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An Evaluation of Hybrid Arc and Volumetric Modulated Arc Therapy Plans for Brain Fractionated Stereotactic Radiotherapy

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D To*, H Liu , V Gunn , W Shi , Y Yu , J Li , Thomas Jefferson University, Philadelphia, PA


SU-I-GPD-T-425 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Hybrid arc (HA) technique, combining dynamic conformal arc with intensity modulated radiotherapy (IMRT) beams, is routinely used at our institution to treat brain fractionated stereotactic radiotherapy (FSRT) patients. Volumetric modulated arc therapy (VMAT) technique has not been accepted to replace HA due to dosimetric deficiencies, although it has clear advantage on delivery efficiency. A retrospective study was performed to evaluate their plan qualities.

Methods: Twenty FSRT patients treated with HA plans were replanned with VMAT. HA and VMAT plans were generated using Brainlab iPlanDose (Version 4.5) and Varian Eclipse (Version 11.0), respectively, with 6MV photon beams on a Varian TrueBeam STx linear accelerator. Fractional dose ranged from 1.8 to 3.5 Gy and fraction size ranged from 10 to 30. Planning target volume (PTV) ranged from 1.2 to 54.6 cc. Dose conformity index CI (ratio of prescription isodose volume to PTV), gradient index GI (ratio of 50% prescription isodose volume to PTV), homogeneity index HI (ratio of maximum dose in PTV to prescription dose), and maximum dose in the brainstem (percentage of prescription dose) were compared. Wilcoxon signed rank test was used to determine statistical significance in paired comparison.

Results: CIs of HA and VMAT plans are 1.11±0.12 and 1.13±0.08, respectively, not significantly different (p>0.2); GIs are 4.73±1.32 and 5.15±1.56, respectively, favor HA plans (p = 0.0026); HIs are 1.03±0.02 and 1.05±0.01, respectively, favor HA plans (p = 0.0003); and brainstem maximum doses are 63.6 ± 35.6% and 60.3 ± 38.8 %, respectively, not significantly different (p>0.2).

Conclusion: HA plans demonstrated better dose homogeneity within PTV and steeper dose falloff outside PTV. VMAT plans showed equivalent conformity and slightly less brainstem maximum dose, although not significant. HA plans are still favorable for indications such as pituitary adenoma and skull based meningioma near orbital region due to its superior dose homogeneity.

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