ESTRO 36 Abstract Book

S901 ESTRO 36 2017 _______________________________________________________________________________________________

performed. The maximum (D 2%

), minimum (D 98%

) and

differences up to 900 (in bone) can be obtained leading to systematic dose differences up to 6 % (DVH shift). Using an “average” IVDT still leads to dose uncertainties > 2 %. Results can be CT scanner specific. Conclusion Uncertainties on pCT images used for MRI-only treatment planning should be compared to those on tCT images. The uncertainties on tCT images (even when not considering CT artifacts) are non-negligible and are of the same order as those on pCT images generated by e.g. atlas-based methods. Electronic Poster: Physics track: (Quantitative) functional and biological imaging EP-1677 Multicentre initiative for standardisation of image biomarkers A. Zwanenburg 1 , Image Biomarker Standardisation Initiative IBSI 2 1 OncoRay - National Center for Radiation Research in Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus - Technische Universität Dresden - Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany Purpose or Objective Personalised cancer treatment has the potential to improve patient treatment outcomes. One particular approach to personalised treatment is radiomics. Radiomics is the high-throughput analysis of medical images. There are several challenges within the radiomics field which need to be overcome to translate findings into clinical practice. The Image Biomarker Standardisation Initiative (IBSI) addresses the challenge of reproducing and validating reported findings by comparing and standardising definitions and implementation of several image feature sets between participating institutions. Material and Methods A 5x4x4 voxel digital phantom was devised, with a super- imposed region-of-interest (ROI) mask (Figure 1). This volume has characteristics similar to real patient volumes of interest, namely voxels outside of the ROI and missing grey levels. The phantom is moreover sufficiently small to manually calculate features for validation purposes. Because no pre-processing steps (e.g. discretisation) are necessary for calculations on the phantom, feature values may be standardised across all institutions.

median (D 50%

) doses were registered.

Results The maximum HU average difference over all the patients was observed in the thyroid (81.37 ± 36.01 HU) followed by the PTV50 (10.76 ± 15.70 HU) and the parotids (9.39 ±16.01 HU). The differences found with the AAA® algorithm were below 0.1% for D 2% , D 98% and D 50% in target volumes and between -0.11 and 0.36% in OAR. The differences observed with Acuros XB® Algorithm were less than 0.2% in target volumes and 0.31% in OAR. Moreover, the differences between two algorithms were statistically insignificant (p > 0.4). Conclusion This study shows that the use of intravenous contrast during CT simulation does not significantly affect dose calculation in head and neck VMAT plans using AAA and Acuros XB algorithms. EP-1676 Comparison of accuracy of Hounsfield units obtained from pseudo-CT and true CT images N. Reynaert 1 , P.F. Cleri 1 , J. Laffarguette 1 , B. Demol 1 , C. Boydev 1 , F. Crop 1 1 Centre Oscar Lambret, PHYSIQUE MEDICALE, Lille, France Purpose or Objective Quality of pseudo-CT (pCT) images used for MRI-only treatment planning is often evaluated using the so-called MAE (Mean Average Energy) curve. Furthermore, a dosimetrical comparison is performed by comparing DVHs using pCT and true CT (tCT). The tCT is always considered as the reference, while uncertainties on these images are neglected. The purpose of the current work is to compare MAE curves for tCT images by varying different scanning parameters and to compare the results with uncertainties on our pCTs. Material and Methods A Toshiba Large Bore CT was used. Different IVDT curves were determined, for different energies (100-135 kV), FOVs, reconstruction kernel, phantom size, insert positions, using an in-house phantom, with variable size. The IVDT curves were used in our in-house Monte Carlo platform for recalculation of Cyberknife and Tomotherapy plans. pCT images were generated from MRI images (3D T1 sequence) using an atlas-based method. Image quality was determined using MAE, ME and gamma curves. Results Three parameters for tCT had an important impact on the HUs, namely the energy, patient size and reconstruction kernel. These parameters individually modified image values with up to 300 HUs in bone inserts. Furthermore, patient size and energy are often correlated as, it is specifically for small patients that lower energies are used, both leading to higher HUs in bone. The impact of the reconstruction kernel was a surprise (e.g. comparing the FC64 and FC13). For the energy and the reconstruction kernel one can consider introducing specific IVDTs. It becomes more complicated when the IVDT should be modified as a function of patient diameter though. Furthermore, in some TPSs (e.g. Masterplan, Nucletron) only one predefined IVDT is used. Another important problem is the fact that the HUs in the air surrounding the patient are increased when using large phantom sizes (changing from -1000 HU to -910 HU). Depending on the IVDT, this can lead to a largely overestimated air density around the patient (0.2 g/cm 3 ) with a possible dosimetric impact. The dosimetric impact of using different IVDTs when modifying energy, reconstruction kernel and patient size individually are below 2 %, for all Cyberknife and Tomotherapy plans considered. This is also the case for most of our pCT images. In extreme cases for tCT, e.g. when comparing a small patient scanned at 100 kV using the FC64 reconstruction kernel compared to a large patient scanned at 135 kV using the FC13 kernel, HU