ESTRO 36 Abstract Book

S913 ESTRO 36 _______________________________________________________________________________________________

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.

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 performed. The maximum (D 2% ), minimum (D 98% ) and 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

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