Pre-clinical evaluation of respiratory-gated delivery of volumetric modulated arc therapy with RapidArc.

A study was carried out to evaluate the possibility of delivering volumetric modulated arc therapy with the RapidArc technology under respiratory-gated conditions. The experiments were performed in the framework of a non-clinically released environment. Plans of six patients, all realized for a single arc, were used for the experiments. The Real-time Position Management (RPM) respiratory gating system from Varian was used to generate gate-open signals of different durations. Arcs were delivered applying the different gates creating sequences of beam-hold/beam-on during the dose delivery: the average number of interruptions for a single arc ranged from 0 to 45. Dose prescription was set to 2 Gy and different gate-open periods of 30, 15 and 5 s to keep gantry speed constant at maximum. 5 Gy and 15 Gy doses were then applied to gate-open signals of 5 and 8 s, respectively, to mimic the most challenging conditions of slow gantry rotation and high-frequency interruptions. The 5 and 15 Gy experiments represent dose conditions of clinical interest for stereotactic treatments. For each patient and gating condition, pre-treatment 2D verification measurements were performed using the PTW-729 array in conjunction with the Octavius phantom (PTW, Freiburg); measurements were performed on different days (one per patient, with the complete setup of phantom and detectors every time), while each gating experiment was repeated seven consecutive times for reproducibility (without a new setup of the measurement equipment). Measurements were compared with dose calculations in the treatment planning system (performed without any gating) to appraise the dosimetric impact of the presence of gating and the eventual dependence from the number of interruptions during a single arc. Analysis of machine-registered log files in terms of average deviations between actual and expected positions (from automatic measurements every 50 ms) resulted in mean Delta MU (monitor units) <0.02% for all gating conditions. Delta(Gantry angle) = 0.38 +/- 0.01 degrees for 2 Gy (all gate periods), 0.24 +/- 0.01 degrees for 5 Gy, and 0.10 +/- 0.01 degrees for 15 Gy deliveries. Average deviations for multileaf collimator (MLC) positions (root mean square over all 120 leaves) were 0.45 +/- 0.01 mm for 2 Gy (all gate periods), 0.32 +/- 0.01 mm for 5 Gy and 0.14 +/- 0.01 mm for 15 Gy. Results in terms of dose measurements confirmed that the application of gating to RapidArc delivery does not affect the quality of the dose delivery. With criteria of Delta D = 3%, DTA = 3 mm, the gamma test was passing in a range of 99 to 100% of the measured points for most of the cases (with maximum number of interruptions of about 20 per arc) and from 97 to 98% for the extreme case of 15 Gy and 8 s gate-open signal (corresponding to almost 50 interruptions per arc). In conclusion, RapidArc delivery proved, in a pre-clinical phase and non-clinically released framework, to be reliable and dosimetrically accurate also when applied in conjunction with gating procedures.