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Flattening Filter Free VMAT Delivery of Total Body Irradiation at An Extended SSD: A Treatment Planning Feasibility Study

R Frederick*1,2, A Hudson1,2, G Pierce1,2, (1) University of Calgary, Calgary, AB, (2) Tom Baker Cancer Centre, Calgary, AB


(Tuesday, 7/31/2018) 3:45 PM - 4:15 PM

Room: Exhibit Hall | Forum 4

Purpose: The challenge of total body irradiation (TBI) treatment lies in the difficulty of delivering a homogeneous dose distribution due to unconventional patient set-up, and the consequent protracted treatments that exacerbate patient discomfort. This project evaluates the use of flattening filter free (FFF) volumetric modulated arc therapy (VMAT) delivery to increase the time efficiency of TBI treatments while attaining acceptable dose homogeneity.

Methods: Seven patients with comparable AP widths (18-20.5cm) treated with our institution’s current AP/PA VMAT technique (4Gy/2 fractions, extended SSD with beam spoiler) were retrospectively examined using Varian Eclipse™. Midplane lateral dose profiles at eight cranial-caudal locations were collected from three patients, with dose calculated from our standard 6MV 40x10cm² open field arc delivery TBI plan substituted with 6MV FFF. Custom Python™ code was developed to automatically position the MLC leaves to provide a homogenous dose using the open field profiles in conjunction with relative arc output factors. These MLC positions were then applied to all seven patients. Dose-volume histogram (DVH) parameters and beam-on times for our clinical and FFF VMAT plans are compared.

Results: The DVH parameters and beam-on times are reported as median (range). The Body-5mm D98% for the clinical and FFF VMAT plans were 95.1% (91.5-95.4) and 94.3% (91.7-97.9) respectively. The Body-5mm D2% for the clinical and FFF VMAT plans were 114.5% (109.2-118.0) and 116.2% (112.6-125.5) respectively. The mean lung dose (MLD) for the clinical and FFF VMAT plans were 104.4% (101.6-105.7) and 105.7% (103.4-109.5) respectively. Our calculated clinical and FFF VMAT beam-on times were 19.3min (18.8-20.4) and 8.9min (8.6-9.6) respectively.

Conclusion: The forward-planned FFF VMAT TBI treatment plans provide comparable dose homogeneity to our clinical VMAT technique for this patient cohort. Beam-on time decreased by 54% on average through using FFF delivery, providing a possible means of reducing patient discomfort, motion, and radiotherapy resources.

Funding Support, Disclosures, and Conflict of Interest: This work was supported by the Natural Sciences and Engineering Research Council of Canada and the Alberta Cancer Foundation. There are no relevant financial disclosures or conflicts of interest to declare.


TBI, Treatment Techniques, Treatment Planning


TH- External beam- photons: treatment planning/virtual clinical studies

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