Abstract Book

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fractions (hypo-fractionation) or 66Gy in 33 fractions (post-op). Our current standard (STD) bladder filling protocol requires a patient drink 500cc of water 1 hour prior to each treatment and bladder filling is evaluated on daily CBCT prior to treatment. When the bladder is less than 50% of the planning CT volume, treatment is postponed and given later the same day when the bladder at least 50% full. Our goal was to develop an evaluation volume that could ensure a safe minimal bladder filling. Our dose constraints require that no more than 50% of the bladder can receive 60-65Gy with standard fractionation (2Gy/day) or 50Gy with the hypo-fractionation regimen (3Gy/day). We created a structure defined as a Virtual Bladder (VB) using the V60 isodose line from each patient’s respective treatment plan. The V50 isodose was used for patient receiving the hypo-fractionation regimen. This structure was than used to prospectively evaluate the bladder volume during each treatment and this was compared to our STD protocol. The VB protocol requires that the radiation therapist must evaluate the sagittal plane of the CBCT containing the largest bladder volume. The therapist than ensures that less than 50% of the bladder is found within the VB volume. The radiation therapists were instructed to follow the STD protocol in our center; however, they were also asked to prospectively determine whether the VB protocol would have changed their conduct. Results Between June and September 2017, 38 patients were evaluated with the VB protocol. Of these, 9 received 78Gy, 17 received 60Gy and 12 received 66Gy. With our STD protocol, 18 instances occurred where the criteria were not met in 6 (16%) patients. Of the 6 patients, there were 2 from each fractionation regimen. With our VB protocol, only 2 (5%) patients did not meet the criteria. Both received 66Gy and had no bladder filling issues with the STD protocol. The VB protocol changed the radiation therapists conduct in 8 (21%) of patients. Conclusion The Virtual Bladder protocol is a novel method to evaluate bladder filling, customized to each patient based on dose constraints derived from their treatment plan. Application of this protocol would likely result in a reduced number of postponed treatments and decrease the need for repeat imaging. This tool could allow us to target patients needing stricter bladder constraints with the goal of reducing bladder dose and toxicity. Further studies are warranted. OC-0312 Inter- and Intra-fraction setup errors in patients of HCC with PVTT treated with SBRT using ABC system T. Teekendra Singh 1 , T. Kataria 1 , K. Narang 1 1 Medanta The Medicity, DIVISION OF RADIATION ONCOLOGY, Gurgaon, India Purpose or Objective With the technologic advancements in image guidance and dose delivery, stereotactic body radiotherapy (SBRT) is being widely used in Hepato cellular carcinoma with Portal Vein TumourThrombosis (HCC with PVTT) cases as a noninvasive alternative to surgery. The purpose of this study was to evaluate the daily setup accuracy, inter & intra-fraction motion in these patients treated with Linac based SBRT using Active Breathing Co-ordinator (ABC), and to calculate the PTV margins required to account for the same. Material and Methods 22 Treatment fraction datasets for 5 patients of HCC with PVTT treated with SBRT to a total dose of40–60 Gy in 3-5 treatment fractions, were available. All patients were immobilized in supine position with Blue Bag cushion (Vacloc). Active Breathing Control (ABC) system was used for all patients to minimize tumor motion due to

respiration. Pre- and post-treatment CBCT scans were registered with the bony anatomy of the planning CT to find inter- and intra-fractional patient positioning errors. These CBCT images were registered with the Gray Value (T+R) matching algorithm available in the XVI® CBCT software. The translational displacements in Medio- Lateral, Supero-Inferior and Antero-Posterior directions were recorded as X, Y and Z (cm) respectively while rotational displacements in Pitch, Roll & Yaw directions were recorded as X,Y & Z (º) respectively for all pre and post CBCT scans. Mean and standard deviations were calculated for displacements in each direction, and resulting PTV margins were calculated based on Van-Herk formula. Results Total 44 (22 pre & 22 post) CBCT scans were analysed during the study. The pre treatment mean ± 1 SD translational displacements were 0.22±0.23cm left-right, 0.34±0.39cm supero-inferior and 0.32±0.27cm in antero- posterior direction, and rotation displacements were 0.79º±0.62º pitch,1.00º±0.68º roll and 1.2º±0.73º yaw, while the post-treatment (Intra-fraction) residual errors were 0.13±0.10cm left-right, 0.19±0.14cm supero-inferior and 0.22±0.14cm antero-posterior direction, and rotation displacements were 1.30º±0.67º pitch, 1.20º±0.80º roll and 1.2º±0.97º yaw. The PTV margins were calculated based on post treatment residual errors using Van-Herk Formula and the evaluated margins were 3mm , 4mm & 4mm in Medio-Lateral, Supero-Inferior and Antero- Posterior directions respectively. Conclusion Both setup errors (pre-treatment displacements) and intrafractional movement (post-treatment residualerror) exist to a finite degree during treatment of HCC patients with ABC technique. Online correctionwith CBCT image guidance should be applied to reduce pre-treatment displacements. The intrafractional movement can be accounted for by giving a 4mm uniform PTV margin. These measures may together help in accurate RT delivery and the minimization of toxicity PV-0313 Tract-Crawler: A Computational Tool to Analyze Regional White Matter Dose Effects after Brain RT J. Houri 1 , M. Connor 2 , R. Karunamuni 2 , C. McDonald 2 , T. Seibert 2 , N. White 3 , N. Pettersson 2 , A. Dale 3 , J. Hattangadi-Gluth 2 , V. Moiseenko 2 1 University of Oxford, Department of Physics, Oxford, United Kingdom 2 University of California San Diego, Department of Radiation Medicine and Applied Sciences, La Jolla, USA 3 University of California San Diego, Department of Radiology, La Jolla, USA Purpose or Objective To develop a computational neuroimaging tool to slice individual white matter tracts in the brain normal to the tract’s medial axis and use it to analyze local changes in mean diffusivity (MD) and fractional anisotropy (FA) following brain radiotherapy (RT). Material and Methods RT dose, diffusion metrics, white matter tract structures, and censoring masks were extracted and mapped to a reference brain for 49 patients who received fractionated brain RT from 2010 to 2014. The patients underwent diffusion tensor imaging (DTI) pre-RT and one-year post- RT. 23 of 48 white matter tracts were selected for this analysis based on their elongation. The Tract-Crawler software was developed in MATLAB to analyze changes in MD and FA as a function of dose. This is done by slicing Poster Viewing : Poster viewing 6: Radiobiology