Dosimetric evaluation of radiation dose rate effect in respiratory gated intensity modulated radiation therapy.

PURPOSE: To investigate the dosimetric accuracy of the sliding window gated IMRT compared with the static treatment, using varying dose rates. MATERIALS AND METHODS: This study measured changes in output and diode array response with changing dose rate, verified the precision of the motion table, and measured changes in dose distribution accuracy with film and diodes at two depths with changing dose rate. During 4DCT (4 Dimensional Computed Tomography), the patient's respiratory signals and target motion were recorded and imported to the XY4D simulation table of SUN NUCLEAR Corporation to simulate the patient's respiration and tumour motion. A single field of each sliding window IMRT plan with 30o wedge and one for lung cancer were used in this study. Three irradiating conditions, static and moving target with and without gating, were applied to both plans. RESULTS: The standard deviations of output, with the dose rates changing from 300-600 MU/min, were 0.065 cGy and 0.169 cGy for the ionisation chamber and diode, respectively. The verification of the motion table shows very good precision with 9.98 ± 0.02 cm (true value = 10.0 cm). The measurements by MapCheck show the gamma index of the planned absolute dose distribution in static and moving targets with gating, resulting in more than 96% passing for all dose rates. The absolute dose distribution measured by film for the static target was agreeable with the value of moving target with gating. CONCLUSION: The sliding window gated IMRT technique is able to deliver an accurate dose to a moving target with the dose rate of 300-600 MU/min that is suitable for clinical treatment.