ESTRO 2020 Abstract Book

S65 ESTRO 2020

internal movements remain unaccounted for. The aim of this study was to analyze the dosimetric benefit of adaptation and evaluate the robustness of the adapted plan over the time period of one treatment fraction. Material and Methods Baseline MR-scans at the MR-linear accelerator (MRIdian Linac System Version 5, ViewRay Inc.) were acquired for ten healthy male volunteers. A mock-prostate SBRT plan was generated with 5mm CTV-to-PTV safety margins and dose prescription of 5 x 7.25Gy. On a separate day, the volunteers were positioned on the couch for one hour and repetitive MR images were acquired every 15 minutes to assess the stability of the adapted plan of the day. PTV- and CTV-coverage as well as dose to organs at risk were analyzed. Results We observed a dosimetric benefit in 90% of volunteers in the adapted plan compared to the baseline plan (D 95 CTV increased 0.41-9.4 Gy, average 2.8Gy). The median D 95 CTV- and D 95 PTV coverage was improved from 34.8Gy to 35.5Gy and 30.7Gy to 34.6Gy, respectively. This benefit was not associated with higher dose to the organs at risk, most importantly the rectum. The median D 1cc rectum was lower in the adapted plans compared to the baseline plans copied onto the MR of the day with 33.33Gy vs. 32.76Gy. The benefit of online adaptation remained stable over 45 minutes for 90% of volunteers. The median D 95 CTV remained sufficient (timepoint 0: 35.5Gy, timepoint 60mins: 35.3Gy; p = 0.44). However, at 60 minutes, CTV- coverage in 30% of volunteers was below a threshold of 32.5Gy (29.3Gy, 30.6Gy, 32.0Gy). Importantly, in those three cases D 95 CTV of the adapted plan still exceeded the baseline-plan copied onto the MR at timepoint 30 minutes in two cases and at 45 minutes in one case outlining the timeframe during which adaptation should be completed. Conclusion The dosimetric benefit of MR-guided online adaptation for prostate cancer was robust over 45 minutes in most volunteers. However, individual volunteers showed linear time drifts starting at 30 minutes. We therefore recommend a second MRI scan before dose delivery for verification purposes. PH-0126 Online 3D prostate cine-MRI on the MR-linac: dosimetric impact of intrafraction motion C. Kontaxis 1 , D. De Muinck Keizer 1 , T. Willigenburg 1 , M. Den Hartogh 1 , L. Kerkmeijer 1 , J. Van der Voort van Zyp 1 , B. Raaymakers 1 , J. Lagendijk 1 , H. De Boer 1 1 UMC Utrecht, Radiotherapy department, Utrecht, The Netherlands Purpose or Objective To investigate the effect of intrafraction translation and rotation motion extracted from online 3D cine-MRI on the delivered dose distributions of the first prostate patients treated on the Elekta Unity MR-linac at our institute. Material and Methods Five low/intermediate prostate patients were treated on a 1.5 T MR-linac (20 × 3.1 Gy) using daily contour/plan adaptation based on online MRI. During the delivery of each fraction, intrafraction 3D cine-MRI using a balanced 3D gradient echo sequence was acquired at 8 or 16 sec intervals. For each fraction a soft tissue tracking algorithm (ESTRO 38) was used to obtain the rigid prostate intrafraction motion of subsequent dynamics relative to the first cine-MRI of that session. The prostate position from the first cine-MRI was then registered to the daily MRI, transferring the local cine-MRI motion to the reference coordinate system. The linac treatment log files of each fraction were used to reconstruct the delivered dose on the respective cine-MRI dynamics. The necessary parameters —including MLC/gantry positions and Monitor Units— corresponding to the timepoint of each dynamic were extracted yielding several partial plan/3D volume combinations. For each partial plan, a pseudo-CT volume

was created by bulk density assignment of the corresponding cine-MRI. Then the partial dose was calculated using our research treatment planning system and was warped back to the reference volume by using the inverse rigid transformation for the purpose of dose accumulation. For each fraction the partial doses were summed leading to the accumulated fraction dose (INTRA) which was compared to the daily reference (REF) dose. Results Dose delivery took 5.5 min per fraction, corresponding to 22 cine-MRI dynamics on average. The mean±SD translations (mm) were 0.1±0.7 (LR), 1.0±1.9 (AP), - 1.0±2.0 (CC) and rotations (degrees) were -0.9±2.5 (LR), - 0.2±0.8 (AP), 0.01±1.2 (CC) among all timepoints of these 100 fractions. Figure 1 shows the D99% point of the target structures between REF and INTRA dose. The average drop in D99% coverage for the PTV, EBV and CTV was 10.9%±9.3%, 7.3%±7.3% and 2.1%±2.8% respectively. Figure 2 shows boxplots of the V62Gy DVH point for bladder and rectum, undergoing an average increase of 0.6cc ± 1.0cc and 0.1cc ± 1.0cc respectively.

Conclusion We present the first dosimetric impact analysis due to rigid prostate intrafraction motion for prostate MR-linac patients. This dose reconstruction based on soft-tissue tracking from online 3D cine-MRI and linac log files, enables accurate dosimetric evaluation with high spatial and temporal resolution. We are now evaluating the adequacy of the clinical margins in these treatments. These results allow us to investigate the optimal inter- and intrafraction adaptation methods for MRI-guided prostate radiotherapy, towards delivering individualized dynamic patient treatments in every fraction. PH-0127 Quantifying intra-fractional gastric wall motion for MR-guided radiotherapy T. Driever 1 , A. Van der Horst 2 , J. Teuwen 1 , M. Fast 3 , J. Sonke 1 1 Netherlands Cancer Institute, Radiation Oncology, Amsterdam, The Netherlands ; 2 Academic Medical Center, Radiotherapy, Amsterdam, The Netherlands ; 3 University Medical Center, Radiotherapy, Utrecht, The Netherlands Purpose or Objective Pre-operative radiotherapy is a novel approach in gastric cancer management. The stomach, however, is an organ