Analyzation of the local confidence limits for IMRT and VMAT based on AAPM TG119 report

The aim of this study was to generate a local confidence limit (CL) for intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT) techniques for Clinac IX linear accelerator using the American Association of Physicists in Medicine (AAPM) Task Group (TG119) protocol. The results were compared with the published studies to test the capability and quality of the VMAT technique in our clinic. We used TG119 cases to create plans for IMRT and VMAT in Eclipse Treatment Planning System for Clinac IX using 6 MV and 10 MV photons. Two preliminary and 5 clinical test cases were created based on the dose prescriptions and planning objectives provided by TG119. Verification plans were created in a planning slab phantom, 2D matrix dosimetry system (MatriXX) with multicube phantom and a volumetric phantom (Delta⁴). Radiation absorption doses to high-dose points in the planning target volume region and low-dose points in avoidance structures were measured with a 0.125 cc semiflex thimble ionization chamber (PTW). The measured and planned doses were normalized with respect to their prescription doses and intercompared with each other. The gamma analysis was carried out for MatriXX and Delta⁴ adopting the acceptance criteria of 3% dose difference and 3 mm distance to agreement with 10% threshold dose, respectively. The local CLs with the bench mark set by TG119 were obtained for point, composite planar and field-by-field measurements for IMRT and VMAT with different energies. In this study, the CLs for the high-dose regions of IMRT with 6 MV and 10 MV were 0.025 and 0.014, respectively. For VMAT, they were 0.032 and 0.018. The counterpart CL was 0.045 in TG119. And in organs at risk region, the CLs of IMRT with 6 MV and 10 MV beam were 0.022 and 0.019, respectively, with the counterpart CL indicated by TG119 was 0.047. For VMAT with 6 MV and 10 MV photon beams, the CLs were measured 0.030 and 0.027 with Delta⁴, respectively. The CLs of the maximum gamma passing for all values were 2.0 in 6 MV VMAT plan, which however recommended in TG119 was 12.4. The data presented here showed all the CLs in our clinic meet the criteria of TG119 report well. All these local CLs reached the goals mentioned in AAPM TG119, which indicated that the local clinic had commissioned IMRT and VMAT techniques with adequate accuracy. Prior to the clinical application practice, it is essential to verify with the TG119 test cases for IMRT and VMAT, which allows us to better understand the basic capability of VMAT technology.