Abstract Book

ESTRO 37

S514

Results

of the gating latency, as well as insight in its dosimetric effects is of utmost importance for gated treatments. The purpose of this research is to measure the dosimetric impact of latency, and to used these results for estimation of the latency time. Material and Methods A prototype MR-compatible motion phantom (ModusQA, Canada) was adopted to hold a sphere for MR based tracking and a piece of radiochomic film (GafChromic EBT) for dose profile measurements. Static as well as gated profiles were acquired on a 60 Co MRIdian system. For all gated profiles a sinusoidal phantom movement was used, with amplitude of 15 mm and a period of 3.14 s. The gating boundary was set as the extreme position of the sphere plus an isotropic margin of 5 mm. The maximum area of the tracked sphere allowed to exceed the boundary (ROI) was varied between 5, 10 and 15%. During the gated delivery the duty cycle was recorded. A model was developed in Matlab to simulate the dosimetric effects of the gated delivery, which has the measured static profile, the target motion, the gating window and the gating latency as input. The model handles the latency independent of the 250 ms frame rate. The output of the model is the gated dose profile as well as the simulated duty cycle. The ROI setting translates into an effective gating window size which was determined by matching the simulated duty cycle to the recorded duty cycle. Then, the gating latency was optimized in order to achieve the best fit between the measured and the modeled gated dose profile. Results For ROI settings of 5, 10 and 15% the recorded duty cycles were 0.35, 0.34 and 0.38, respectively. Using the corresponding effective gating window settings, latency values of 240, 230 and 210 ms were found to closely reproduce the measured dose profiles when used in simulations. Examples of a measured dose profile as well as the simulated gated profiles are shown in Figure 1.

Figure 1 shows the cranio-caudal marker position for one marker during an entire treatment fraction in mm. Patient 1, 2 and 5 showed a good reproducibility in marker position, the marker is within a few mm in all fields in good agreement with the external gating window of 2 mm. Patient 3 had a baseline shift between the intra-treatment images and the post-CBCT. Patient 4 had large caudal marker drift during all treatment fields, even though the external marker block remained inside the gating window. The mean position and standard deviation for all patients and markers are summarized in Table 1. Conclusion Examples of the reproducibility of LN marker positions in five lung cancer patients during IMRT in DIBH was shown. Three patients show reproducibility within a few mm, but one patient shows large marker drift during breath-hold and one patient shows large baseline shift between setup CBCT and post-CBCT. When treating lung cancer patients in DIBH the external position of the gating box, may not be a good surrogate for some patients, since target drift occurs. We recommend intra-treatment imaging when treating lung cancer patients in DIBH. These can after the first fractions be used to select patients, which should not be treated in DIBH. PO-0944 Estimation of gating latency in MR-guided radiotherapy by modeling the dosimetric effects D. Hoffmans 1 , J. Cuijpers 1 1 VU University Medical Center, Radiation Onclogy, Amsterdam, The Netherlands Purpose or Objective The MRIdian system (Viewray inc., Cleveland) provides gating directly on the target volume, based on tumor tracking in 4 frames per second cine-MRI. Quantification

Conclusion A method to determine the latency of a gating system based on modeling of dosimetric effects has been developed. Measurements on an MR guided gating system using various clinically relevant gating settings showed a latency between 210 and 240 ms which is in line with the manufacturers measurements. The weak correlation between duty cycle and ROI settings, might be attributed to factors such as tracking quality which are not specifically accounted for in the model.