Respiratory gating of sliding window IMRT

Breathing motion may limit sites of applicability for intensity modulated radiation therapy (IMRT). Respiratory gating reduces motion by activating the beam during a chosen phase of respiration. We investigated the effect of gating on the dose distribution of sliding window dynamic multileaf collimator (DMLC) fields. A Varian 2100C beam was gated by the Varian real time position management system that tracks moving infrared reflective markers with a wall-mounted camera. A rotating eccentric wheel simulated marker breathing motion. Dose distributions from normal (ungated) and gated DMLC fields were compared using film dosimetry and also by calculating dose distributions from the leaf motion log files. Fields included prostate (15 MV) and H&N (6 MV) treatment fields and special test fields. Kodak V films under polystyrene phantom were exposed separately for gated and ungated operation. Additional ungated fields were filmed as controls. Log files for gated and ungated deliveries were copied to a program written in-house which uses the clinical treatment planning system to calculate the dose distribution delivered by the recorded leaf motion. The films were digitized, optical densities converted to dose, and matched pairs of films were registered. The largest differences (≈5% or ~1 mm) occurred in high dose gradient regions and were consistent with uncertainties in aligning pairs of films. Gated vs. ungated dose distributions calculated from the fog files agreed to better than 0.5%. These preliminary results show no deleterious effects of gating on DMLC dose distributions