Intra-fractional stability of MR-guided online adaptive SBRT for prostate cancer
PH-0125
Abstract
Intra-fractional stability of MR-guided online adaptive SBRT for prostate cancer
Authors: Heitmann|, Jana(1)*[jana.heitmann@usz.ch];Chamberlain|, Madalyne(1);Wilke|, Lotte(1);Baumgartl|, Michael(1);Krayenbühl|, Jerome(1);Zamburlini|, Mariangela(1);Mayinger|, Michael(1);Andratschke|, Nicolaus(1);Tanadini-Lang|, Stephanie(1);Guckenberger|, Matthias(1);
(1)University Hospital Zürich, Radiation Oncology, Zurich, Switzerland;
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Purpose or Objective
MR-guided online adaptation for prostate stereotactic body radiation therapy (SBRT) aims to decrease safety margins and consequently reduce toxicity by compensation of inter-fractional non-rigid target and organ-at-risk variations. However, the process of online adaptation currently takes approximately 45 minutes during which 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 (D95CTV increased 0.41-9.4 Gy, average 2.8Gy). The median D95CTV- and D95PTV 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 D1ccrectum 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 D95CTV 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 D95CTV 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.