Stereotactic ablative radiation therapy with volumetric modulated arc therapy in flattening filter-free mode for low-, intermediate-, and high-risk prostate cancer patients: Are 2 arcs better than 1?

PURPOSE - In preparation for a phase 2 clinical trial of prostate cancer treatment with stereotactic ablative radiation therapy (SABR), the quality of volumetric modulated arc therapy (VMAT) plans was investigated to determine the preferred delivery technique.

METHODS - VMAT treatment plans were generated with version 11 of the Eclipse treatment planning system for a Varian TrueBeam linear accelerator operating with 10-MV x-rays in flattening filter-free mode (FFFM). Plans were designed with pelvic computed tomography scans from 10 patients with prostate cancer with an assumption of low-, intermediate-, and high-risk target volumes. The prescription dose was set to 36.25 Gy to be delivered in 5 fractions of 7.25 Gy each. Dose-volume constraints imposed during optimization to protect organs at risk (OARs) were based on data from published studies and current SABR clinical trials. One-arc and 2-arc plans were compared in terms of dose homogeneity and conformity to the target volumes, dose to the OAR and to the surrounding normal tissue, the total number of monitor units required, and overall treatment time.

RESULTS - Clinically acceptable VMAT-FFFM-based SABR regimens were produced for all low-, intermediate-, and high-risk target volumes using both 1-arc and 2-arc deliveries. No significant dosimetric differences in terms of homogeneity, conformity, or dose to the OAR were observed between 1-arc and 2-arc deliveries, but treatment times were twice as long for 2-arc deliveries and consistently required more monitor units.

CONCLUSIONS - Given the similar dosimetry between 1- and 2-arc plans, single-arc delivery of VMAT-FFFM may be preferable to minimize the risk of intrafraction motion and reduce leakage and scatter radiation to the patient.

Pract Radiat Oncol. 2015 Jun 5. pii: S1879-8500(15)00150-2. doi: 10.1016/j.prro.2015.04.002. [Epub ahead of print]
Stereotactic ablative radiation therapy with volumetric modulated arc therapy in flattening filter-free mode for low-, intermediate-, and high-risk prostate cancer patients: Are 2 arcs better than 1?
Fortin D1, Mestrovic A2, Alexander A3.

1 Department of Medical Physics, BC Cancer Agency-Vancouver Island Centre, Victoria, BC, Canada.
2 Department of Medical Physics, BC Cancer Agency-Vancouver Island Centre, Victoria, BC, Canada; Department of Physics and Astronomy, University of Victoria, Victoria, BC, Canada.
3 Department of Radiation Oncology, BC Cancer Agency-Vancouver Island Centre, Victoria, BC, Canada; Department of Surgery, University of British Columbia, Vancouver, BC, Canada.

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