EP-1425 PRE-TRIAL PLANNING QA FOR THE IDEAL-CRT TRIAL: DOSE ESCALATION IN NSCLC

Abstract

using the linac control system (LCS) software 'Elekta RTD 7'. Later the LCS was replaced by Integrity 1.1 which allows Elekta users to introduce continuous variable dose rate (CVDR) on their linac instead of dose rate variation in discrete steps. Starting from the settings used in the clinical plan, different plans were generated by changing TPS and LCS VMAT-parameters. All plans were delivered to investigate their influence on linac behaviour during delivery. Parameters varied in the TPS were 1) minimum and maximum allowed dose rate, 2) constant or continuous variable gantry speed and 3) maximum gantry speed change per segment. Parameters varied in the LCS were 1) discrete dose rate variation or CVDR, 2) maximum allowed gantry speed and 3) gantry inertia compensation distance (ICD). All plans were generated using the same dose and dose-volume objectives. Differences in target coverage and organ at risk sparing were negligible, and the resulting dose distributions met in all instances the planning criteria. Plan efficiency was analyzed by recording the delivery time. Linac behaviour was characterized by comparing the actual gantry angle with the requested gantry angle. Results: Introduction of CVDR improved delivery efficiency and linac behaviour most. When using maximum allowed gantry speed, occasionally the delivery was still inhibited by the LCS. Reducing both the maximum allowed gantry speed from the initial 6°/s to 5°/s and changing to CVDR resulted in delivery without interruptions keeping the delivery time the same. Changes made to the other variables did not result in faster delivery or behaviour changes of the gantry rotation. The only exception was increasing the ICD value which resulted in a smoother gantry rotation motion but also in a prolonged delivery time. Conclusions: Introduction of CVDR minimized most of the irregular gantry rotation resulting in faster plan delivery. Depending on the gantry acceleration or deceleration requested by the LCS inhibits may still occur, although less frequent compared to delivery using discrete dose rates. For inhibit-free plan delivery a reduction of the maximum allowed gantry speed is also required. Clinics which do not have the CVDR option available may already improve linac behaviour by reducing the maximum allowed gantry speed or increase the ICD distance. Both options will result in slightly prolonged delivery times. Although much improvement has been made using CVDR and gantry speed limitations, more data is needed to clarify the influence of LCS parameters. This knowledge will ease and speed up the introduction of smooth and inhibit-free VMAT delivery for other disease sites.