Poster - Thur Eve - 40: Dynamic arc sliding window tests for checking MLC gap consistency in rapid arc delivery.

MLC gap control is critical for dosimetric accuracy in rotational IMRT (RapidArc, VMAT) treatments. Systematic MLC gap change of 1 mm may cause 3-4% change of EUD to PTV for a typical H&N RapidArc plan. Therefore it is important to monitor MLC gap through QC procedures. For this purpose, we have created dynamic arc sliding window (SW) plans with fixed width MLC slits sliding across a jaw defined field. Plans with MLC slit widths of 5, 10, 15, and 20 mm, respectively, and the same length of 20 cm (in Y direction) were created with 6MV photons in a single arc of gantry angles from 182° to 178°. Dose delivered from these SW plans was measured using an ion chamber in a cylindrical phantom placed at isocentre, and values for dosimetric leaf gap (DLG) were derived based on relative dose measurements. DLG measured in dynamic arc SW tests agrees with that measured in fixed gantry angle SW fields to within 0.02 mm. We also extracted the MLC leaf gaps during MLC travels in these dynamic arc SW deliveries from MLC positions recorded in dynalog files, and compared to the MLC gaps in fixed gantry SW fields. We found that MLC leaf gaps were maintained excellently constant whether in dynamic arc or fixed gantry angle SW delivery, with typical standard deviation of MLC gaps of only ∼0.01mm for all involved leaf pairs. We believe these dynamic arc SW tests are very useful for checking MLC leaf constancy for RapidArc delivery.