient calibration method for fast and reliable daily QA routine was established for the prototype.

Methods

First, a calibration routine was designed and evaluated, which characterizes the array response and allows for a conversion of the measured signal to clinically needed QA parameters. Finally, a time and resource efficient daily QA routine was developed and tested.

Results

The prototype array can distinguish spot position deviations with sub-millimeter accuracy, as well as changes in the spot size in terms of FWHM with a 2$\%$ sensitivity. The range and thus the energy can be evaluated at different depths also with sub-millimeter precision. After some training, the setup of the prototype device took roughly two minutes and the total beamtime was about one minute on cyclotron site and five minutes for synchrotrons.

Conclusions

A prototype for daily QA in spot scanning proton therapy was evaluated, which features a fast and easy setup and allows for measuring relevant beam parameters, typically within less than a minute of beam time. All QA parameters as recommended by the AAPM TG 224 report can be analyzed with sufficient accuracy.