Clinically evaluating directional dependence of 2D seven29 ion-chamber array with different IMRT plans
Purpose: This study aims to clinically evaluate the directional dependence of a 2D seven29 ion-chamber array with different intensity-modulated radiotherapy (IMRT) plans.
Methods: Twenty-five patients who had already been treated with IMRT plans were selected for the study. Verification plans were created in an Eclipse treatment planning system (TPS) for each treatment plan. The verification plans were executed twice for each patient. The first IMRT plan used a true gantry angle (plan-related approach), and the second plan used a 0° gantry angle (field-related approach). Measurements were performed using a Varian Clinac 2100 iX linear accelerator. The fluence was measured for all the delivered plans and analyzed using Verisoft software. A comparison of the fluence was performed between IMRT with a static gantry (0° gantry angle) and real gantry angles.
Results: The results indicate that the Gamma average was 98.8 % for IMRT with a 0° gantry angle and 96.616% for IMRT with a true gantry angle. Average percent difference of normalized doses for IMRT delivered with zero degree gantry angle and IMRT with actual gantry angles is 0.15 and 0.88 respectively.
Conclusion: The ion chamber of the 2D array used in IMRT verification has angular dependence, reducing the verification accuracy when the 2D array is used for measuring the actual beams of the treatment plan.
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