Estimation of local confidence limit for 6 MV photon beam IMRT system using AAPM TG 119 test protocol

Abstract

Purpose: The aim of this study was to estimate local confidence limit for 6 MV photon beam based intensity modulated radiation therapy (IMRT) using TG119 test protocol.

Methods: The American Association of Physicists in Medicine (AAPM) Task Group 119 (TG119) prescribed a protocol to evaluate overall accuracy of IMRT system rather than independent uncertainty in dose calculation, dose delivery and measurement system. Two preliminary and five clinical test cases were created based on dose prescriptions and planning objectives given by TG119 report. Verification plans were created in a planning slab phantom, 2D Matrix dosimetry system (I’MatriXX) with multicube phantom and aS-1000 electronic portal imaging device (EPID). Radiation absorbed doses to high dose points in the planning target volume (PTV) region and low dose points in avoidance structures were measured using CC13 ionization chamber having sensitive volume of 0.13 cm³. The measured and planned doses were normalized with respect to their prescription doses and intercompared. The gamma analysis was carried out for both I’MatriXX and EPID, adopting the acceptance criteria of 3% DD (dose difference) and 3 mm DTA (distance to agreement) with 10% threshold dose.

Results: For the point dose measurements with ion chamber, the average dose difference ratio in high dose low gradient PTV region was -0.0133 ± 0.012 corresponding to a confidence limit of 0.037. The average dose difference in low dose region (avoidance structure) was -0.00004 ± 0.010 corresponding to a confidence limit of 0.021. The average percentage of points passing the gamma criteria of 3% DD and 3 mm DTA for composite planar dose distribution measured by I’MatriXX was 99.47 ± 0.43 which corresponds to a confidence limit of 1.38 (i.e. 98.62% passing). Similarly, the average percentage of points passing the gamma criteria of 3% DD and 3 mm DTA for per-field dose distribution measured by EPID was 98.00 ± 2.49 which corresponds to a confidence limit of 6.87.

Conclusion: Our results were well within action level given by AAPM TG119 report through multi-institutional study providing us adequate confidence in delivering IMRT treatment.