ESTRO 2023 - Abstract Book

S1684

Digital Posters

ESTRO 2023

PO-1930 The potential benefit of adaptive MR-Linac radiotherapy over proton and photon for rectal cancer

A. Christou 1 , D. Tilly 2 , N. Tilly 2

1 Skandion Clinic, Department of Medical Radiation Physics, Uppsala, Sweden; 2 Uppsala University, Department of Immunology, Genetics and Pathology, Uppsala, Sweden Purpose or Objective The aim of this work is to perform comparative treatment planning to investigate the potential dosimetric benefits of daily adaptive treatments, utilizing step-and-shoot MR-Linac (MRL) plans, over current standard non-adaptive treatment plans, conventional VMAT photons and scanned proton beams, for rectal cancer patients receiving a short-course of radiotherapy (5Gy x 5 fractions). Materials and Methods Anonymised data from twelve female rectal cancer patients treated at Uppsala University Hospital (UUH) with conventional VMAT photon plans were utilised. One reference plan per patient was created for proton and VMAT plans, while three reference plans were created for MRL each with a different PTV margin. The PTV A was a 7 mm isotropic margin coinciding with the current clinical practice for both MRL and conventional Linac treatments. The PTV B was 6 mm cranio-caudal, 6 mm anterior, 4 mm posterior, 4 mm left-right and the PTV C was 3 mm cranio-caudal, 3 mm anterior, 2 mm posterior, 2 mm left-right. The proton plans were created according to the PRORECT clinical study and the VMAT plans to the clinical practice at UUH. Moreover, for all proton and MRL plans the electron density of rectal gas was overridden to the value of the surrounding tissue. To study the effect on the delivered dose by the daily anatomical changes, synthetic CTs were created from the daily CBCTs through image registration and post processing techniques. The proton and VMAT reference plans were then recalculated, whereas the MRL plans were adapted on the daily synthetic CTs. We evaluated how many fractions failed to fulfil the target and OARs constraints as well as the potential dosimetric benefits of the daily adaption. Dose was calculated as dose to water since it affected the evaluation of the dose to the sacrum bone. A qualitative evaluation of the daily anatomy variation based on the synthetic CTs was performed using the Dice similarity coefficient. Results The analysis of the fraction plans showed that every MRL plans fulfilled the target and OAR constraints in contrast to the proton and VMAT plans that failed to fulfil the target constraints for some of the fractions. The VMAT fraction plans also failed to fulfil the constraints in the OAR in 60% of the cases. The dosimetric comparison of the three MRL plans to the VMAT plans showed that although there is a decrease in dose to the bowel bag for MRL PTV A, the doses to both the bladder and the sacrum were increased. A decrease in dose to the OARs is present in the reduced PTV margin plans, although the OARs in the proton plans received the lowest dose compared to the other two modalities. Future work will show if the results are sensitive to the synthetic CT quality and if so, how much. Conclusion This study shows that daily adaptive radiotherapy using the MRL delivers full target coverage and that OAR doses may be reduced by using reduced PTV margins. 1 National Institute of Oncology, Radiotherapy Center, Budapest, Hungary; 2 Semmelweis University, Department of Oncology, Budapest, Hungary; 3 Budapest University of Technology and Economics, The Institute of Nuclear Techniques, Budapest, Hungary Purpose or Objective Ethos online adaptation uses deformed planning CT based on acquired CBCT image to calculate the dose of the online adapted treatment plan. Thus, non-deformation based changes in anatomy such as body cavities filled with material of different density daily can cause differences in calculated and delivered dose. Our aim was to investigate the effect of appearance or disappearance of gas in body lumens on dose calculation using Ethos Treatment Planning 1.1 system (dose to medium). Materials and Methods Cirs Thorax IMRT phantom was scanned with a Siemens Somatom Go.Sim CT for planning purposes. 5 cm length of central inserts with a uniform extension of 5 mm were used as target structure (insert 3) and organs at risk in the proximity of the target (insert 1,2,4,5)(Figure 1.). Scanning the phantom with removed insert simulated the appearance of air either in the target or in a nearby healthy organ (insert 5). Treatment plans were generated with 9-field, 12-field IMRT and 2-Arc VMAT. The same optimization goals were used for each arrangement, and the original plans were recalculated on the CT images using inserts with different filling to examine the change of target coverage (V95% and V100%), dose maximum, mean dose and OAR doses (V50% and mean dose). We delivered these plans replacing only a single insert during online adaptation. Ethos used a simulation CT for dose calculation with or without air as on the original planning CT and not with removed or added air seen on CBCT. Thus, we could consider the difference in doses calculated on CT with or without air filling as the difference of calculated and delivered dose for the adapted fraction in this theoretical set-up. PO-1931 Effect of tissue density variation on adaptive radiation treatment planning D. Szegedi 1 , Á. Gáldi 1 , T. Major 1,2 , C. Pesznyák 1,3