Establishment of quality assurance for respiratory-gated radiotherapy using a respiration-simulating phantom and gamma index: Evaluation of accuracy taking into account tumor motion and respiratory cycle

Abstract

The purpose of this study is to present a new method of quality assurance (QA) in order to ensure effective evaluation of the accuracy of respiratory-gated radiotherapy (RGR). This would help in quantitatively analyzing the patient’s respiratory cycle and respiration-induced tumor motion and in performing a subsequent comparative analysis of dose distributions, using the gamma-index method, as reproduced in our in-house developed respiration-simulating phantom. Therefore, we designed a respiration-simulating phantom capable of reproducing the patient’s respiratory cycle and respiration-induced tumor motion and evaluated the accuracy of RGR by estimating its pass rates. We applied the gamma index passing criteria of accepted error ranges of 3% and 3 mm for the dose distribution calculated by using the treatment planning system (TPS) and the actual dose distribution of RGR. The pass rate clearly increased inversely to the gating width chosen. When respiration-induced tumor motion was 12 mm or less, pass rates of 85% and above were achieved for the 30–70% respiratory phase, and pass rates of 90% and above were achieved for the 40–60% respiratory phase. However, a respiratory cycle with a very small fluctuation range of pass rates failed to prove reliable in evaluating the accuracy of RGR. Therefore, accurate and reliable outcomes of radiotherapy will be obtainable only by establishing a novel QA system using the respiration-simulating phantom, the gamma-index analysis, and a quantitative analysis of diaphragmatic motion, enabling an indirect measurement of tumor motion.