ESTRO 2021 Abstract Book

S355

ESTRO 2021

Results Only R:1/L:7 automatic plans were unacceptable in terms of PTV coverage and/or right-lung and heart constraints. After manually refinement of the start/stop angles shifting them by 5°/10°, KB-ViTAT automatic plans were acceptable for all cases. Overall, KB-ViTAT plans were equivalent to TF plans (Fig.1). PTV coverage was similar; PTV D1% was improved by R:0.44/L:0.15 Gy (p<0.05); ipsilateral lung mean dose (Dm) was similar (p>0.05 R:6.8/L:5.7 Gy for both TF and ViTAT) with a slight improvement/worsening in the 20-35 Gy/2-15 Gy range, respectively. In L-sided plans, heart showed the same behaviour in the range 15-35Gy/2-10 Gy with a Dm of 2.7 Gy (p>0.05). On average, KB-ViTAT better spared contralateral OARs: Dm of R-heart, lung, breast was 0.1 Gy lower (p<0.05) than TF and integral dose was 5% lower (p<0.05) than TF for both R and L sided plans.

Conclusion Fully automatic plan optimization of ViTAT can replace the manually optimized TF technique. The KB-ViTAT approach was able to generate automatic plans comparable to TF or slightly better in terms of PTV dose distribution and Dm to body and contralateral OARs. Slight worsening concerns only 2-15 Gy range for the ipsilateral lung and “L-side” hearth, although without any relevant clinically increase of Dm. In R:3%/L:23% the standard KB-ViTAT setup required a manual adjustment of starting angles. The technique has relevant potentials for large-scale replacement of manually optimized TF for whole breast irradiation. Acknowledge: The study is supported by an AIRC grant (IG23150). OC-0469 inter-institute variability of kb-models for whole breast irradiation with tangential field A. Tudda 1 , R. castriconi 1 , E. cagni 2 , G. benecchi 3 , F. dusi 4 , P. esposito 1 , M. guernieri 5 , A. ianiro 6 , V. landoni 6 , A. mazzilli 7 , E. moretti 5 , L. placidi 8 , V. trojani 2 , A. scaggion 9 , C. fiorino 1 1 IRCCS San Raffaele Scientific Institute, Medical Physics, Milano, Italy; 2 Departement of Advanced Technology, IRCCS Reggio Emilia, Medical Physics, Reggio Emilia, Italy; 3 University Hospital of Parma AOUP, Medical Physics, Parma, Italy; 4 IRCCS Veneto Institute of Technology IOV , Medical Physics, Padova, Italy; 5 University Hospital of Udine, Medical Physics, Udine, Italy; 6 IRCCS Istituto Nazionale dei Tumori Regina Elena , Medical Physics, Roma, Italy; 7 University Hospital of Parma, Medical Physics, Parma, Italy; 8 Foundation University Hospital A. Gemelli IRCCS, Medical Physics, Roma, Italy; 9 IRCCS Veneto Institute of Technology IOV, Medical Physics, Padova, Italy Purpose or Objective Whole breast (WB) irradiation with tangential fields (TF) is a still largely used technique in post-operative radiotherapy of breast cancer. Knowledge-based (KB) plan prediction offers large opportunities in efficiently replacing manual planning, time-consuming and prone to inter-planner variability and sub-optimal planning. The expected, relatively high, consistency in contouring may translate into models usable on a large scale. First step of this process concerns the evaluation of inter institute model’s variability. Materials and Methods Seven Institutions delivering WB according to national guidelines with TF manually optimized using few segments per field (1-4, with/without wedges) joined a national funded project. The first aim was to model right breast TF plans by using RapidPlan (Varian Inc.), installed in each of the participating centers. Methods to generate the models, including minimum number of pts (>70), “fake”-arc geometry definition, outlier