ESTRO 37 Abstract book

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ESTRO 37

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 Poster Viewing : Poster viewing 6: Radiobiology

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 the tract normal to a computed medial axis (MA), thereby enabling such analysis of regional effects along the tract length. The MA is calculated via a MA transform by thinning followed by a 3D parametric cubic spline fit. MD, FA, and RT dose data were mapped to the normal slices after restricting these data to the censored tracts for each patient. The sensitivity measure was calculated as the percent change in mean FA or MD per patient and slice divided by the mean RT dose per patient and slice, scaled by the mean over all the slices, and averaged over all 49 patients. Results Distinct patterns of FA/MD sensitivity to dose, relative to the tract’s mean, were seen for specific white matter structures, in particular at their terminal ends. For example, for the corticospinal tract, FA sensitivity at terminal ends, right hemisphere, was -39.1±28.5 and - 7.6±11.4 percent change per Gy. For the corresponding left hemisphere, this FA sensitivity was -29.5±32.0 and - 7.1±12.4 percent change per Gy. The sensitivity pattern persisted for corresponding tracts in both the left and right brain hemispheres (Figure 1). Structures which exhibited marked variation seen in both hemispheres included the corticospinal tract, medial lemniscus, and inferior cerebellar peduncle. Conclusion Our analysis suggests that some tracts exhibit significant local variations in sensitivity of diffusivity changes to radiation dose. These results could not have been obtained via axial slicing of the tracts, as very curved or hooked tracts would be sampled in two locations at once during axial slicing, regardless of rotation. Tract-Crawler is a novel tool to visualize and analyze white matter structures.

PV-0314 Model Based Radiotherapy: Submandibular Dose-Response NTCP-curve Based on Objective Measurements. C. Terhaard 1 , J. Vermaire 1 , T. Dijkema 2 , M. Philippens 1 , P. Braam 2 , J. Roesink 1 , C. Raaijmakers 1 1 UMC Utrecht, Radiation Oncology Department, Utrecht, The Netherlands 2 UMCN Radboud, radiation oncology, Nijmegen, The Netherlands Purpose or Objective Submandibular gland (SMG) normal tissue complication probability (NTCP) curves will be part of model based indication for proton therapy in the Netherlands. Subjective measurements of xerostomia and sticky saliva reflect whole saliva, and are not suitable for the determination of the SMG NTCP curve. We performed direct measurements of salivary flow, including the SMG flow. Based on a large data base with a broad mean SMG dose distribution we obtained NTCP curves 6 weeks and 1 year after therapy.