Analysis of organ motion effects on the effective fluences for liver IMRT

An effective fluence concept was employed to make forward dose calculations to investigate the effects of a distorted fluence map on dose plans. Fluence changes caused by organ motion were calculated using Chui's algorithm (2003 Med. Phys. 30 1736). In two test cases with various fluence maps, the effects of motion were simulated using a maximal displacement from 5 mm to 25 mm; 108 fluence maps that were calculated from 16 IMRT plans for eight liver cancer patients were analyzed and compared with and without gating. Fluoroscopic measurements were made of a moving diaphragm in this study. Fluence changes associated with superior-inferior organ motion, perpendicular to the moving MLC, were also examined. The effects of motion on the fluence maps were evaluated from both the fluence differences between static and motion and the χ function. The maximum displacements of the organs in all of these cases were analyzed and correlated with the change in fluence generated from the liver IMRT plans. The dosimetric effects on the target coverage were evaluated for each plan. The results indicate that, for the same fluence map, the mean fluence intensity error or the percentage of the fluence points that have an unacceptable error is linearly related to the extent of motion. For different fluence maps, the degree to which the fluence is distorted by motion is strongly related to the product of the motion extent and the fluence gradient in the direction of diaphragm motion. For eight liver patients and 16 IMRT plans in this work (with gated technique, motion extent from 0.5 cm to 1.0 cm; without gated technique, motion extent from 0.9 cm to 1.8 cm), the fluence modulations are mild, such that the respiratory motion of each patient did not strongly affect the CTV coverage. The mean dose error is 1.5% for free motion (0.9-1.8 cm) and is around 1% for gated motion (0.5-1 cm).