ESTRO 38 Abstract book

S249 ESTRO 38

by performing square field measurements under a variety of different phantom setups (varying SSD, phantom thickness and field size), which were semi-empirically fitted to EPID images of the corresponding fields. The algorithm was commissioned for three gantry angles (0, 90 and 180 degrees) and a solution to adapt the back- projection to arbitrary intermediate gantry angles was introduced and validated with phantom experiments. Phantom verification was performed for 75 IMRT fields irradiated to a solid water slab phantom at gantries 0, 90 and 180 degrees. The EPID images were back-projected to the isocenter at 10 cm depth using the model commissioned at its corresponding gantry angle. The reconstructed dose distributions were compared to 2-D measurements acquired at 10 cm depth at isocenter with an ionization chamber (IC) array (OCTAVIUS 1500 PTW array). The same IMRT fields were back-projected using a model commissioned at a different gantry angle and incorporating the gantry angle adaptation in the algorithm, and compared to 2-D measurements. Results In the phantom study, the gamma results (global, 3%, 2mm, within the 10% isodose) averaged over 75 IMRT fields were γ mean = 0.37 ± 0.07 and γpassrate=98.1 ± 2.4, with an average dose difference in reference point of ∆DRP=-0.5% ± 1.8%. Gamma results for the fields reconstructed from a different commissioned gantry angle were γ_mean = 0.39 ± 0.08 and γpassrate=97.6 ± 3.3, with an average ∆DRP=- We have successfully demonstrated the feasibility of back- projection portal dosimetry for the MR-Linac. Verification of IMRT fields irradiated to a phantom shows good agreement between EPID reconstructed and IC array dose distributions. The first MR-Linac prostate and rectum patients are being treated in our clinic and EPID images acquired during treatment for validation of the described method. This research was partly sponsored by Elekta AB, Stockholm, Sweden. PV-0484 In vivo dosimetry using CBCT and EPID device: analysis of sources of errors in VMAT treatments S. Bresciani 1 , L. Botez 1 , A. Miranti 1 , M. Stasi 1 1 Candiolo Cancer Institute - FPO- IRCCS, Medical Physics, Candiolo TO, Italy Purpose or Objective In vivo dosimetry (IVD) using an electronic portal imaging device (EPID) combined with CBCT imaging may provide an effective means for quantify dose discrepancies and preventing errors in radiotherapy. The aim of this work is to evaluate the daily dose delivery discrepancies catching sources of errors in VMAT treatments with a 3D EPID-based IVD system Material and Methods The IVD software PerFRACTION (Sun Nuclear Corporation) was used. PerFRACTION is based on the information concerning MLC and collimator positions obtained by EPID images and on the information of log files, such as the MUs and the gantry angles. From these informations, a collapsed- cone/superposition algorithm (SDC) is used to calculate the dose distribution on daily CBCT images. It also uses the SDC to produce an entire dose volume to check against that from the TPS. First of all we calculated the dose discrepancies (DD%) at the isocenter (ID) and the 3D 2%/2mm gamma passing rate (% GP) between our reference (AAA) and SDC algorithm for 180 plans. About IVD, 3D dose distributions were reconstructed for 961 fractions of 133 patients, including prostates (17%), lungs (15%), Head & Neck (6%), PBI (17%), and palliatives (28%). We evaluated DD% between reference and daily 0.1% ± 1.8%. Conclusion

Conclusion The dosimetric accuracy of the MR-linac online adaption workflow was tested for the first time using PRESAGE® dosimeters and a phantom mimicking a tumour in the lungs. The dose distributions were accurately delivered to the target by positioning the phantom at the isocenter and by shifting it and applying the adaptive planning workflow. Complex IMRT plans will be evaluated in the future using larger samples of PRESAGE®. PV-0483 Pre-treatment portal dosimetry for the MR- Linac I. Torres Xirau 1 , I. Olaciregui-Ruiz 1 , A. Mans 1 , U. A. van der Heide 1 1 Netherlands Cancer Institute, Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective The implementation of MRI-guided radiotherapy systems has become a reality in our clinic with the first patients being treated in the Elekta’s Unity MR-Linac (Elekta AB, Stockholm, Sweden). The system introduces the possibility of real-time adaptation of the plan based on the actual anatomy of the patient during treatment. Given the complexity of the system, in combination with novel techniques for daily re-planning, the verification of the dose delivered to the patient is very important. For this purpose, the use of an Electronic Portal Imaging Device (EPID) for independent in vivo dose verification in the MR- Linac is being developed and the first phantom validation An existing back-projection algorithm used with conventional linacs was adapted for the Unity system, accounting for the increased amount of scatter received by the panel and the extra attenuation by the MR elements in the beam. The algorithm parameters were determined results are presented. Material and Methods