ESTRO 2020 Abstract Book

S903 ESTRO 2020

For γ mean OOT results were obtained for: 1,3% of IMRT and 7,35% of 3DCRT, 1,3% of IMRT and 6,25% of 3DCRT fields and for 6,25% IMRT, 32,5% 3DCRT and 6,25% IMRT, 27,5% 3DCRT fractions. Based on these results, the 2%, 2mm criteria have been chosen for further application. The procedure allowed to detect and correct set-up problems and did not yet result in re-planning. However, sensitivity of the IVD test allowed us to develop a protocol for ART, presented in fig. 1. Fig. 1. ART protocol algorithm.

based on clinically observed deformations as ground truth to investigate the dose accumulation uncertainty (DAU) in head-and-neck radiotherapy (HNRT). Material and Methods A planning CT ( pCT ), cone beam CT (CBCT) from week one of treatment and three later CBCTs were used to generate three corresponding in silico reference CBCTs serving as ground truth (GT) for 12 HNRT patients. To assess the DAU, the pCT was first matched to each GT CBCT using the same reference DIR algorithm (B-spline) to generate a deformed pCT ( dCT ). The treatment plan was then recalculated on the dCT and the resulting dose distribution mapped back to the pCT space using the backward deformation vector field (DVF) to produce a fraction-specific GT reconstructed dose (RD). The process was repeated using the DIR algorithm under investigation (demons) to generate a demons RD which was compared to the GT RD to calculate the voxel-specific dose error Δ tot . Inverse consistent c and inconsistent i voxels were identified by successive application of the forward and backward demons DVF, and comparing the net shift to the dose calculation grid size. The fraction-specific RD uncertainties for each structure S and both voxel distinctions (i.e., uSc and uSi ) were estimated using the 95% percentile range of Δ tot . The feasibility of incorporating the DAU as a confidence interval in dose-volume histograms (DVHs) of the delivered dose was demonstrated for various structures. For each structure, the voxel-specific uncertainties in the accumulated dose (AD) over all fractions uAS were first calculated by summing uSc or uSi in quadrature. Subsequently, the 95% level of confidence for each dose level bin in a DVH of the AD was calculated by averaging the uAS of all voxels with an AD equal to or larger than that dose level. Results Overall, the IC rate of voxels was 98.5% (Fig. 1) with uSc per fraction equal to [-2.3%; +2.1%], [-10.2%; +15.2%] and [-9.5%; +12.5%] relative to their planned dose for target structures, critical OARs and non-critical OARs, respectively. Inverse in consistent voxels generally showed a higher level of uncertainty. At the end of treatment, uAS was [-0.4%; +0.4%] for target structures. The impact of DAU is demonstrated in the DVH of Fig. 2.

Conclusion Detection by means of EPID IVD of set-up errors overlooked in daily IGRT indicates that basing on the sole IGRT protocol may be insufficient in ART procedure as important indications for re-planning may be missed in the heavy workload. Combining the existing protocol with EPID dosimetry provides powerful algorithm for the decision- making process. PO-1646 Evaluation of the feasibility of EPID-based in vivo dosimetry system for prostate cancer patients PO-1647 Quantifying the dose accumulation uncertainty after DIR in head-and-neck radiotherapy N. Lowther 1 , S. Marsh 2 , R. Louwe 1 1 Wellington Blood & Cancer Centre, Department of Radiation Oncology, Wellington, New Zealand ; 2 University of Canterbury, School of Physical and Chemical Sciences, Christchurch, New Zealand Purpose or Objective Deformable image registration (DIR) facilitated dose reconstruction and accumulation can be applied to assess delivered dose in the presence of anatomical changes and verify the validity of a treatment plan during treatment. However, displacement field errors (DFEs) in the forward registration or inverse consistency (IC) errors in the backward registration introduce uncertainties in the reconstructed dose. This study uses an in silico model Abstract withdrawn