ESTRO 2023 - Abstract Book

S219

Saturday 13 May

ESTRO 2023

Figure 2. The average of all 10 patients’ percentage difference in mean dose; when calculated between both sCTD and IndD, and sCTD and PopD. The average dose difference calculated for each organ is presented, with error bars representing the standard deviation found between all patients.

Conclusion Given the comparable dose differences observed between PopD and IndD (current clinical method), using a population- based ED was deemed feasible in an MRL workflow. References: 1. Lui et al. Precis Radiat Oncol. 2022; 6:75-84 2. Karotki et al. J Appl Clin Med Phys. 2011;12:97-104 3. Hsu et al. Phys Med Biol. 2018;63:155003 4. Prior et al. Phys Med Biol. 2016;61:3819-3842

Proffered Papers: Proton treatment planning

OC-0284 Multi-center analysis shows inconsistency of the Dutch robustness protocol for Head and Neck IMPT J. Rojo Santiago 1,2 , S.J. Habraken 1,2 , M. Unipan 3 , S. Both 4 , G. Bosmans 3 , Z. Perkó 5 , E. Korevaar 4 , M.S. Hoogeman 1,2 1 Erasmus MC Cancer Institute, Department of Radiotherapy, Rotterdam, The Netherlands; 2 HollandPTC, Department of Medical Physics & Informatics, Delft, The Netherlands; 3 Maastricht University Medical Center, Department of Radiation Oncology (Maastro), Maastricht, The Netherlands; 4 University Medical Center Groningen, Department of Radiation Oncology, Groningen, The Netherlands; 5 Delft University of Technology, Department of Radiation Science and Technology, Delft, The Netherlands Purpose or Objective In the Netherlands, the Dutch Proton Therapy group has a standardized plan evaluation protocol to assess robustness of CTV dose for intensity-modulated proton therapy (IMPT). However, its performance to ensure adequate CTV dose might be impacted by (i) differences in dose prescription and treatment planning and (ii) differences between the treatment delivery machines (UMCG/IBA, HPTC/Varian, Maastro/Mevion). Our aim is to assess inter-patient and inter-institutional variation in terms of CTV dose, for a cohort of head-and-neck cancer (HNC) patients treated in the three Dutch proton centers. Materials and Methods Sixty clinical IMPT HNC plans were included, divided in subcohorts of 20 plans per center. Patients were treated to 70 GyRBE and 54.25 GyRBE for the primary and elective CTVs respectively, using 3mm/3% setup/range robustness (SR/RR) settings. Dose was prescribed to different prescription dose levels (L) per center, to the voxel-wise minimum dose of 28 evaluation scenarios: VWmin-D98%,CTVs ≥ L (%) Dpres (Table I). Polynomial Chaos Expansion (PCE) was applied to generate a fast patient- and plan-specific model of voxel doses, enabling a probabilistic robustness evaluation of 100,000 simulated treatment courses for each plan. Setup systematic ( Σ ) and random ( σ ) errors were sampled from Gaussian distributions with errors (1SD) of (i) Σ =0.92mm and σ =1.00mm and, consistent with a 3mm margin based on van Herk’s recipe: M=2.5 Σ +0.7 σ . A systematic range error of 1.5% (1SD) from literature was used, in line with the 3% RR setting. For each plan, scenario D98%, D99.8%, D2%, D0.2% and V95% CTV distributions were determined from the simulated treatment courses. Based on a prior photon plan calibration, per center population D99.8% histograms were derived from the simulated treatment courses to probabilistically assess adequate CTV dose and, subsequently, to be compared to their prescribed near minimum dose. Results Figure 1 shows population D99.8%,CTVs dose histograms for each center. The other probabilistic dose metrics are summarized in Table I. For center 1 and 2, the 10th-90th percentile range of D99.8% was above their prescription dose levels, with the smallest inter-patient variation for the elective CTV in center 1. In contrast, the D99.8% values of center 3 were partly (primary CTV) or completely (elective CTV) below the 94%/93% prescription level used in that center. Compared to the prescribed VWmax-D2%, CTV dose inhomogeneity was consistent and smaller for center 1 and 2 according to their D0.2% values. For center 3, a higher D0.2% of 75.3 GyRBE was achieved on average, which exceeded the clinical prescribed VWmax-D2% value.