Effects of lung tumor motion on delivered dose distribution during RapidArc treatment technique

The purpose of this investigation was to quantify the effect of lung tumor motion on dose distribution delivered by volumetric arc modulation with RapidArc (RA) treatment technique. An in-house-developed dynamic phantom was set in linear sinusoidal motion in cranio-caudal direction with amplitude of 2 cm and at a frequency of 15 cycles per minute. A RA treatment plan (with gantry arc from 210° to 150°) was optimized for and delivered to the phantom containing a simulated target and critical organs. The treatment plan was executed for 30 fractions to the dynamic phantom, started at random initial breathing phases. GaF-Chromic films which were embedded in the moving phantom in the coronal plane at the isocenter level were used to capture the dose distribution. (i) Region of interest (ROI) as planning target volume (PTV): when the daily dose distributions were compared against the static distribution, dose variation of 10% to 20% was observed near the field edges and 5% to 10% dose difference was observed elsewhere. Similar results were observed when comparing the dose distribution averaged for 30 fractions against the static distribution. (ii) ROI as clinical target volume: it was observed that on a day-to-day basis, the standard deviation of the dose to a given pixel could be as high as 4.5% to 6.5%. Also, the mean, median, and modal doses varied inter-fractionally with standard deviations of 5.62%, 5.61%, and 5.73%, respectively. When comparing the averaged dose distribution with the static distribution, dose variation of 5% to 10% was observed. Also, the mean, median and modal doses were reduced by 6.35%, 6.45% and 6.87%, respectively, compared with static distribution. The result shows that, if respiratory management techniques were not implemented, RA treatment for lung cancer could induce underdosing, so this error should be measured as part of the patient-specific quality assurance.