Evaluation of SRS MapCHECK™ for SABR patient-specific QA
PO-1399
Abstract
Evaluation of SRS MapCHECK™ for SABR patient-specific QA
Authors: Miao|, Yun(1)[yun.miao@nhs.net];Kidane|, Ghirmay(1)*;Ifthaker|, Ahmed(1);Crees|, Liz(1);Almond|, Ewan(1);
(1)Queen's Hospital - Barking Havering and Redbridge University Hospitals NHS Trust, Radiotherapy Physics, Romford, United Kingdom;
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Purpose or Objective
Patient-specific quality assurance (PSQA) for stereotactic ablative radiotherapy (SABR) is currently performed using an ion-chamber and GAFchromic™ films. The development and analysis of films causes delay between QA delivery and the availability of results. Recently, the SRS MapCHECK™ detector (Sun Nuclear Corporation, Melbourne, FL, USA) was reviewed for the pre-treatment verification of SABR treatments. The detector is a diode array, offering high spatial resolution and results processing in real time. This work verifies the SRS MapCHECK detector for SABR PSQA to replace film measurement.
Material and Methods
12 SABR treatment plans on a range of sites were retrospectively analysed in this study. The plans were produced on the Eclipse™ (v15.6) treatment planning system (Varian Medical Systems, Palo Alto), using the Anisotropic Analytical Algorithm (AAA). The plans were delivered using a Varian Edge® radiosurgery system, and dose was measured by EBT3 GAFchromic film and SRS MapCHECK for each plan. Gamma analysis was used to compare the delivered dose distribution to the planning system’s calculation, and the two detectors’ results compared. Local tolerance for MapCHECK was determined based on gamma analysis results. Bland Altman (B-A) statistical analysis was used to analyse interchangeability between the two detector systems, with film as the current standard for SABR PSQA. A SABR plan was scaled with a 2.1% drop in plan dose, for sensitivity testing of local tolerances.
Results
The B-A plots (figures 1, 2) show good agreement between the two detector systems, where all results lie within the 95% confidence level.
Figure 1. B-A plot for 2%/2mm gamma criteria
Figure 2. B-A plot for 3%/2mm gamma criteria
The mean SRS MapCHECK gamma pass rate is 99.6% and 99.9% on 2%/2mm and 3%/2mm respectively, with all 12 plans scoring above 99.0% and 99.7%, on the two gamma criteria. Local tolerance for MapCHECK was set at 98% on 2%/2mm, based on two standard deviations away from the mean gamma pass rate. For 11 out of 12 patients, MapCHECK results were higher than the film results, which is evident in the negative bias of the B-A plots in figures 1 and 2 (-0.94 for 2%/2mm and -0.69 for 3%/2mm). Only one outlier sits outside of two standard deviations of the bias, on 3%/2mm.
Sensitivity testing using the erroneous plan produced results of 96.0 % and 92.7%, on 2%/2mm, for SRS MapCHECK and film, respectively. These results are below the local set tolerance of 98% and 96%, on 2%/2mm, for the two devices, respectively.
Conclusion
SRS MapCHECK and GAFchromic film are interchangeable detector systems for SABR PSQA, and a local tolerance based on gamma analysis was determined for SRS MapCHECK.