MapPHAN for conformal arc SABR: an analysis of normal tissue dosimetry
Abstract
Stereotactic Ablative Body Radiotherapy (SABR), when delivered to the lung, provides several unique challenges from a quality assurance (QA) standpoint. To date, there has been little research performed on the efficacy of QA at detecting significant dosimetric errors in organs at risk (OARs). In addition, the feasibility of using the MapCHECK phantom housed in the MapPHAN accessory for SABR QA is not fully appreciated. The aim of this study is to investigate the sensitivity and specificity of MapCHECK in MapPHAN to introduced errors in conformal arc SABR plans, with a specific focus on OAR dosimetry. Multi-Leaf Collimator (MLC) class shift errors up to 2 mm, and isocentre shift errors up to 1 mm, were introduced to 26 simulated 6 MV X-ray conformal arc lung SABR plans on Raystation. 10 of these plans had errors introduced, and were delivered on a Varian Clinac iX linear accelerator to the MapCHECK phantom housed in MapPHAN. In order to commission the phantom for this project, the inherent angular dependency of the MapCHECK diodes had to be corrected to within ± 2%. Gamma analysis was used to compare the measurements on MapCHECK to the simulated Raystation plans, using 1%/1 mm, 2%/1 mm, 2%/2 mm, 3%/2 mm, 3%/3 mm and 5%/1 mm gamma criteria. Based on these results, Receiver Operating Characteristic (ROC) curves were generated to determine the sensitivity and specificity of MapCHECK in MapPHAN to the introduced errors. These ROC curves were then used to determine an optimal set…
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