ESTRO 37 Abstract book

S155

ESTRO 37

Material and Methods Eight patients affected by locally advanced pancreatic cancer were considered. GTV, stomach, duodenum, liver, bowel bag, spinal cord and kidneys were contoured on the simulation CT scan by two radiation oncologists. CTV was considered equal to GTV. PTV was generated from CTV, adding an isotropic 3 mm margin and excluding areas overlapping with OARs. IMRT plans with 20 equal distant angular beams were elaborated for the MR-Linac, while dual arc VMAT plans were optimised for the SBRT Linac. Prescription dose was 40Gy in 5 fractions (normalisation= 40Gy at 50% of PTV). All plans had to strictly respect the following OARs constraints: V33Gy<1cc and V25Gy<20cc for duodenum, stomach and bowel bag, V12Gy<50% for liver and kidneys, V25Gy for spinal cord. The dosimetric comparison evaluated the target coverage (D98 for CTV and V95 for PTV), dose homogeneity (HI) according to ICRU 83, conformity index (CI) according to ICRU 62 and dose diffusion (V10Gy, V20Gy). Statistical significance was estimated calculating the Wilcoxon Mann Whitney test for paired samples. Results Seven plans were considered clinical acceptable (V95 PTV>95%). No statistical significance was observed for D98 CTV (p-value =0.13) and V95 PTV (p=0.11) (see Figure 1).

VMAT clin , which was borderline significant, p=0.06. In contrast, the side-by-side scoring showed a difference in plan quality of +18.1 ± 41, pointing at an enhanced quality for VMAT auto , which was statistically significant (p<0.001) and in line with DVH comparisons between VMAT auto and VMAT clin . The large difference in standard deviations (above: 12 vs. 41) shows that in the side-by- side comparisons, the clinicians were much more convinced of plan quality differences. Figure 1 shows the overall poor correlation between separate and side-by- side plan scoring (R 2 =0.22), with clinician-specific R 2 - values of 0.02, 0.12, 0.27, 0.33 and 0.75.

V105 values were always inferior to 2% for both CTV and PTV in all cases analysed. No significant difference was observed for OARs irradiation, dose constraints were always met. The analysis of dose diffusion is reported in Fig.2.

Conclusion Large inconsistencies were observed in clinicians‘ plan evaluations. Such evaluations may often not prevent treatment with a suboptimal plan. Automated planning and/or automated plan QA are needed to better guarantee high quality treatment. OC-0300 Linac MRI guided SBRT treatment in pancreatic cancer: dosimetric evaluation of a new technology D. Cusumano 1 , S. Menna 1 , L. Boldrini 2 , S. Teodoli 1 , E. Placidi 1 , G. Chiloiro 2 , L. Placidi 1 , F. Greco 1 , G. Stimato 1 , F. Cellini 2 , V. Valentini 2 , M. De Spirito 1 , L. Azario 1 1 Fondazione Policlinico Universitario A.Gemelli, Unità Operativa Complessa di Fisica Sanitaria, Roma, Italy 2 Fondazione Policlinico Universitario A.Gemelli, Area di Radioterapia Oncologica, Roma, Italy Purpose or Objective Hypofractionated RT today plays a relevant role in different moments of the clinical management of pancreatic cancer patients. MR guided adaptive RT (MR- ART) is considered a new promising resource in this context, as it allows to online modify the dose distribution according to therapy volumes position of the day. Aim of this in silico study is to evaluate the dosimetric performance of the first 6MV MR-Linac authorised for clinical treatments (MRIdian Linac, ViewRay) in case of SBRT pancreas treatments. The results have been compared with those obtained with a 6MV Linac specifically designed for SBRT treatments (TrueBeam Edge, Varian).

In six cases the low dose values are more spread in the MR-Linac plans, even if no statistical significance was observed. The plan quality indicators are also comparable: the CI mean value calculated for PTV was 1.21±0.16 for MR- Linac and 1.17±0.7 for SBRT Linac. The PTV HI mean value was 0.10±0.03 for MR-Linac, 0.07±0.04 for SBRT Linac. Conclusion The dosimetric performance between MR and SBRT Linac are comparable for SBRT pancreas treatment. Plans calculated with SBRT Linac show less dose diffusion, probably due to the different optimisation software, that is more complex but also slower in processing in comparison to MR-Linac one (optimisation mean time is 2 min for MR-Linac, 20 min for SBRT Linac). Considering the MR-ART need to optimise the treatment plan while the patient is on the couch, MRIdian Linac allows to elaborate SBRT plans clinically and dosimetrically satisfying.