Abstract
PurposeLinac parameters potentially influencing the delivery quality of IMRT and VMAT plans are investigated with respect to threshold ranges, consequently to be considered in a linac based quality assurance procedure. Three commercially available 2D arrays are used to further investigate the influence of the measurement device.MethodsUsing three commercially available 2D arrays (Mx: MatriXXevolution, Oc: Octavius1500, Mc: MapCHECK2), simple static measurements, measurements for MLC characterization and dynamic interplay of gantry movement, MLC movement and variable dose rate were performed. The results were evaluated with respect to each single array as well as among each other.ResultsSimple static measurements showed different array responses to dose, dose rate and profile homogeneity and revealed instabilities in dose delivery and profile shape during linac ramp up. Using the sweeping gap test, all arrays were able to detect small leaf misalignments down to ±0.1 mm, but this test also demonstrated up to 15% dose deviation due to profile instabilities and fast accelerating leaves during linac ramp up. Tests including gantry rotation showed different stability of gantry mounts for each array. Including gantry movement and dose rate variability, differences compared to static delivery were smaller compared to dose differences when simultaneously controling interplay of gantry movement, leaf movement and dose rate variability.ConclusionLinac based QA is feasible with the tested commercially available 2D arrays. Limitations of each array and the linac ramp up characteristics should be carefully considered during individual plan generation and regularly checked in linac QA. Especially the dose and dose profile during linac ramp up should be checked regularly, as well as MLC positioning accuracy using a sweeping gap test. Additionally, dynamic interplay tests including various gantry rotation speeds and angles, various leaf speeds and various dose rates should be included.
Highlights
Implementing modulated radiation therapy techniques like step-andshoot intensity modulated radiation therapy, dynamic sliding window IMRT, or volumetric modulated arc therapy (VMAT) into clinical routine requires generating a correct beam model of the linac in the treatment planning system (TPS) used
With regard to delivered dose rate levels in modulated techniques, this might contribute to less influence of the dose rate linearity of the arrays to the delivery quality of IMRT and VMAT plans
The dose delivered by varying gantry speed, dose rate, and leaf speed was compared to the dose delivered by the same sweeping gap, but without gantry movement at gantry angle 0°
Summary
Implementing modulated radiation therapy techniques like step-andshoot intensity modulated radiation therapy (sIMRT), dynamic sliding window IMRT (dMLC), or volumetric modulated arc therapy (VMAT) into clinical routine requires generating a correct beam model of the linac in the treatment planning system (TPS) used. For beam modeling careful measurements of quadratic and rectangular open fields with a single detector in a large water tank and some simple tests for MLC parameter definition are required. For these simple tests, point detector arrays or electronic portal imaging devices (EPIDs) are frequently used. After establishing the beam model, the regular plan-individual and linac QA measurements are conducted with these devices
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