ESTRO 36 Abstract Book

S956 ESTRO 36 2017 _______________________________________________________________________________________________

Results This method was tested on 7 linacs of different makes and models (Elekta Synergy Platform, Elekta Versa HD, Varian Unique, Novalis Tx) in the authors' radiotherapy centres. The average distance by which the isocentre moves between both gantry positions was found to be 1.04 mm (SD 0.30 mm), with the whole range covered by the [0.53, 1.48] interval. The two lowest values were achieved on the two single energy Varian Unique linacs. It was found out that the longitudinal isocentre shift is largely independent of the gantry isocentre wobble determined by the star-shot test. We also tested the alignment of the collimator 0° setting with the gantry rotation plane. The average deviation was found to be 0.16° (SD 0.10°), range [0.04°, 0.31°]. Conclusion The results appear consistent, but it would be helpful to test the method on a wider pool of treatment machines over a longer period of time. The longitudinal isocentre shift during gantry rotation is a non-negligible parameter which needs to be incorporated into the uncertainty budget which is the basis for the CTV-PTV margin. EP-1761 Workflow development for the clinical implementation of an MR-guided linear accelerator T. Stanescu 1 , A. Berlin 2 , L. Dawson 2 , J. Abed 2 , A. Simeonov 2 , T. Craig 2 , D. Letourneau 2 , D. Jaffray 2 1 Stanescu Teodor, Radiation Physics- PMH, Toronto, Canada 2 Princess Margaret Cancer Centre, RMP, Toronto, Canada Purpose or Objective Development of clinical workflows for the implementation of a new external beam radiation therapy environment which relies on hybrid MR-CBCT in-room imaging guidance. Material and Methods A standard radiation therapy 6X linac (TrueBeam, Varian Medical System, Palo Alto, CA) was integrated with a 1.5 T diagnostic MR scanner (IMRIS, Minnetonka, MN). The MR can move on rails and was tuned up to perform optimal imaging inside the treatment room in the proximity of the linac. The patient load is transferred directly between the MR diagnostic table and the linac IGRT couch via a hovercraft system (Zephyr XL, Diacor, Salt Lake City, UT). No special MR safety requirements were employed regarding the curation of the linac room – the linac/couch can be freely operated mechanically when the MR magnet is present - only typical MR room screening for ferromagnetic content was implemented. Comprehensive testing was completed to confirm negligible magnetic field coupling between the MR and the TrueBeam system (linac and patient table). Since the linac retains its default features and an MR imager is available in the linac vault a combined MR-kV approach can be employed for the patient setup verification and treatment delivery. A new software tool was developed in collaboration with Varian to provide the computation and implementation of treatment couch shifts based on soft-tissue information, i.e. image matching between plan MR and guidance MR. In this study, clinical workflows for liver and prostate were developed and tested. Each site posed challenges from patient image data planning and acquisition to RT planning and in-room guidance. The approach was to integrate the capabilities offered by the new technology in existing processes. Results MR imaging protocols for planning and guidance were established. The guidance scans were optimized to minimize session time with negligible penalty on the accuracy of the image matching process (planned vs. on demand). MR acquisition was also designed to allow for collection of data needed for CT+MR as well MR-only (or synthetic CT) workflows. This is expected to facilitate access to the in-house adaptive RT implementation in RayStation (RaySearch, Stockholm, Sweden). All MR image

data was validated for RT use by means of comprehensive testing for global image quality and correction of spatial distortions. In particular for liver, the management of motion was adapted for the MR environment in the case of potential subjects undergoing either breathhold or abdominal compression (AC). Breathhold was evaluated using Medspira (Minneapolis, MN) whereas AC was implemented using pneumatic pressure belts. Staff requirements and safety for in-room operations was evaluated via simulations and dry runs. Conclusion Workflows for MR guidance in prostate and liver were designed and preliminary tested. The next step is to enable imaging-only clinical trials in patients. Based on feedback the workflows will be refined to allow for the full implementation of MR-guidance studies. EP-1762 A Comparative Study of 4D and 3D CTSimulation in Esophageal Carcinoma J. Li 1 , G. Lai 1 1 Fujian Cancer Hospital, radiation oncology, Fuzhou- Fujian, China Purpose or Objective To compare the internal gross target volume (IGTV) and its displacement of primary esophageal carcinoma (EPC) on the base offour-dimensionaland three-dimensional computed tomography (4D-CT and 3D-CT) simulation technology. Material and Methods Twenty-two esophageal cancerpatients with pathological proved diagnosiswererecruited in this prospectively study. Every patient sequentially received contrast enhancement free-breathing 3D-CT and respiration-synchronized4D-CT simulationchest.Then the target volume and the displacement on three orthogonal directionsof three different IGTV planningmethods (including IGTV 4D, IGTV 4D' and IGTV 3D . The dice similarity coefficient (DSC) and overlap index (OI) between IGTV 4D and IGTV 4D' , between IGTV 4D and IGTV 3D for different segments of esophagus were calculated. Statistical analysis included theFriedman test, analysis of variance on the base of repeated measurement data and paired-samples student t test and the P <0.05 was set as statistically significant. Results There were statistical significance of displacement at left- right, anterior-posterior, and superior-inferior directions of primary esophageal tumor GTV of the ten phases originated from medium-thoractic segments and medium- lower-thoractic segments ( P = 0.005 and P = 0.001). There presented a significant differences of primary tumor volume where appeared IGTV 3D > IGTV 4D > IGTV 4D' ( P <0.05).In other words, the implementation of GTV by means of extending position error from based of 3D-CT wouldlead to unnecessary radiation of the surrounding normal tissues (about 9~24%); However, the application of GTV only by means of integrating end-inhalation and end-exhalation phases would result in uncover target area (about 10~34%). Conclusion 4D-CT simulation technology is superior to 3D-CT simulation technologyfor IMRT in esophageal cancer. EP-1763 Acute toxicity and in-vivo dosimetry of a two week hypofractionated schedule within the HYPORT study A. Saha 1 , G. Goswami 2 , S. Mandal 2 , A. Mahata 2 , D. Midha 3 , R. Ahmed 4 , S. Agarwal 4 , S. Ray 5 , J. Das 5 , S.S. Datta 6 , S. Sinha 7 , S. Chatterjee 2 1 Tata medical center, Radiotherapy, Kolkata, India 2 Tata medical center, Radiation Oncology, Kolkata, India 3 Tata medical center, Oncopathology, Kolkata, India 4 Tata medical center, Breast Surgery, Kolkata, India 5 Tata medical center, Nuclear Medicine, Kolkata, India