ESTRO 36 Abstract Book

S900 ESTRO 36 2017 _______________________________________________________________________________________________

DECT measurements of typical tissue-surrogate phantoms and evaluated its uncertainty. Results The methodological uncertainty of electron-density assessment for the alpha-blending method was found to be below 0.15% for arbitrary mixtures of human tissue. In the case of small abundance of high-Z elements, electron- density results are positively biased, e.g. 0.5% for thyroid containing 0.1% iodine (Z=53) by mass, which is due to the K edge of the photoelectric effect. The calibration parameters obtained from various published data sets, showed very little variation in spite of diverse experimental setups and CT protocols used. The calibration uncertainty was found to be negligible for soft tissue while it was dominated by beam hardening effects for bony tissue. Conclusion The alpha-blending approach for electron-density determination shows universal applicability to any mixture of human tissue with a very small methodological uncertainty (< 0.15%); and a robust and bias-free calibration method, which is straightforward to implement. We conclude that further refinement of algorithms for DECT-based electron-density assessment is not advisable. EP-1674 Experimental investigation of CT imaging approaches to deal with metal artefacts in proton therapy S. Belloni 1,2 , M. Peroni 1 , S. Safai 1 , G. Fattori 1 , R. Perrin 1 , M. Walser 1 , T. Niemann 3 , R.A. Kubik-Huch 3 , A.J. Lomax 1 , D.C. Weber 1,4,5 , A. Bolsi 1 1 Paul Scherrer Institut, Center for Proton Therapy, Villigen PSI, Switzerland 2 University of Bologna, Department of Physics and Astronomy, Bologna, Italy 3 Cantonal Hospital Baden, Department of Radiology, Baden, Switzerland 4 Inselspital, Radiation Oncology, Bern, Switzerland 5 University Hospital Zurich, Radiation Oncology, Zurich, Switzerland Purpose or Objective Metal implants are challenging for proton therapy, mainly because of beam hardening artefacts severely compromising image quality of the planning CT. In fact, they result in non-negligible uncertainties in Stopping Power (SP) evaluation and significantly affect VOI delineation accuracy. The aim of this study was to compare different approaches to minimize the artefacts: a manual approach based on delineation of the visible artefacts, which was developed and is used clinically at the Center for Proton Therapy (PSI), and the new tools recently introduced in CT, such as SIEMENS Iterative Metal Artefact Reduction (iMAR) and Sinogram Affirmed Iterative Reconstruction (SAFIRE). Moreover, an experimental verification of direct SP calculation from Dual Energy (DE) images with iMAR has also been considered. Material and Methods A clinical treatment of a cervical chordoma patient was reproduced on a head and neck anthropomorphic phantom, which presents metal implants (titanium screws and cage) in the area where the PTV was defined. An IMPT plan with two anterior oblique and two posterior oblique fields (dose per fraction 2 GyRBE) was optimized and calculated on 7 different CTs which corresponded to the different imaging approaches: no correction of artefacts, manual correction, iMAR (each of these reconstructed using Filtered Back Projection (FBP) and SAFIRE) and DE together with iMAR. The delivered dose was measured with EBT3 Gafchromic films, inserted in three sagittal planes of the phantom included in the PTV area, and was compared with the dose calculated on the different CTs from machine log files. Local dose differences and gamma maps were used to evaluate the results, taking into

account residual positioning errors, daily machine dependent uncertainties and film quenching. Results We restricted the analyses to the 50% isodose and defined A +10% and A -10% as the percentage area having percentage differences higher (lower) than 10% (-10%). In general, A +10% between calculated and measured dose distributions were below 10% for plane 1 and 2 with the DE approach combined with iMAR (Table 1). Maximum differences were mainly located in the areas of steep dose gradients. Focusing on the SAFIRE algorithms, the three methods showed comparable results to the corresponding FBP algorithms for plane 2 and 3. For plane 1, A +10% increased to 24.8% for uncorrected approach, but SAFIRE was again comparable to FBP when iMAR is used. Conclusion DE combined with iMAR shows potential for predicting SP values and reducing metal artefacts. However, all approaches provided comparable, and clinically acceptable, results in terms of dosimetry accuracy. This could be related to the uncertainties in the experimental setup and in the measurements method (mainly use of gafchromic films), which might be comparable to the differences introduced by the metal artefacts correction approaches. The planning approach with multiple fields was robust against errors introduced by metal implants.

EP-1675 Influence of CT contrast agent on head and neck VMAT dose distributions L. Obeid 1 , J. Prunaretty 1 , N. Ailleres 1 , L. Bedos 1 , A. Morel 1 , S. Simeon 1 , P. Fenoglietto 1 1 Institut Régional du Cancer de Montpellier, Radiotherapy, Montpellier, France Purpose or Objective Intravenous contrast agent injection during the patient CT simulation facilitates radiotherapy contouring in the case of head and neck cancers. However, the image contrast enhancement may introduce discrepancy between the planned and delivered dose. The aim of this retrospective study is to quantify the variations of Hounsfield unites (HU) and to investigate their effect on Volumetric Modulated Arc Therapy (VMAT) dose distributions. Material and Methods Ten patients previously treated by VMAT techniques with identical dose levels (70/60/50 Gy) were selected. For each patient, two CT scans were performed, 2 min. (CT inj ) and 12 min. (CT delay ) after Iomeron® 350 biphasic intravenous injection (60 mL, 1mL/s followed by 90 mL, 2 mL/s after 30 s). The treatment planning (optimization and calculation) was performed with CT inj using the Eclipse TPS and two calculation algorithms (AAA® and Acuros XB®). Two other treatment plans were recalculated with the same parameters and CT delay . The mean HU and the iodine distribution were compared between the two scan images in the PTV50, the parotids and the thyroid. A dosimetric comparison using dose-volume histograms in target volumes and OAR (thyroid, parotids) was