ESTRO 36 Abstract Book

S893 ESTRO 36 2017 _______________________________________________________________________________________________

Two CT scans were acquired and registered (empty/full bladder). The bladder CTVs and GTVs were delineated on both CTs. An in-house developed script was developed to calculate intermediate CTVs and GTVs based on the empty and full bladder delineations. The script, which utilizes a Robust Point Matching (RPM) algorithm (Osorio, 2012), yields a deformation vector field that can transform the target structure to the reference structure. The algorithm can be tuned with the following parameters: stiffness, density of points, number of iterations and the final 'temperature”. To create intermediates, the deformation can be applied partially, e.g., to create a structure in the middle of the two input structures, a 50% deformation would be applied. Dividing the maximum spacing required between consecutive intermediate plans by the maximum distance between reference and target structure, will give the excitation percentages required to get to equidistant intermediate structures. Bladder CTV and GTV need to be handled by separate RPM processes because the required parameters are very different due to large discrepancy of deformation and size. The number of plans is set by the maximum distance between full and empty CTVs. Therefore first the intermediate structures for CTV are created and then the same excitation percentages are applied to GTV. Results Figure 1 shows an example of a generated library of plans for CTVs and GTVs.To evaluate the results we create a structure with 100% deformation, which should coincide with the target structure. Using the default stiffness parameter for 10 patients (1000 for CTV and 250 for GTV) we found a success rate of 60%. By tuning the stiffness parameter, intermediate structures were created successfully for the remaining cases. On average it takes 3.50 and 2 minutes for the CTVs and GTVs to be created, respectively.

Conclusion Using a two-step clinical verification of the dose deformation confirms the feasibility to perform accurate dose accumulation for CTV, bladder and rectum during LACC RT. These values are within the range of uncertainties originated from dose calculation, residual positioning errors or anatomical changes, confirming the reasonable clinical usage. EP-1665 Library of plans approach for bladder cancer radiotherapy including a simultaneous integrated boost S. Nakhaee 1 , L. Hartgring 1 , M. Van der Burgt 1 , F. Pos 1 , P. Remeijer 1 1 Netherlands Cancer Institute Antoni van Leeuwenhoek Hospital, Radiotherpay, Amsterdam, The Netherlands Purpose or Objective With image guided radiotherapy the positioning of patient can be corrected accurately by a table shift after a registration procedure. However, for large deformations of the target area, for example due to inter-fractional changes in bladder filling, table shift might not fully compensate the variation. Compared to full bladder treatments, the need for accuracy in dose delivery is even more profound for bladder patients receiving simultaneously increased dose to the gross tumor volume (GTV). A daily plan selection from a library of plans is a strategy to tackle this challenge. With this approach, a number of radiation treatment plans are made for a set of anticipated shapes and positions of the target prior to treatment. At every fraction the most suitable plan can then be selected. The purpose of this study was to develop an interpolation method to generate a library of plans for bladder treatments with a combined target of the total bladder and the GTV. Material and Methods

Conclusion We have developed a robust, quick and straightforward method to generate a library of plans for a combined bladder CTV and GTV using delineations of full and empty bladder CTs. The method is able to generate plans at every cm from full bladder. EP-1666 Adaptive radiotherapy in prostate cancer: when and why? R. Muelas 1 , R. García 2 , L. Vidueira 2 , J. Bonaque 2 , A. Conde 1 , C. Ferrer 1 1 Consorcio Hospitalario Provincial de Castellón, Oncología Radioterápica, Castellón, Spain 2 Consorcio Hospitalario Provincial de Castellón, Radiofísica y protección radiológica, Castellón, Spain