ESTRO 36 Abstract Book

S448 ESTRO 36 2017 _______________________________________________________________________________________________

Sweden) for both initial treatment planning and online plan adaptation. Next to the presence of a magnetic field, also several MRL-specific beam and collimator properties need to be taken into account that could influence plan quality. Our aim was to investigate the influence of MRL- specific characteristics on plan quality for rectum cancer and benchmark MRL plans against current clinical practice. Material and Methods Eight rectum cancer patients treated on a conventional CBCT-based linac (25 x 2.0 Gy) were included in this retrospective study. For each patient, the clinically acquired planning CT, delineated structures and treatment plan generated with Pinnacle 3 (dual-arc VMAT, 10MV, collimator 20°, SAD: 100.0 cm) were available. The same CT and structure set were used to create two MRL treatment plans with Monaco: one plan with (MRL + ) and one plan without (MRL – ) the presence of a 1.5 T magnetic field. Both MRL plans were created using a 7-beam IMRT technique incorporating MRL-specific properties (7MV, collimator fixed at 90°, FFF, SAD: 143.5 cm). Plan optimization was based on a class solution and objective values were individually optimized. Also, a quasi MRL plan was generated with Pinnacle 3 using a 7-beam IMRT technique and comparable MRL properties (6MV, collimator 90°, FFF, SAD: 143.5 cm). After rescaling (PTV V 95% = 99.2%), plans were accepted when the clinical acceptance criterion was fulfilled (PTV D 1% < 107%). Quality differences between MRL + , MRL – and quasi MRL plans were assessed by calculating PTV D mean , PTV D 1% , bowel D mean and bladder D mean . Also, D mean and D 1% to the patient excluding PTV 2cm (i.e. PTV + 2.0 cm) were determined. All MRL plans were benchmarked against the clinically delivered treatment plans and tested for significance (Wilcoxon signed-rank test). Results All MRL plans were clinical acceptable after rescaling. Figure 1 shows an example of dose distributions for the MRL plans and the clinical plan of one patient. The 7-beam IMRT technique used for all MRL plans resulted in a minor decrease in plan homogeneity, indicated by an increased PTV D mean (Table 1). Also, all MRL plans showed a significant increase in D mean for the bladder, bowel and body compared to clinical practice. However, the clinical relevance of these differences is expected to be limited. Given the similar quality of MRL – and quasi MRL plans, differences between MRL + plans and clinical practice are mainly induced by the MRL-specific properties. The small difference between MRL + and MRL – plans indicated limited influence of the magnetic field on plan quality.

Conclusion This study demonstrates the ability of creating high- quality MRL treatment plans for rectum cancer. Given the differences in machine characteristics, some plan quality differences were found between MRL treatment plans and current clinical practice. These results support a well- prepared clinical introduction of the MRL. PO-0839 Personalized VMAT optimization for pancreatic SBRT I. Mihaylov 1 , L. Portelance 1 1 University of Miami, Radiation Oncology, Miami, USA Purpose or Objective Inverse IMRT planning is a very labor intensive, trial-and- error process, aiming to find a middle ground between the conflicting objectives of adequate tumor coverage and sparing nearby healthy tissues. Even if a plan is clinically acceptable, that plan is unlikely to be the best solution, where the healthy tissue is spared as much as possible. To a large extent the optimization process is user and treatment planning system specific, where more experienced users generate better quality radiotherapy plans. This work introduces a fully automated inverse optimization approach and its application to pancreatic SBRT. Material and Methods Ten cases, treated breath-hold, were retrospectively studied. The outlined anatomical structures consisted of a PTV, and OARs including duodenum, stomach, bowel, spinal cord, liver, and kidneys. In each case the prescription was set to 35 Gy (to 95% of the PTV) in 5 fractions. The treatment plans were created by experienced dosimetrists, following national and international clinical protocols. Those treatment plans were generated for VMAT delivery. For each case an additional plan was generated with the newly proposed automated inverse optimization. This optimization is based on unattended step-wise reduction of DVHs, where several DVH objectives were specified for each OAR. The automated plans utilized the same number of arcs, with the same parameters as the treatment plans. The treatment and the automated plans (Treatment and Auto hereafter) were compared on commonly used clinical dosimetric parameters. Those parameters included D PTV 95% (dose to 95% of the PTV), D Duodenum 1% , D Bowel 1% , D Stomach 1% , D Cord 1% , D Liver mean , D rt_kidney mean , and D lt_kidney mean . The doses to 1% of the volumes of duodenum, bowel, stomach, and spinal cord were used as surrogates for maximum doses. The prescriptions for the Auto plans matched the prescriptions of the Treatment plans. Results The first row in the table below summarizes the average values of the tallied quantities (over the ten patients) as derived from the treatment plans. The second row outlines the average differences (in per-cent) between the dosimetric endpoints as well as the range of the differences between the Treatment and the Auto- optimized plans. The negative differences indicate that the Auto plans result in lower absolute doses and vice-