ESTRO 36 Abstract Book

S905 ESTRO 36 _______________________________________________________________________________________________

Material and Methods Eleven LACC patients were included in this study treated between 06/2015 and 06/2016. Before each of the 25 treatment session, online corrected CBCT acquisition was performed (XVI 5.0, Elekta Ltd., Crawley, UK). Using the daily CBCTs the CTV, bladder and rectum were delineated (actual position), and the actual dose volume histogram (DVH_actual) was calculated using the reference dose matrix (rigidly transferred). For a topological co- registration a constraint-based deformation using Radial Basis Function with Robust Point Matching (RBF-RPM) was performed between the current and the reference position of each given organ using Mirada RTx (1.6.3, Mirada Medical ltd, Oxford, UK). Hausdorff-distance distributions (HDDs) from the reference volume towards the initial and deformed positions were assessed and the accuracy of the RBF-RPM deformation was evaluated. Further two DVHs were generated by deforming the dose matrix (transferred previously to the CBCT) in combination with the actual contour deformed (DVH_deformed) or with the reference delineation (DVH_reference). Differences between the relative DVHs were assessed in two steps: 1) the residual error of the deformation (DVH_actual vs. DVH_deformed) and 2) the volumetric mismatch sourced from the constraint-based RBF-RPM approximation (DVH_deformed vs. DVH_reference). Volume-specific confidence intervals were determined for the separated and combined steps. Results A total of 621 DVHs were generated. The HDDs (Figure 1, from reference) were reduced from the initial 30.5 mm (standard deviation, SD = 16.6) to a reasonably good 10.4 mm (SD = 6.4) confirming a good performance of the constraint-based RBF-RPM (Figure 2, bladder). The initial deformations were responsible for maximum of 3.8%/6.9% and 5.7% errors for CTV, bladder and rectum respectively, reaching a total combined maximum discrepancy of 4.6/7.2/6.2%. For CTV deviations are observed between 40-55 Gy, while fore bladder and rectum after 25 Gy errors can be seen. The interquartile errors remained within +/- 5% deviations for the entire dose range.

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