Dosimetric evaluation of the interplay effect for non-respiratory-gated VMAT treatment of moving targets with high dose rate FFF beams

Abstract

Dose deviations caused by the interplay effect can be reduced by decreasing dose rate for intensity modulated radiation therapy. It is questionable whether FFF dose rate will increase dose deviations for VMAT. This work evaluates the interplay effect using a moving phantom to simulate tumor motion for non-respiratory-gated VMAT using FFF beams, and investigates the impact of dose rate. Two VMAT plans were selected from lung patients treated with 6 MV FFF beams. Dose distributions were measured using a 2D ion chamber array placed on a motor-driven motion phantom. Respiratory phase was accounted for by dividing the breathing cycle into 8 phases and irradiation was initiated at each of the phases. Delivery was done with and without phantom motion using 3 dose rates: 1400, 600, and 400 MU/min. Measured dose was compared to planned dose using gamma analysis (3%/3mm), percent of pixels within 5% of pixel dose, and percent of pixels within 10% of pixel dose. Calculations were done for 8 starting phases and repeated for a random sampling of starting phases for 1000 trials. Target coverage was evaluated by the dose difference for four points at the center of the target. Four points at the periphery of the target were used to evaluate planning target volume margin. A degradation of dose distribution was seen for the moving compared to the static condition. Results didn’t vary significantly for different dose rates. Target coverage was within 5.0% and 7.4% (center of target), and within 13.0% and 39.7% (periphery) for the static and moving conditions, respectively. Our results indicate the interplay effect had an impact on non-gated VMAT, however using high FFF dose rate did not increase dose deviations. The impact of the interplay effect on target dose is acceptable, as long as sufficient margin is given.