ESTRO 2020 Abstract book

S819 ESTRO 2020

with normally distributed motion amplitudes with a standard deviation (σ) of 5 mm (and limited to 3σ). Fraction doses are sampled into a treatment course scenario and accumulated onto the planning CT using deformable image registrations between the simulated CTs and the planning CT. Worst-case robust optimization is subsequently performed over 1000 treatment course scenarios. To assess the performance of plans created with the new robust optimization method, they are compared to traditional PTV plans with increasing margins in steps of 1 mm between 5 and 15 mm. The plans are evaluated for three prostate patients treated with seven-beam SMLC plans. The plans were optimized with a constraint on the prescription dose (77 Gy in 35 fractions) to 100% of the target (CTV for robustly optimized plans; PTV for PTV plans). Evaluation is performed in an adapted version of the Robust evaluation module of RayStation. The evaluation is fully independent from the robust optimization taking 125 simulated CT images (randomly sampled organ motion amplitudes from a Gaussian distribution with σ = 5 mm), and creating 200 evaluation scenarios by accumulating deformed doses on the planning CT. Results For each patient case, the PTV plan that achieves equivalent target coverage as the robustly optimized plan, is used for comparison with respect to clinical goals. This results in a margin of 9 mm in two cases and 7 mm in one case. For all patients, the doses to organs at risks are highly reduced for the robustly optimized plan as compared to the PTV plans, as is exemplified for one of the patients in Figure 1. The reduction of dose to healthy tissue comes at the expense of a higher maximum dose in the target. The margins in the compared PTV plans are higher than normally used in clinical practice and the dose to surrounding tissue could be reduced with smaller margins. However, with reduced margins, the target coverage would be compromised during the course of treatment, and it is likely that adaptive replanning would be needed, which would not be the case for the robustly optimized plans.

Comparison of a robustly optimized plan (a) and a PTV plan with 9 mm margin (b). Conclusion Robust optimization based on accumulated dose over the full treatment course provides excellent target coverage for the investigated prostate patients, while lowering the dose to organs at risk by a substantial amount and limiting the need for adaptive replanning. PO-1445 A feasibility study: can a vol/dose model in HNC standardize plans and optimize planning time? P. Ferrari 1 , H. Stefan 1 , M. Haller 1 , J.M. Waskiewicz 2 , S. Bou Selman 2 , M. Rosa 2 , M. Maffei 2 1 Azienda Sanitaria dell'Alto Adige, Servizio di fisica sanitaria, Bolzano, Italy ; 2 Azienda Sanitaria dell'Alto Adige, Servizio di radioterapia oncologica, Bolzano, Italy Purpose or Objective To assess the feasibility of an automatic planning (AP) strategy, based on python script implemented in our treatment planning system (TPS), that minimize planning time reducing parotid glands toxicity on Head Neck Cancer (HNC) radiotherapy treatment planning. Material and Methods 15 patient underwent VMAT treatment planning techniques using simultaneous integrated boost (SIB) in head neck cancer (HNC), were retrospectively analyzed. The mean dose of parotid glands were correlated with fraction of volume outside PTV. Plots described two areas, green and red as in figures. In the green area were represented plans with total mean dose of parotid glands that respect the constrains and in red area plans that were unable to respect the constrains (Merlotti et al. Radiation Oncology 2014). Considering the middle area between the green and the red one, were possible to fit data linearly. The linear model was implemented in a python script integrated in our TPS as priority settings for new treatment plans: with Volume% of parotids outside PTV and with eq.1 we calculated the mean dose limit for parotid gland outside PTV. 6 arcs were implemented in order to avoid shoulders (from arc angle 186° to 235°, 270° to 90°, 125° to 174°, clockwise and counterclockwise), help structure as spine expanded of 5 cm subtracted with PTV low dose expanded of 0.6 cm was defined to minimize low dose distribution outside PTV and around the spine. Planning time was measured and compared with manual planning time. Results The linear dose/volume models obtained for parotid glands, was eq.1 y=4103.2*x - 1666.6 R2=0.7519. Equation