ESTRO 2020 Abstract book

S1008 ESTRO 2020

end-to-end test of patient-specific SPR predictions with spectral CT was developed and conducted, investigating the potential of beam range determination improvements for proton and helium ion treatment planning. Material and Methods Spectral CT imaging based on a dual-layer detector-based approach enables the direct generation of spectral information (i.e., electron density relative to water (ED), effective atomic number (EAN)) covering the full standard FOV of 500 mm. Obtained results were compared against conventional single-energy CT (SECT) predictions in three different experimental and clinical set-ups, in homogeneous tissue-equivalent surrogates (Gammex 467, Gammex-RMI, USA), in a heterogeneous patient-like phantom (CIRS Proton Therapy Dosimetry Head, CIRS, USA) attached to an ionization chamber array, and in a retrospective cohort analysis of 20 radiological head patients evaluating homogeneous tissue regions as well as proton and helium ion beam treatment plans on a Monte Carlo treatment planning platform. Due to its minimal fragmentation tail and narrow lateral penumbra, helium ions were chosen for validations of range prediction in clinical-like settings. Results Using calibrated and validated maps of ED and EAN from spectral CT data, predicted SPR values in tissue surrogates were within a mean accuracy of 0.6 % compared to measured SPR and showed substantially better agreement with measured data compared to standard CT-number-to- SPR calibration. Dosimetric validation of the developed method using a half-head anthropomorphic phantom in a clinical-like setting indicated that SPR prediction with spectral CT outperforms the clinical SECT standard with a range prediction improvement of about 1 mm. Furthermore, significant SPR and range differences of about 1 mm to the standard approach were observed in patients, highlighting clinical relevance (Fig. 1). Fig. 1: (a) SPR histogram for ROIs in brain tissue. (b) Axial plane of radiological head patient showing predicted SPR. (c) Absolute BEV range difference in a hypothetical PTV between spectral CT and SECT. Conclusion In sum, spectral CT imaging has the ability to predict SPR for particle therapy more accurately by using a projection- based calculation of both ED and EAN. Results obtained during this study demonstrate the superiority of spectral CT over conventional CT imaging in this context with improved range estimates for high-precision particle therapy. Further comparative studies of SPR maps derived from spectral CT to those determined by conventional CT imaging for treatment planning and range measured in biological tissue samples should evaluate its potential in realistic tissue compositions. PO-1727 The geometric and dosimetric effect of algorithm choice on propagated contours from CT to CBCTs D. Nash 1 , A. McWilliam 2 , A.L. Palmer 3 , E. Vasquez Osorio 4 1 Queen Alexandra Hospital- Portsmouth, and the University of Manchester, Manchester, United Kingdom ; 2 The Christie NHS Trust- Manchester, and the University of Manchester, Manchester, United Kingdom ; 3 Queen Alexandra Hospital- Portsmouth, and the University of Surrey, Guildford, United Kingdom ; 4 University of Manchester- Manchester, and The Christie NHS Foundation Trust, Manchester, United Kingdom

Purpose or Objective Efficient adaptive radiotherapy requires rapid plan assessment based on the anatomy of the day to determine whether plan adaptation is required. To determine the relevant DVH parameters, contouring the organs at risk (OARs) on the daily image is required. Manually contouring in such a short timescale puts a significant burden on oncologists. Using software to propagate the planning contours onto the daily images can theoretically remove this burden. Since there are many commercially available options, the geometric and dosimetric differences of the propagated contours should be assessed, before evaluating their role in plan adaptation. Material and Methods Planning CT (pCT) contours and 5 shading-corrected CBCTs [1] (for more accurate dose calculation) acquired weekly throughout treatment for 10 randomly selected head and neck cancer patients were used. The patients were re- planned with VMAT in RayStation v6.99 (RS). The spinal cord (SC), brainstem (BS), parotids and larynx were propagated using 2 strategies: from pCT to each CBCT (pCT2CBCT) and in cascade, i.e. from the pCT to CBCT1 to CBCT2. The pCT2CBCT strategy was used in ADMIRE, Mirada and RS, and the cascade in ADMIRE and RS, for a total of 5 methods. Mean distance to agreement (mDTA) between propagated contours was computed between pairs of contours for all propagating methods. BS and SC were expanded to form PRVs. For the dosimetric impact, the plan was calculated on each CBCT after setup correction. The relevant DVH parameters were extracted (D1cc for BS and SC PRVs, mean dose parotids and larynx) for each contour and each method, and compared using the Kolmogorov-Smirnoff test to determine statistically significant differences. The doses were compared against the clinical constraints for BS and SC PRVs (D1cc< 54 and 46 Gy respectively). Results All methods produced similar contours evidenced by the range of the mDTA (up to 1.7 mm) (table 1). Figure 1 shows the dose differences for the OARs. There were no statistically significant differences except for SC PRV, with no particular method outperforming the others. The presence of significant differences on the SC is indicative of the high dose gradients present in the area, confirming that small variations on the propagated contours cause larger dose changes. The median dose for both left and right parotids showed a small increase for most methods. The larynx dose remained similar with time. There were 6 instances of the spinal cord failing against clinical constraint, and 5 for the brainstem - all were in cases of high dose gradients near to the OAR. No particular method failed consistently.