ESTRO 2020 Abstract Book

S888 ESTRO 2020

(c) deformable radiobiological accumulation In (a) image sets were merged initially using rigid registration. In (b) and (c) DIR was applied to deform anatomical information from a previous image set to the current image set. These DIR based transformation matrices were then used to deform the corresponding dose matrix (dose warping) from previous to current anatomy to allow deformable dose accumulation. In (a) and (b), physical dose accumulation was performed, assuming all contributing dose plans were prescribed as the standard 2Gy per fraction. In (c) correction for non- standard dose fractionation, was achieved using radiobiological calculation of individual dose plans to create its EQD2-based DVHs prior to dose accumulation. See figure 1. The Dice Similarity Coefficient (DSC) was used as a parameter for assessing the result of DIR on multiple patient studies as recommended by AAPM TG132 1 . 1 AAPM Task Group 132: Brock KK, et al. Med Phys. 2017 Jul;44(7)

Results Addition of dose by combining multiple treatment plans for retreatment patients was best achieved using method (c) deformable radiobiological accumulation. Quantitative DIR assessment in terms of DSC values showed that DIR performed on rigid structures such as the spinal cord (0.8) and mandible (0.8) were clinically acceptable as per AAPM TG132. OARs affected by motion such as the heart (0.82) and normal lung tissue (0.97) also showed acceptable DIR. The bowel (0.4) and bladder (0.35) were classified as complex OARs to perform DIR. Correction for hypofractionated dose prescriptions in examined patients showed clinically significant differences when compared with corresponding physical dose accumulation methods. Conclusion This study suggests that combining DIR and radiobiological dose recalculations for overall dose accumulation provides more representative dose information when compared to standard rigid registration based analysis. DIR and radiobiological dose in treatment plan evaluation has the potential for optimizing the standard of care delivered to patients requiring retreatment. PO-1626 Cervical cancer IGRT: A comparison of offline, online and MRI guided adaptive planning strategies I. White 1 , D. McQuaid 2 , E. Goodwin 2 , J. Mohajer 2 , S. Lalondrelle 3 1 Institute Of Cancer Research And Royal Marsden Hospital NHS Trust, Clinical Oncology, London, United Kingdom ; 2 Royal Marsden Hospital NHS Trust, Physics, Sutton, United Kingdom ; 3 Royal Marsden Hospital NHS Trust, Radiotherapy, Sutton, United Kingdom Purpose or Objective Quantify benefit of different adaptive radiotherapy (ART) strategies, using dose accumulation to assess target and OAR doses. Define optimal adaptive planning solutions for

Conclusion Convolutional neural networks trained on the combined objective of maximizing prostate overlap and minimizing registration errors or on contour overlap only resulted in the best performance against networks trained on minimizing registration errors only. Convolutional networks are significantly faster than conventional DIR methods while achieving superior accuracy. Obtaining deformation fields and contours within a second is essential to facilitate intra-fraction adaptations. PO-1625 Deformable image registration (DIR) and radiobiological recalculation for retreat plan evaluation L. Stenzel 1 , R. Flynn 2 , M. Moore 3 1 National University of Ireland Galway, Dept. of Physics, Galway, Ireland ; 2 Mid-Western Radiation Oncology Centre- University Hospital Limerick, Radiotherapy Physics, Dooradoyle- Limerick, Ireland ; 3 University Hospital Galway, Medical Physics & Clinical Engineering, Galway, Ireland Purpose or Objective The study aimed to evaluate the application of DIR and radiobiological dose calculations when applied to multiple treatment plans for patients requiring retreatment. Retreatment was defined as patients who had undergone previous radiotherapy treatment and who returned for further treatment due to recurrent cancer or tumour growth localised near a previously treated area. To evaluate the potential clinical significance of DIR and radiobiological recalculation in retreat plan assessment, three methods of dose accumulation were investigated during this study. Material and Methods DIR and dose accumulation analysis were carried out on six patient retreat cases which involved non-standard fractionation. Dose accumulated in specific OARs was investigated. The dose accumulation methodologies varied in terms of image registration and dose calculation: (a) rigid physical accumulation (b) deformable physical accumulation

cervical cancer EBRT. Material and Methods

Two hundred and fifty VMAT plans were created in RayStation 7.0R (RaySearch, Stockholm) to model