Prostate volumetric-modulated arc therapy: dosimetry and radiobiological model variation between the single-arc and double-arc technique.

This study investigates the dosimetry and radiobiological model variation when a second photon arc was added to prostate volumetric-modulated arc therapy (VMAT) using the single-arc technique. Dosimetry and radiobiological model comparison between the single-arc and double-arc prostate VMAT plans were performed on five patients with prostate volumes ranging from 29-68.1 cm3. The prescription dose was 78 Gy/39 fractions and the photon beam energy was 6 MV. Dose-volume histogram, mean and maximum dose of targets (planning and clinical target volume) and normal tissues (rectum, bladder and femoral heads), dose-volume criteria in the treatment plan (D99% of PTV; D30%, D50%, V17Gy and V35Gy of rectum and bladder; D5% of femoral heads), and dose profiles along the vertical and horizontal axis crossing the isocenter were determined using the single-arc and double-arc VMAT technique. For comparison, the monitor unit based on the RapidArc delivery method, prostate tumor control probability (TCP), and rectal normal tissue complication probability (NTCP) based on the Lyman-Burman-Kutcher algorithm were calculated. It was found that though the double-arc technique required almost double the treatment time than the single-arc, the double-arc plan provided a better rectal and bladder dose-volume criteria by shifting the delivered dose in the patient from the anterior-posterior direction to the lateral. As the femoral head was less radiosensitive than the rectum and bladder, the double-arc technique resulted in a prostate VMAT plan with better prostate coverage and rectal dose-volume criteria compared to the single-arc. The prostate TCP of the double-arc plan was found slightly increased (0.16%) compared to the single-arc. Therefore, when the rectal dose-volume criteria are very difficult to achieve in a single-arc prostate VMAT plan, it is worthwhile to consider the double-arc technique.