ESTRO 38 Abstract book

S223 ESTRO 38

the manual plans. The clinical protocol was leading in both manual and automated plan generation and evaluation. For the healthy tissues the goal was to maximally reduce the mean dose in a composite OAR consisting of small- bowel-bag + bladder excluding overlaps with the PTV. For consistent plan comparisons based on the OAR sparing, all plans were rescaled such that 95% of the PTV received 99% of the prescribed dose, as required by the clinical protocol. The two-sided paired Wilcoxon signed rank test was applied for statistical analyses. Results Differences in plan quality are presented in Table 1. With equal PTV coverage and PTV D 99% , automated plans had favorable V 107% and homogenity index (HI). Compared to the manual plans, the OAR D mean was reduced in the automated plans by 1.2 Gy (6.4%) on average and a maximum reduction of 4.2 Gy (21.3%). A large reduction in planning time was obtained, going from ~4-6 hours per patient for manual plans (mainly hands-on time) to ~1-2 hours per patient (mainly computational time).

Conclusion The novel method for automatic configuration of the LRPM resulted in clinically favourable Pareto-optimal treatment plans. Due to the vast reduction in configuration workload, a major obstacle for introducing large-scale automated multi-criterial planning with the LRPM has been overcome. PV-0426 First system for fully automated multi- criterial planning for an MR-Linac applied to rectal cancer R. Bijman 1 , L. Rossi 1 , T. Janssen 2 , P. De Ruijter 2 , C. Carbaat 2 , B. Van Triest 2 , S. Breedveld 1 , J.J. Sonke 2 , B. Heijmen 1 1 Erasmus MC Cancer Center, Radiation Oncology, Rotterdam, the Netherlands ; 2 The Netherlands Cancer Institute, Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective High-quality Pareto-optimal treatment plans are mandatory for MR-Linac (MRL) treatment units to maximally benefit from the offered high geometrical accuracy. In this study we have adapted our system for fully automated multi-criterial planning on regular linacs for plan generation on a 1.5T MRL. We evaluated automated MRL planning for rectal cancer by comparing automatically and manually generated plans regarding planning workload and plan quality. Material and Methods A fully automated planning workflow for generation of clinically deliverable Pareto-optimal plans was implemented by coupling the in-house Erasmus-iCycle optimizer to a dedicated version of the Monaco TPS (Elekta AB, Sweden). The integrated system can handle the dosimetric impact of the magnetic field, enlarged source- isoc. distance, etc. 15 rectal cancer patients were manually planned for a 9-beam Step and Shoot IMRT treatment, delivering 50 Gy in 25 fractions. The 9 fixed beam directions were selected to avoid irradiation through the MRL cryostat pipe and the high attenuation regions of the MRL treatment couch. Automated plans were generated with the same beam directions as used for

Conclusion A system for fully automated multi-criterial planning for a high magnetic field MR-Linac has been developed and tested for rectal cancer patients. Plan quality of automated plans was better with an improvement in OAR dose. Automated planning resulted in a major reduction in manual planning workload. PV-0427 Improving cumulative dose evaluation for re- irradiation: first results from the STRIDeR project L. Murray 1,2 , S. Gregory 3 , M. Nix 3 , M. Aldred 3 , L. Aspin 3 , J. Uzan 4 , J. Lilley 3 , B. Al-Qaisieh 3 , A. Appelt 1,3 1 University of Leeds, Leeds Institute of Cancer and Pathology, Leeds, United Kingdom ; 2 Leeds Cancer Centre, Department of Clinical Oncology, Leeds, United Kingdom; 3 Leeds Cancer Centre, Department of Medical Physics, Leeds, United Kingdom ; 4 Raysearch Laboratories, Service Department, Stockholm, Sweden Purpose or Objective Accurately evaluating cumulative doses in the re- irradiation (reRT) setting is challenging due to anatomical and positional changes between radiotherapy courses. Furthermore, treatment planning systems (TPS) do not routinely incorporate radiobiology (e.g. fraction size correction) into dose summation. The STRIDeR ( S upport T ool for R e- I rradiation De cisions guided by R adiobiology) project aims to develop a software tool for use in a commercially available TPS to address these issues and so facilitate more informed reRT. We evaluated three approaches to dose summation, incorporating anatomical change and radiobiology to differing extents. Material and Methods Ten patients who received pelvic reRT were included. All originally had conventionally fractionated radiotherapy. ReRT was Stereotactic Ablative Radiotherapy (SABR, 30Gy/5 fractions) to oligometastatic nodal disease in the previously irradiated pelvis. For clinical reRT planning, permitted doses to organs at risk (OAR) were at the