E. Onjukka1,2, L. Södergren1, G. G. Y. Kim3, J. Söderström1, M. Westermark1, and M. Ghazal1,2; 1Medical Radiation Physics and Nuclear Medicine, Karolinska University Hospital, Stockholm, Sweden, 2Oncology and Pathology, Karolinska Institutet, Stockholm, Sweden, 3Department of Radiation Medicine and Applied Sciences, University of California San Diego, La Jolla, CA
Purpose/Objective(s): The prototype of a new commercial treatment plan optimizer, including new degrees of freedom, was evaluated with regard to deliverability. The new optimizer produces volumetric modulated arc therapy with the integration of static angle ports and dynamic collimator rotation during gantry rotation (referred to as VMATp). Materials/
Methods: VMATp combines rotational VMAT delivery with one or more static angle ports, as selected by the planner. The collimator angle at each control point is optimized taking the projection of the target into account. The feasibility of reproducing the resulting dose distribution with a linac was investigated using a cylindrical verification phantom with integrated diodes. The measurements were compared to regular VMAT plans (without static angle ports) optimized with the current clinical version of the optimizer. Treatment plans were optimized with 1) the current version and 2) the VMATp prototype, for each patient in a cohort of mixed anatomical sites. Having aimed for similar plan quality, plan 1 and plan 2 were also compared with regard to plan complexity metrics (number of MU, average aperture area, average aperture per leaf couple, beam irregularity*, beam area* and beam modulation*). A t-test was performed to test for differences (p<0.05 was considered statistically significant). *As defined in Du et al. Med Phys. 2014;41(2):021716 Results: Fourteen previously treated patients, from two institutions, were simulated with both techniques (6 gynecological, 4 head and neck, 2 glioblastoma and 2 spine SBRT). The mean gamma pass rate at 3%/3mm was excellent: 98.8% (std 1.8%) for the current version and 98.9% (std 1.4%) for the new version (p = 0.96). The pass rate at 2%/2mm was also equivalent (90.7% (std 8.6%) vs. 89.8% (std 9.2%), p = 0.79). A variation of beam arrangements were selected for the different anatomical sites. While two arcs were typically used in Plan 1, a single arc with some static ports were often sufficient in Plan 2. The mean number of monitor units was 1192 (std 1417) and 1171 (std 1312) for Plan 1 and Plan 2, respectively, and the corresponding mean delivery time was 153 (std 37) seconds vs. 141 (std 52) seconds. Beam irregularity was the only complexity metric which differed significantly between the two sets of plans, with reduced irregularity for the new version (p < 0.01). Conclusion: This work is the first to investigate and confirm the accuracy of the dose delivery for the plans optimized with the new degrees of freedom offered by the new treatment plan optimizer.
