Local confidence limits for IMRT and VMAT techniques: a study based on TG119 test suite.

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 used at Waikato Regional Cancer Centre. This work was carried out based on the American Association of Physicists in Medicine (AAPM) Task Group (TG) 119 report. The AAPM TG 119 report recommends CLs as a bench mark for IMRT commissioning and delivery based on its multiple institutions planning and dosimetry comparisons. In this study the locally obtained CLs were compared to TG119 benchmarks. Furthermore, the same bench mark was used to test the capabilities and quality of the VMAT technique in our clinic. The TG 119 test suite consists of two primary and four clinical tests for evaluating the accuracy of IMRT planning and dose delivery systems. Pre defined structure sets contoured on computed tomography images were downloaded from AAPM website and were transferred to a locally designed phantom. For each test case two plans were generated using IMRT and VMAT optimisation. Dose prescriptions and planning objectives recommended by TG119 report were followed to generate the test plans in Eclipse Treatment Planning System. For each plan the point dose measurements were done using an ion chamber at high dose and low dose regions. The planar dose distribution was analysed for percentage of points passing the gamma criteria of 3%/3 mm, for both the composite plan and individual fields of each plan. The CLs were generated based on the results from the gamma analysis and point dose measurements. For IMRT plans, the CLs obtained were (1) from point dose measurements: 2.49% at high dose region and 2.95% for the low dose region (2) from gamma analysis: 2.12% for individual fields and 5.9% for the composite plan. For VMAT plans, the CLs obtained were (1) from point dose measurements: 2.56% at high dose region and 2.6% for the low dose region (2) from gamma analysis: 1.46% for individual fields and 0.8% for the composite plan. All these CLs were well within the TG119 recommended bench marks. Based on these analysis which were performed in line with the TG119 recommendations, it is evident that the local clinic has commissioned IMRT and VMAT techniques with adequate accuracy. These results compliment our clinical confidence of using IMRT and VMAT routinely and expanding to different clinical sites.