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A Rapid 1D in Vivo Dose Measurement Technique for VMAT Treatments Using Single Integrated Portal Images

James Roberts

J Roberts1*, W Ansbacher2, (1) University of Victoria, Victoria, BC, (2) BC Cancer Agency - Vancouver Island Centre, Victoria, BC

MO-B-Salon EF-1 Monday 10:00:00 AM - 12:00:00 PM Room: Salon EF

Purpose: To develop a rapid, accurate in vivo dose measurement technique for use in volumetric-modulated arc therapy (VMAT) treatments using single arc-integrated electronic portal images.

Methods: The dose measured along axis of gantry rotation can be directly related to the signal along the y-axis of EPIs in integrated mode. A simple back-projection algorithm was developed using a composite transmission factor averaged over the gantry angles subtended by the treatment arc. Corrections were applied in order to account for beam hardening, scattered radiation within the EPID, phantom-to-EPID scatter, and phantom scatter. Four clinical prostate VMAT treatment plans were exported for delivery to an anthropomorphic phantom and integrated EPIs were recorded.

Results: Portal image reconstructed doses at isocenter agreed with treatment planning system calculations (mean difference of 4.4% and standard deviation 2.7%). 1D dose profiles along the axis of gantry rotation showed good agreement with planned doses (percentage of gamma-index values less than unity of 88.3% and standard deviation 9.5%). Upon the introduction of an empirical global small-field correction factor, the agreement improved greatly (percentage of gamma-index values less than unity of 94.4% and standard deviation 5.2%) Verification time per plan is approximately one minute for the first fraction and approximately five seconds for each fraction thereafter.

Conclusion: Single integrated EPI in vivo dosimetry has been successfully extended to VMAT treatments, albeit with the absence of true dosimetric information in the cross-plane direction. We conclude that 1D dosimetric verification using single EPIs can supplement, but not replace, pre-treatment patient specific dosimetric QA.

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