ESTRO 2022 - Abstract Book

S393

Abstract book

ESTRO 2022

Conclusion Real-time dosimetry during PDR BT treatments has been performed in a large cohort, showing good agreement with the expected dose for the MVC. The larger deviation seen with TR is expected to originate from positional offsets of the detector. The measured dose rates enabled a more thorough investigation of the treatment, including determining positional offsets in the detector position. Correcting for detector position based on the irradiation of the first source channel increased the reliability of the subsequent dose rate comparison. The next step is to test this method in the full cohort including TR+needle treatments.

Proffered Papers: Optimisation & algorithms in proton & ion radiotherapy

OC-0449 Uncertainty analysis shows equivalence of PTV-based VMAT and robust IMPT for model-based selection

J. Rojo Santiago 1,2 , E. Korevaar 3 , Z. Perkó 4 , S. Both 3 , S.J. Habraken 1,2 , M.S. Hoogeman 1,2

1 Erasmus MC Cancer Institute, Radiotherapy, Rotterdam, The Netherlands; 2 HollandPTC, Medical Physics & Informatics, Delft, The Netherlands; 3 University Medical Center Groningen, Radiation Oncology, Groningen, The Netherlands; 4 Delft University of Technology, Radiation Science, Delft, The Netherlands Purpose or Objective In the Netherlands, head-and-neck cancer (HNC) patients are referred for proton therapy through model-based selection. For each patient, a photon (VMAT) and a proton (IMPT) plan are made. The plans are compared in terms of normal tissue complication probabilities (NTCPs) for grade II and III xerostomia and dysphagia. However, the differences between the modalities may impact CTV dose and NTCPs. Our aim is to assess: (i) the consistency and robustness of CTV dose for VMAT and IMPT and (ii) the sensitivity of NTCPs to beam and patient alignment errors (geometric) and, for IMPT, stopping-power prediction errors (range). Materials and Methods Thirty oropharyngeal HNC patients, treated to 70 Gy(RBE) and 54.25 Gy(RBE) for the primary and elective CTVs respectively, were included. Clinical VMAT and IMPT plans for all patients were available from the plan comparison. A 3mm PTV margin and 3mm/3% geometric and range robustness settings were used for VMAT and IMPT planning, respectively. For VMAT, dose was prescribed to the PTV-D 98%,PTV ≥ 95%D pres , while, for IMPT, it was prescribed to the voxel-wise minimum dose of the 28 clinical robustness evaluation scenarios: VWmin-D 98%,CTV ≥ 94%D pres . Polynomial chaos expansion (PCE) was applied to generate a fast patient- and plan-specific model of voxel doses. PCE enabled a robustness evaluation of 100,000 error scenarios for each plan. Systematic and random geometric errors were sampled from Gaussian distributions with errors (1SD) of Σ = 0.94mm and σ = 1.14mm, consistent with a M = 2.5 Σ +0.7 σ = 3mm margin based on van Herk's recipe. A systematic range error of 1.5% (1SD) from literature was used, in line with the 3% range setting. For each patient and each plan, the PCE model was used to calculate the median and the 5 th and 95 th percentiles of the D 98% to both CTVs and the NTCPs. Zero baseline toxicities were assumed for all patients. Results Figure 1 shows a correlation plot of the D 98% to both CTVs with VMAT and IMPT planning. For IMPT, the population median D 98% was 69.1 (range 68.5-69.4) GyRBE and 53.6 (range 53.3-53.8) GyRBE for the primary and elective CTVs. For VMAT, values of 68.7 (68.4-68.9) Gy and 53.2 (52.9-53.3) Gy were found respectively. Figure 2 shows similar NTCP spreads for the grade II and III xerostomia and dysphagia in both modalities. A median NTCP spread of 2.39 (1.16-2.93) p.p. and 2.47 (0.28- 3.86) p.p. for IMPT and 1.86 (1.28-2.33) p.p. and 2.37 (0.90-3.46) p.p. for VMAT were found for grade II xerostomia and