ESTRO 37 Abstract book
S1142
ESTRO 37
Conclusion Our results show that for harder beam qualities (in the air) the beam softening is larger within CTDI phantoms and vice versa. Proposed method for CTDI measurements using radiochromic film dosimetry protocol corrected by the beam quality change within the phantom shows better agreement between calculated CTDIvol and tabulated values with maximum difference 11% for Head and 14% for Body phantom, as opposed to 16% for Head and 28% for Body phantom when the beam quality correction is not performed. EP-2077 A critical look at the stoichiometric single- energy CT calibration for proton therapy C. Gomà 1 , I.P. Almeida 2 , F. Verhaegen 2 1 KU Leuven, Department of Oncology- Laboratory of Experimental Radiotherapy, Leuven, Belgium 2 Maastricht Radiation Oncology MAASTRO Clinic, Physics Research, Maastricht, The Netherlands Purpose or Objective Despite extensive research in dual-energy CT imaging, single-energy CT (SECT) is still the standard imaging modality in proton therapy treatment planning. In this context, the stoichiometric calibration is, with more than 500 citations, considered to be the most accurate method to establish a unique relationship between CT numbers and proton stopping power. This work revisits the SECT calibration for proton therapy treatment planning, with a critical look at the stoichiometric method. Material and Methods Three different sets of tissue-substitutes of known elemental composition (Gammex, CIRS and Catphan) were scanned with the same scanning protocol. A stoichiometric fit was performed for each set of tissue- substitutes. Based on that, the CT number and proton stopping power (relative to water) were calculated for different sets of biological tissues and tissue-substitutes. Results The figure shows different SECT calibration curves. The disconnected symbols correspond to experimental SECT calibration curves based on tissue-substitutes, i.e. HU are measured. The connected symbols correspond to stoichiometric SECT calibration curves based on biological substitutes, i.e. the HU of biological tissues are calculated using the stoichiometric coefficients obtained using different calibration phantoms (Gammex, CIRS, Catphan).
that obtained by simple interpolation of experimental data. Conclusion The stoichiometric method for SECT calibration seems to depend on the tissue-substitutes used in the calibration, which may be regarded as an additional source of systematic uncertainty in proton range for bone tissues. Furthermore, Gammex tissue-substitutes appear to be a good representative of biological tissues within the energy range relevant to computed tomography—making in this case the stoichiometric method unnecessary. EP-2078 Experimental validation of a synthetic 4DCT- MRI approach using an anthropomorphic breathing phantom M. Krieger 1 , K. Klucznik 2 , C. Emma 1 , M. Peroni 1 , O. Bieri 3 , Z. Celicanin 3 , D.C. Weber 1 , A.J. Lomax 1 , Y. Zhang 1 1 Paul Scherrer Institute, Centre for Proton Therapy, Villigen PSI, Switzerland 2 ETH Zürich, Department of Physics, Zürich, Switzerland 3 University Hospital of Basel, Department of Radiology, Basel, Switzerland Purpose or Objective To validate the synthetic 4DCT-MRI approach for lung proton treatment planning using an anthropomorphic phantom under well controlled conditions. Material and Methods Temporally resolved CT and MR images of an anthropomorphic moving phantom were acquired. The pressure applied to the phantom followed a sin 4 curve to simulate human respiration, which resulted in mean motion amplitude of 7mm for the embedded tumour. The motion vector fields of the resulting 4DCT and 4DMRI were extracted using a B-splines based deformable image registration (DIR), with the full exhale phase as a reference. The quality of DIR is assessed for selected landmarks whose locations were defined manually. A rigid registration was applied to match the coordinates of CT and MR images of the exhale phase. The extracted vector fields from 4DMRI were resampled to obtain the same resolution as CT, and were then applied to the full exhale CT image to achieve the simulated 4DCT-MRI images, as depicted in Figure 1. The motion vector fields of the 4DMRI were interpolated in time to match the phases of the 4DCT. The extent of motion extracted from 4DCT and 4DMRI was analysed by comparing the motion vectors in the region of the tumour (as shown in Figure 2a).
Results Figure 2a) shows a comparison of the synthetic 4DCT-MRI for two phases, with respect to full exhale and the corresponding phase of the original 4DCT. The differences between the real and the simulated CT are rather limited, in comparison to the initial differences between the phases and the full exhale CT. The extent of motion of the selected region (mean +/- standard deviation) for the 4DCT (red circles) and the resampled 4DMRI (blue crosses) is shown in Figure 2b), where the differences of the vector fields is given by the black dashed line. Although the reconstruction principle for the time resolved images is fundamentally different for 4DCT and 4DMRI, very similar motion vector fields could be
It was found that, contrary to common belief, the stoichiometric fit depends on the elemental composition of the tissue-substitutes used in the calibration, leading to CT calibration curves which differ up to 15% in predicting the relative stopping power (RSP) of bone tissues. For a given RSP, only Gammex tissue-substitutes were found to reproduce the CT numbers of biological tissues within the whole energy range relevant to computed tomography. Finally, it was found that, for Gammex tissue-substitutes, the CT calibration curve resulting from the stoichiometric method agrees with
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