ESTRO 36 Abstract Book

S895 ESTRO 36 2017 _______________________________________________________________________________________________

planning strategies were explored: 1) A new optimized IMRT plan on each weekly MR image (WeeklyAdapt) 2) A new optimized IMRT plan on week four MR image only (OneAdapt) and 3) Calculating the plan from the planning MR on each weekly MR image (NoAdapt). The PTV coverage for all ViewRay MR-IGRT plans were such that 95% of the PTV received 100% the prescription dose. The differences between accumulated doses on planning MR for all three ART strategies were evaluated using the dose-volume constraints for targets and critical structures. Results For PTV70, PTV60 and PTV54, as compared to the D95 coverage on planning MR, the differences in D95 accumulated doses between three ART strategies were <2%. The maximum dose to both cord and brainstem between WeeklyAdapt and only OneAdapt were very similar and <1% as compared to the planning MR values. However, NoAdapt Dmax for brainstem and cord were >27% and 25% than the original planning MR Dmax values respectively. The mean left and right parotid dose remained very similar both WeeklyAdapt and OneAdapt strategies although there were up to 10.7% increase in the mean dose to the parotids with NoAdapt strategy. Conclusion This study demonstrated that no significant differences in accumulated doses were observed between weekly ART and only one ART at week four during MR IGRT of H&N cancer patients. Further studies are needed to evaluate benefits of daily online ART during MR IGRT.

using its standard-dose head protocol (100 kV and 2.5 slice thickness). On the other hand, planning CT images were acquired in a Toshiba Aquilion LB using the same phantom and with our own clinical head and neck protocol (120 kV and 3 mm slice thickness). Different OAR (Body, spinal cord, parotids, mandible, oropharynx, dermis, an inner ring with 1 cm thickness and shoulder) and a PTV were delimitated. Additionally, reference points were inserted over all these structures. All defined structures and points were registered with the CBCT images by means of the Varian rigid registration software. Both the delimitation of volumes and the design of the treatment plan have taken into account the limited field of view of CBCT (length 16 cm, diameter 25 cm). A head and neck VMAT plan has been calculated in Eclipse (v10) using both sets of images. For CT images, we only used a standard calibration curve and 3 different calibration sets of curves for CBCT images, i.e., standard, measured with a CATPHAN 504 phantom and measured with a CIRS 062M head phantom placed between head and neck RANDO slices. Dose and HU were calculated in all reference points as well as dose-volume-histograms for the anatomical locations for both CT and CBCT. A gamma analysis was used for HVD comparison. Results The mean HU differences are less than 50 UH and the relative dose differences are less than 3% for all the calibration curves (Table 1) on every reference point over all the structures. The gamma (2%, 2 mm) DHV analysis shows an excellent agreement for almost all the structures (>95%) (Image1). Ring and dermis have gamma >85% .The non-pass regions correspond to very low dose regions. The worst gamma (>50%) corresponds to the left parotid because it is a very small structure (10 cc) into a high gradient dose zone. Furthermore, there is a difference of 1.8% on its volume as measured on the CBCT and the CT images, probably due to interpolation errors. These results are similar for all the calibration curves analysed.

EP-1668 Dose calculation accuracy using CBCT images for head and neck VMAT M.A. Carrasco Herrera 1 , B. Quintana 2 , J.M. Nieto 2 , F.J. Luis Simon 1 , C. Santa Marta Pastrana 3 1 Hospital Universitario Virgen del Rocio, Medical Physics department, Sevilla, Spain 2 Hospital Universitario Virgen del Rocio, Radiation Oncology department, Sevilla, Spain 3 Universidad Nacional de Educación a Distancia UNED, 2UDepartment of Mathematical Physics and of Fluids, Madrid, Spain Purpose or Objective Assessment of the differences between CT and CBCT based dose calculation for a volumetric modulated arc therapy (VMAT) in head and neck radiotherapy treatment. Material and Methods CBCT images of an Alderson RANDO phantom with tissue- equivalent material were acquired in a Varian`s On-Board Imager OBI (v1.5) installed on a Varian DHX accelerator,