engleski

EVALUATION OF PATIENT SPECIFIC IMRT QA GAMMA PASSING RATES TO MLC PENUMBRA MODELLING ERRORS

Intensity modulated radiation therapy (IMRT) is nowadays a treatment of choice for multiple cancer sites, especially for head and neck due to high doses and many near organs at risk (OARs). In these cases, each treatment field can be highly complex and that justifies the need for patient specific quality assurance (QA) to verify treatment planning system (TPS) calculation and to exclude other planning errors. Since IMRT fields consist of many segments, accurate penumbra modelling for small fields is crucial. The aim of this study was to evaluate the sensitivity of single field gamma analysis in detecting realistic penumbra modelling errors. Considering that beam modelling is based on dose profile measurements, errors can be made by using inappropriate detectors for measuring small fields. There have been several studies questioning predictive power of gamma analysis [1-3]. In this study we will show that common employed IMRT QA “3%/3mm” (3% dose difference, 3mm distance to agreement (DTA)) gamma passing rates are not capable of detecting beam models with erroneous penumbra. Majority of institutions perform their gamma analyses with 3% dose difference and 3 mm DTA criteria, with IMRT plan acceptance criteria requiring 90% - 95% of points passing the gamma analysis. Our study shows that by using these criteria, it is impossible to distinguish between the two beam models, i.e. wrongly modelled beam could easily slip unnoticed. Our results showed that only 4 out of 126 beams calculated with erroneous penumbra beam model failed to meet above mentioned criteria. Therefore, when performing per beam planar patient specific QA, it is recommended also to check 2%/2mm gamma passing rates to exclude serious errors which could easily go unnoticed when performing „standard“ “3%/3mm” gamma analysis. This is crucial when performing IMRT commissioning where more strict criteria should be used including measurements with multiple dosimetry systems to catch subtle errors in beam modelling.

IMRT ; Gamma passing rate ; MLC ; Quality assurance

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