ESTRO 36 Abstract Book

S892 ESTRO 36 2017 _______________________________________________________________________________________________

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.

Figure 1: a) A schematic representation of the planning strategy applied in this study to reduce optimization times. b) Resulting dose distribution with isodoses (cGy). Results For 30 (91%) of the 33 cases no clinical dose constraints are violated in combination with sufficient PTV dose coverage. In the other 3 (9%) cases PTV coverage is reduced by 5.4 ± 3.0 % to meet all dose constraints of the OAR. The average time required for optimization is 158 ± 95 s. The estimated dose delivery time, as reported by Monaco, is 198 ± 32 s. This leads to a total average optimization and delivery times of 357 ± 124 s, which fits well within the proposed 30 minute time limit for treatment on the MR-linac. Both the optimization and delivery time are dependent on the volume of the PTV and increases with increasing PTV. The average PTV is 6.4 ± 5.1 cc (range, 1.8 – 28.3 cc). Conclusion We have shown that automated full-online replanning for the MR-linac to account for inter-fraction motion is feasible for SBRT of lymph node oligometastases. With the planning strategy as applied in this study we are able to automatically generate treatment plans, suitable for clinical use, within a timespan which is clinically acceptable for treatment on the MR-linac. EP-1664 Two-step verification of dose deformation in presence of large inter-fraction changes during LACC RT A. Gulyban 1 , M. Baiwir 1 , S. Nicolas 1 , M. Enescu 2 , V.P. Nguyen 1 , M. Gooding 2 , T. Kadir 2 , J. Hermesse 1 , V. Baart 1 , P.A. Coucke 1 , F. Lakosi 3 1 Liege University hospital, Department of Radiation Oncology, Liege, Belgium 2 Mirada Medical Ltd., Department of Research, Oxford, United Kingdom 3 University of Kaposvar, Health Science Center, Kaposvar, Hungary Purpose or Objective Dose accumulation is one of the most challenging parts of modern radiotherapy, especially in the presence of large inter-fraction motion. Determining actual dose to a given organ during external treatment of locally advanced cervical cancer (LACC) is one of the most prominent examples. In our current investigation we aimed to evaluate the residual dose deformation errors during the summation of dose for clinical target volume (CTV),

bladder and rectum. Material and Methods

Eleven LACC patients were included in this study treated between 06/2015 and 06/2016. Before each of the 25