ESTRO 38 Abstract book

S495 ESTRO 38

times and patient discomfort. This project aims to decrease planning and treatment time while attaining acceptable dose uniformity by designing stock TBI plans based on patient anterior-posterior (AP) width at umbilicus, and by evaluating the use of flattening filter free (FFF) delivery. Material and Methods Retrospective examination of 47 TBI patients receiving a prescription of 4Gy/2fx BID was completed using Varian Eclipse TM . All patients were treated using our centre’s clinical AP/PA VMAT technique at extended SSD with beam spoiler. Patients were stratified into 4 AP width categories: 17-19cm, 19-21cm, 21-23cm, and 23-25cm. The latter two groups were additionally sorted into either short or tall heights. To generate standard FFF VMAT TBI plans, the clinical TBI 6MV 40×10cm 2 open field arcs were used as the plan base with the beam energy substituted with 6MV FFF. Dose homogeneity was achieved through planning MLC leaf positions based on AP midplane lateral dose profiles at eight superior-inferior locations for three patients in each category, and calculated output factors. Custom Python TM code positioned the MLCs at their calculated locations. Two plans (supine, prone) were created for each category for a total of 12 standard plans, and applied to all patients depending on their categorization. The clinical and FFF TBI plans were compared by examining dose-volume histogram (DVH) parameters, dose rates, and beam-on times. Results Figure 1 presents DVH parameters for all patient categories for the clinical and FFF plans, and Table 1 lists the associated median (range) for each parameter. Significantly similar dose coverage (D98) is seen between the clinical and FFF plans, but the FFF plans have hotspot (D2) and mean lung dose (MLD) distributions that are significantly shifted higher compared to the clinical plans. This decreased dose uniformity still lies within acceptable clinical bounds, and is expected for the standardized FFF plans versus the clinical plans that are tailored to an individual patient. The presented DVH distributions and summarized plan parameters are representative of those for each AP-width category separately.

PO-0924 Viability of standardized FFF VMAT TBI treatment plans for extended SSD delivery based on AP-width R. Frederick 1,2 , A. Hudson 2,3 , G. Pierce 1,2,3 1 University of Calgary, Department of Physics and Astronomy, Calgary, Canada ; 2 Tom Baker Cancer Centre, Department of Medical Physics, Calgary, Canada ; 3 University of Calgary, Department of Oncology, Calgary, Canada Purpose or Objective Total body irradiation (TBI) treatments require a large radiation field that often necessitates delivery at an extended source-to-surface distance (SSD), resulting in supplementary set-up procedures that increase treatment