Purpose: SRS and SBRT are beginning to be used in proton therapy. Currently, there are no criteria for patient specific quality assurance for proton SBRT. This presents certain challenges due to the small size of the fields and high dose fall off. Film provides high spatial resolution and measures absolute dose but can be cumbersome to use. In this study, we look at clinically available detectors and their ability to address issues.
Methods: Two patients were selected: one with a spinal lesion and one with a brain lesion. The evaluation PTVs were both under 60 cc. Each field was measured with a MatriXX PT (32x32 array, 7.6mm spacing). Additionally, the MatriXX was shifted by half the detector distance in the x and y directions. These two measurements were combined with Matlab code and interpolated to create a measurement with higher spatial resolution. A high resolution relative dose measurement was taken with a scintillation detector (Lynx, IBA). The dose was scaled to the maximum dose of the plane. For volume averaging effects, the planned dose was calculated at a 0.5mm resolution. The dose to each detector was calculated with this fine resolution plane and compared to the MatriXX measurements.
Results: The three measurement methods showed similar passing rates across all criteria. The MatriXX alone had a 2%/2mm passing rate of 99.74%±0.62% across all fields, the MatriXX with interpolation had a passing rate of 98.16%±0.62%, and the Lynx a passing rate of 97.8%±1.34%. The volume averaging showed an average of -40.22±34.02 mGy across all fields. This corresponds to a 1.67%±1.25% dose difference.
Conclusion: The MatriXX with interpolation will provide a satisfactory method for measuring small fields with more heterogeneities. This has the advantage over the Lynx by measuring absolute dose. Volume averaging also provides a promising solution to account for heterogeneities.