ESTRO 2022 - Abstract Book

S1348

Abstract book

ESTRO 2022

Materials and Methods One-dimensional dose profiles of 5x5 mm ² to 60x60 mm ² photon beams (Varian TrueBeam, 10 MV FFF, SSD = 90 cm, 10 cm depth in water) and common planar SRT dose distributions were sampled with high resolution and fitted to interpolate data sets with arbitrary sampling distances. The data sets were transformed by Discrete Fourier Transformation and examined for aliasing effects using the amplitude spectrum. The resulting Nyquist frequencies per field size or dose distribution were validated by evaluating the energy densities of the data sets under the assumption that 99.95 %, 99.97 % or 99.99 % of the spectral energy density can be assigned to the actual signal. In addition, stereotactic VMAT plans were measured with the SRS MapCHECK (Sun Nuclear Corp. Melbourne, FL, USA) at the native sampling resolution (3.5 mm lat./long. and 2.49 mm diagonal), as well as at the double sampling resolution (1.75 mm lat./long. and 2.49 mm diagonal), which was achieved by merging two longitudinally shifted measurements. Both measured dose distributions were compared to that of the treatment plans by γ index analysis (1 mm, 1 %, TH 10%). Results The Nyquist frequencies of static beams averaged over all methods suggest required sampling distances of (2.2±0.8) mm for a 5 mm beam, (2.3±0.6) mm for a 10 mm beam, (2.5±0.6) mm for a 20 mm beam, (3.3±0.9) mm for a 40 mm beam and (4.0±1.1) mm for a 60 mm beam. The required sampling distance for the SRT treatment plans proved to be (2.4 ±0.5) mm, i.e. slightly larger than the results of the smallest beam segments due to beam accumulation. The results of the γ index analysis of the measurements with single and double detector density showed no significant differences. Conclusion The Nyquist frequency of small, static radiation fields suggests a required sampling distance upwards of (2.2±0.8) mm. The required sampling distance of a detector array additionally increases with the distance parameter of the commonly used γ index analysis and signal perturbation effects, such as volume averaging (Poppe et al. 2007), at the cost of sensitivity to errors. The theoretical analysis was validated by comparing measurements with a detector array matching the Nyquist frequency within uncertainty and repeated measurements with oversampling detector density. Consequently, previously published recommendations of 2.5 mm sampling distance for treatment verification measurements (Dempsey et al. 2005, Poppe et al. 2007) could be confirmed for SRT cases by theoretical analysis. Purpose or Objective Output factor (OF) corrections based on IAEA TRS-483 Code of Practice (TRS-483) may be several percent for certain detector and beam types and sizes, and the implementation of the corrections may cause the need to adjust other parameters affecting the calculated dose distributions. The aim of this study was to evaluate the effect of TRS-483 based OF corrections on dose calculation accuracy of 6 MV FFF VMAT and conformal arc (CARC) stereotactic treatment plans of small lesions. Materials and Methods Nine treatment plans (6 VMAT, 3 CARC) were re-calculated using corrected OFs and varying values for dosimetric leaf gap (DLG). A TrueBeam STx linear accelerator equipped with high-definition MLC, and configured in Varian Eclipse treatment planning system (TPS) (version 15.6) (Varian Medical Systems Inc, Palo Alto, CA), was used for the dose calculation. Effective target spot size (SS) was 1.5 and 0.0 mm in X and Y directions, respectively. Objects covered three brain (planning target volumes (PTVs) of 0.5-1.7 cc) and three lung targets (PTVs of 5.2-30.0 cc). The plans were calculated using Acuros XB dose calculation algorithm (version 15.6.04). TPS dose distributions were compared to EGSnrc-based full Monte Carlo (MC) simulations and measurements using Delta4 phantom (ScandiDos, Uppsala, Sweden). Results Difference in clinical target volume (CTV) mean dose between TPS and MC for VMAT and CARC plans and different OF and DLG values are presented in fig 1. Median dose difference between measured and TPS calculated plans are shown in fig 2. Compared to MC simulations and measurements, TRS-483 based OF correction seems to overestimate the resulting dose, which is seen especially in CARC plans, for which the adjustment of DLG does not fully compensate for the increased output. PO-1567 The effect of TRS-483 recommendations on dose calculation accuracy of stereotactic treatment plans A. Vanhanen 1 , M. Partanen 1 , J. Ojala 1 1 Tampere University Hospital, Oncology/Medical Physics, Tampere, Finland