348 - Scintillation Imaging Dosimetry for High-Temporal Resolution Error Detection During UHDR Proton Beam Delivery

M. Clark¹, J. Harms², R. Vasyltsiv¹, D. J. Gladstone³, J. Kraus⁴, R. Zhang¹, and P. Bruza¹; ¹Thayer School of Engineering, Dartmouth College, Hanover, NH, ²Department of Radiation Oncology, University of Alabama at Birmingham, Birmingham, AL, ³Department of Medicine, Geisel School of Medicine, Dartmouth College, Hanover, NH, ⁴Miami Cancer Institute, Miami, FL

Purpose/Objective(s): In-vivo beam monitoring will be necessary for safe clinical translation of ultra-high dose rate (UHDR) proton therapy. Although there is no perfect detector for this extreme radiation environment, scintillation imaging dosimetry (SID) has shown promise for accurate dose and dose rate measurements at UHDR timescales. In this study we characterize the noise resolution of an SID system during delivery of modulated UHDR PBS proton, showcasing its utility as an online error-prevention tool. Materials/

Methods: Delivery of complex, 250 MeV UHDR PBS treatment plans with PBS dose rates above 65Gy/s at treatment isocenter were designed with an in-house planning script. A set of pre-defined delivery errors were added to both simple, square-field plans, and complex plans designed for SBRT in the lungs. Spot intensity errors from 1-5% per spot and spot displacements from 1-3 mm were randomly inserted into plans. Surface dose images were collected at 4.5 kHz/0.23 mm resolution during delivery of the planned beams via an intensified CMOS camera facing a block of solid with a top scintillation layer.
Results: The average spot-to-spot intensity variations for uniform fields was 2.5+/-0.4% and spot positioning accuracy was 0.49±0.15mm. In uniform fields with planned delivery errors, the SID was able to detect spot MU intensity changes of 5%, measuring 2% differences in dose, which agreed with the planned dose distribution, and spot position changes as low as 1 mm were seen on imaging. For the modulated fields, spot intensity errors as low as 2% were detected, and planned spot position errors of 0.5mm were detected. Of note, imaged dose and dose rate maps showed strong agreement between simulation and imaging, with average gamma passings rate of 98% and 99%, respectively, at 3%/2 mm.
Conclusion: In this study, an SID was shown to be able to detect single-spot intensity errors as low as 5%, offering higher sensitivity than gamma analysis. Additionally, the proposed technique allows for monitoring dose-time profiles, potentially alerting the user if an in-delivery error has occurred in real time.

Abstract 348 – Table 1

Plan	Variable/Degree	Planned Error	Meas. Dose (Gy)		Meas. Dose Rate (Gy/s)
			No Error	With Error	No Error	With Error
Baseline	MU Intensity	1%	10.1±0.6	10.1±0.6	131.6±16.5	142.7±59.9
		3%	10.8±0.4	10.9±0.3	63.9±6.6	65.7±16.0
		5%	9.1±0.9	10.1±0.5	185.8±8.9	193.8±14.4
	Position	1 mm	10.4 ± 0.4	11.7 ± 0.4
		3 mm	10.6±0.4	12.0±0.5
Lung Field	Intensity	2%	3.7±0.5	4.3±0.5
	Position	0.5mm	6.0±0.2	5.5±0.5