mARC prostate treatment planning with Varian Eclipse for flat vs. FFF beams

Purpose: The modulated arc (mARC) technique as an alternative to VMAT is a rotational IMRT irradiation with burst mode delivery. Varian has recently implemented an option for mARC-planning into the Eclipse treatment planning system (TPS) and so far mARC-planning with this TPS has not been evaluated systematically. Therefore, for prostate treatment with Eclipse we compare mARC with IMRT using flat (6 MV) and flattening-filter-free (FFF, 7 MV) beam energies. Methods: For ten prostate cancer patients standardized re-contouring and re-planning was performed with a prescription of 76 Gy to the complete planning-target-volume (PTV). IMRT and mARC plans (6 MV vs. FFF 7 MV) were compared pairwise considering indices for plan quality. All plans were delivered on an anthromorphic phantom equipped with thermoluminescent dosimeters to measure out-of-field dose and treatment times. Results: Regarding PTV coverage, there was no marked preference for either technique or energy. The evaluation of organs at risk showed improved bladder sparing of the mARC plans up to about 75 Gy; above this dose the IMRT plans achieved significant better sparing. The use of the FFF-beam-energy and mARC-technique resulted in a significant decrease in out-of-field dose. This combination also led to a drastic reduction of treatment time by factor of three in comparison with 6 MV IMRT. Conclusion: While highly conformal treatment plans could be created by the use of all modalities, the combination of the high dose rate with mARC appears to be the preferable option as it benefits from a marked decrease in treatment time and out-of-field dose. (c) 2016 Associazione Italiana di Fisica Medica. Published by Elsevier Ltd. All rights reserved.