ESTRO 36 Abstract Book

S894 ESTRO 36 2017 _______________________________________________________________________________________________

Purpose or Objective To evaluate if planned doses for prostate and rectum are equal to the doses which are actually delivered and to determine adaptation points for the accumulated dose. Material and Methods Twenty four patients with intermediate and high-risk prostate cancer who were going to be treated with image guided radiotherapy were enrolled. A plan-CT (pCT) and nine treatment kilovoltage conebeam-CT (kvCBCT) scans were acquired prospectively during the first three weeks of a prostate lGRT treatment (a total of 240 CTs). A rectal emptying preparation and a full bladder protocol were used. For each patient, a deformable image registration (DIR) from the pCT to each of the nine kvCBCT was performed with RayStation treatment planning system. All registers were revised and recontoured by a Radiation Oncologist, establishing regions of interest (ROIs) for a second DIR with control of such ROIs. For every patient, a hypofractionated VMAT schedule (15 x 3.82 Gy) was planned and correlated with their kvCBCT images, being able to determine the accumulated and total doses that would have been actually delivered. Since the pCT day, a nutritional evaluation control with anthropometric and biochemical parameters was performed for each of the 24 patients.

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 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 of H&N cancer patients. Material and Methods

Results A significant difference between planned and delivered D98 CTV 57 (p=0.026) and D2 CTV 57 (p=0.005) was observed; however, the average D98 CTV 57 delivered was higher than the prescription dose. Despite not having observed a significant difference in V36.5 of the planned and delivered to the rectum, the delivered doses to 50% of the rectum exceeded the planned constraints in 37.5% of the patients. A significant rectum volume variation was observed during the first week of treatment. An accumulated delivered dose to 50% of rectum > 1194 cGy in fraction five was a significant predictor for exceeding 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.