ESTRO 2021 Abstract Book

S1585

ESTRO 2021

(v5.51.2, Elekta AB, Stockholm, Sweden) based on the Elekta Agility MLC. The dose prescription was 77 Gy and 59 Gy in 35 fractions to the PTV-HD and PTV-LD, respectively, normalized to the median dose of 77 Gy for the PTV-HD. Plan evaluations were performed based on our internal protocol. Furthermore, monitor units, number of volumes and objective functions used in the optimization were also compared. Paired t-tests were used to assess differences between the two approaches, considering p<0.05 as significant. Results Both optimizations resulted in clinically acceptable plans. Targets, bladder and femoral heads showed comparable results. However, for rectum, a majority of DVH parameters showed a statistically significant reduction with the gEUD approach (see figure and table) decreasing the mean rectum dose by 6.7 Gy (range: 2.6-13.0) (p<0.01). Conformity and dose homogeneity indices for the PTV-HD showed no difference between physical and biological optimizations. Mean monitor units were 639.4 (574.8 – 784.4) and 596.7 (503.0 – 696.2) (p=0.33) for DVH and gEUD based plans, respectively. For physical DVH approach, 12 volumes and a total of 23 objective functions were used, in contrast to biological optimization, where only 7 volumes and 14 objectives were required.

Table 1: Comparison of DVH parameters between the two planning approaches. Significant differences indicated in bold

Figure 1: Median DVH of physical (blue) vs. biological (red) plans of the cohort.

Conclusion Biological gEUD-based optimization achieved significant rectum sparing as compared to the physical DVH optimization approach, with comparable target coverage and monitor units, but fewer optimization volumes and objectives, providing a suitable class solution for the treatment planning optimization of intermediate risk prostate cancer using VMAT. PO-1861 Evaluation of VMAT-like planning technique for magnetic resonance guided radiotherapy treatment L. Placidi 1 , M. Nardini 1 , D. Cusumano 1 , L. Boldrini 1 , G. Chiloiro 1 , A. Romano 1 , C. Votta 1 , M.V. Antonelli 1 , V. Valentini 1 , L. Indovina 1 1 Fondazione Policlinico Universitario A. Gemelli IRCCS, Radiation Oncology, Rome, Italy Purpose or Objective Volumetric modulated arc therapy (VMAT) or other dynamic delivery techniques, is currently not applicable to Magnetic Resonance-guided radiotherapy (MRgRT), due to the production of artifacts during the online imaging acquisition, caused by the motion of ferromagnetic material in a magnetic field. Significantly increasing the number of step-and-shoot IMRT beams can lead to deliver a discretized VMAT, with significant advantages in terms of dose conformality, generating a so-called “VMAT-like” (VML) treatment. The aim of this study is to evaluate and quantify the dosimetric and planning advantages of the VML delivery technique in order to further improve MRgRT irradiation workflow. Materials and Methods Three consecutive phases have identified for this in silico study: firstly, planning and dosimetric validation of the VML treatment on 10 different diseases in terms of site, tumour size, dose prescription, and irradiation setting, keeping the same optimisation parameters of the IMRT plans delivered in clinical practice. Phase 2 consists in the evaluation of 10 re- optimised VML plans in case of patients affected by locally advanced pancreas cancer (LAPC), as one of the most