ESTRO 36 Abstract Book

S907 ESTRO 36 _______________________________________________________________________________________________

Material and Methods Patients with locally advanced H&N cancers were selected for this study. For each patient, six weekly MR imaging were acquired on the ViewRay MR-IGRT system during the course of radiotherapy. PTVs, parotids, cord, brainstem, mandible, oral cavity and larynx were contoured on planning MR image and all structures were deformably-mapped on the weekly MR images. Three ART 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.

the rectum constraints. There was an average weight gain of 668 gr between the pCT day and the first day of treatment, but no significant relation with not fulfilling the prescription goals or organ at risk constraints was observed.

Conclusion The significant differences between CTV 57 prescribed doses and those actually delivered do not have a clinical impact because the average D98 CTV 57 is higher than the prescribed dose. The V 36.5 delivered to the rectum in 37.5% of the patients exceed the planned constraints, although this difference is not significant. The subgroup analysis has shown significant anatomical variations. The fraction five adaptation point for the accumulated doses in the rectum (1194 cGy) allows to significantly predict when the risk of not fulfilling the rectum V36.5 constraint is high and a plan adaptation is needed. The significant weight gain between the pCT day and the first day of treatment has no significant relation with not fulfilling the prescription goals or organ at risk constraints . EP-1667 MR-Guided Radiotherapy of Head and Neck Cancers: Adaptive Planning Strategies N. Dogan 1 , K. Padgett 1 , M. Duffy 1 , M. Samuels 1 1 University of Miami- Sylvester Comprehensive Cancer Center, Department of Radiation Oncology, Miami- Florida, USA Purpose or Objective Adaptive Radiotherapy (ART) with frequent imaging has been used to improve dosimetric accuracy by accounting for anatomical variations, such as primary tumor shrinkage and/or body weight loss, in head-and-neck (H&N) cancer patients. MR-guided radiotherapy technology provides daily real time MR images in the treatment room, hence has a great potential for online adaptive radiotherapy. The purpose of this study is to provide an assessment of different adaptive planning strategies using three-source Co 60 and Magnetic Resonance Imaging (MRI) Guided Radiation Therapy (MR-IGRT) System for treatment of H&N cancer patients.

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,