ESTRO 36 Abstract Book

S886 ESTRO 36 _______________________________________________________________________________________________

Physics, Munich, Germany 3 Heidelberg University Hospital, Department of Radiation Oncology, Heidelberg, Germany 4 German Cancer Research Center, Software development for Integrated Diagnostic and Therapy - Department of Radiology, Heidelberg, Germany Purpose or Objective The interplay between respiratory motion of a tumor and dose delivered by complex techniques like IMRT and VMAT can potentially lead to undesirable and non-intuitive deviations from the planned dose distribution. Small field sizes and fluences used in these advanced therapy techniques might amplify the dose deviations. We aim at developing a 4D dose recalculation tool to simulate the dose distribution for a moving target volume more precisely. The impact of interplay effects can be evaluated and compared for different treatment techniques. Material and Methods We developed a workflow combining a Monte Carlo dose calculation and a dose accumulation based on 4DCT images and linac log files. Log data from the linac are retrieved with Delivery Parameters Log File Convertor for Integrity™ R3.2 provided by Elekta. The time information in these log files has a resolution of 0.04 s and is used to divide the original treatment plan into small time intervals correlated to the patient’s respiratory phases. All resulting plan fragments (each corresponding to a certain 4DCT phase) are then recalculated using MCverify/Hyperion V2.4 (research version of Elekta MONACO 3.2). As a final step the single doses are sorted and combined to a total dose distribution. Different respiratory cycles, e.g. changes in the breathing frequency or pattern, and treatment methods, e.g. stereotactic treatment or gating, can be simulated and compared for different treatment techniques. The handling and accumulation of the different dose fragments are performed with AVID, a software framework for radiation therapy data processing developed at Deutsches Krebsforschungszentrum (DKFZ). For a first demonstration, implementation and verification, the 4DCT of a Dynamic Thorax Phantom (CIRS) is used. A 1D- sinusoidal-rigid-motion with frequency 0.25 Hz and amplitude 2 cm was set. Results A 3D-CRT and an IMRT plan were delivered to the phantom. Fig. 1 shows the resulting DVHs for the GTV using the 3D-CRT and the IMRT plan, respectively. In the plots, two different starting phases are marked (“treatment starting in inhale” and “treatment starting in exhale phase”). The DVHs of the 3D-CRT plan remained unaffected by the respiratory phase shift. Whereas for the IMRT plan (optimized on a dose of 10Gy) the maximal dose changed by 0.27 Gy (2.3%) from 11.95 Gy to 11.68 Gy after the 50% phase shift.

(see

flow

chart).

We retrospectively applied this protocol to 88 historical treatments (May 2011 - January 2015) performed within our institute to evaluate its effect. Results Of 88 treatments in 86 patients, 36 were initially selected for 3D CBCT in all fractions. From the remaining 52 treatments 25 would have been suitable for 3D CBCT after comparing and combining 4D and 3D CBCT position verification (see pie chart).

Conclusion With use of the in-treatment decision protocol for 3D or 4D position verification, the number of patients having 3D CBCT for position verification raised from 41% to 69%. This is not only beneficial for patient comfort, it limits motion related treatment degradation and it is also increases treatment capacity. Therefore we propose to use the in- treatment CBCT information and our decision protocol for making optimal use of 3D CBCT. EP-1635 Framework for the evaluation of interplay effects between respiratory motion and dose application A. Von Münchow 1,2 , K. Straub 1 , J. Hofmaier 1,2 , P. Freislederer 1 , M. Reiner 1 , C. Thieke 1 , M. Söhn 1 , M. Alber 3 , R. Floca 4 , C. Belka 1 , K. Parodi 2 , F. Kamp 1 1 LMU Munich - Klinikum der Universität München, Department of Radiation Oncology, Munich, Germany 2 LMU Munich - Faculty of Physics, Department of Medical

Fig 1: Example for the occurrence of interplay effects: IMRT plan segment on two CT Phases