Searching standard parameters for volumetric modulated arc therapy (VMAT) of prostate cancer

Background: Since December 2009 a new VMAT planning system tool is available in Oncentra (R) MasterPlan v3.3 (Nucletron B. V.). The purpose of this study was to work out standard parameters for the optimization of prostate cancer. Methods: For ten patients with localized prostate cancer plans for simultaneous integrated boost were optimized, varying systematically the number of arcs, collimator angle, the maximum delivery time, and the gantry spacing. Homogeneity in clinical target volume, minimum dose in planning target volume, median dose in the organs at risk, maximum dose in the posterior part of the rectum, and number of monitor units were evaluated using student's test for statistical analysis. Measurements were performed with a 2D-array, taking the delivery time, and compared to the calculation by the gamma method. Results: Plans with collimator 45 degrees were superior to plans with collimator 0 degrees. Single arc resulted in higher minimum dose in the planning target volume, but also higher dose values to the organs at risk, requiring less monitor units per fraction dose than dual arc. Single arc needs a higher value (per arc) for the maximum delivery time parameter than dual arc, but as only one arc is needed, the measured delivery time was shorter and stayed below 2.5 min versus 3 to 5 min. Balancing plan quality, dosimetric results and calculation time, a gantry spacing of 4 degrees led to optimal results. Conclusion: A set of parameters has been found which can be used as standard for volumetric modulated arc therapy planning of prostate cancer.