Room: AAPM ePoster Library
Purpose: Proton pencil beam scanning (PBS) systems modulate proton fluence, energy and spot position dynamically during treatment delivery. Current quality assurance focuses on 2-D cumulative dose distribution without specific attention to the delivery dynamics. In this study, through fast scintillation screen imaging, the spatiotemporal dose and dose rate dynamics were quantified. The key advance was optimizing the screen-camera spectrum for maximum signal from a high frame rate camera that allowed rapid readout. Sufficient signal to noise allowed the most challenging spot scanning sequences and depth dose profiles to be detected with high fidelity.
Methods: A fast intensified CMOS camera imaged (100 frames-per-second) a scintillation screen placed at the isocenter either flat on the couch to image PBS dynamics or standing in water to image single spot volumetric dose. PBS dynamics relation to beam parameters in the treatment plan was established (i.e. spot weight, energy, field size, spot-spacing, and minimum spot weight). The single spot was scanned inline through the screen fixed in water. The images were corrected for optical blurring and ionization quenching and stacked to produce volumetric dose.
Results: linearity in scintillation response with respect to varying dose rate, dose, energy and field size were within 4% with no specific correction applied. Large spatiotemporal variations in dose rate were observed, even for plans delivering similar dose distributions (4.89± 0.13 Gy, 1 standard deviation). The volumetric dose had a pass rate above 98% for 2%/2mm gamma analysis.
Conclusion: Spatiotemporal dose and dose rate dynamics were quantified through fast scintillation screen imaging. Dose rate temporal profiles, spatial maps, and cumulative dose-rate histograms provide useful metrics for the potential evaluation and optimization of dose rate in treatment plans. The slice acquisition and reconstruction method can be used for 3-D single spot characterization for all PBS systems and extended to image patient plans.
Funding Support, Disclosures, and Conflict of Interest: Authors acknowledge the Irradiation Shared Resource at the Norris Cotton Cancer Center with NCI Cancer Center Support Grant 5P30 CA023108-41 and the NIH Grant R01 EB023909. Brian Pogue, Rongxiao Zhang, and David Gladstone report financial interest in DoseOptics LLC, a company that manufactures Cherenkov cameras used to monitor radiation therapy.