Assessment of Online Adaptive MR-Guided SBRT of Liver Cancers

Abstract

Online Adaptive Radiotherapy (ART) with daily MR imaging has the potential to improve target coverage by taking into account for anatomical changes during radiotherapy. The purpose of this study is to provide an assessment of potential benefits of online adaptive MRI-Guided Stereotactic Body Radiotherapy (SBRT) for treatment of liver cancer. Five patients with liver cancer who were treated with MR-Guided SBRT were included. Prescription doses ranged between 27-50 Gy in 3-5fx, with the 90%-95% of the PTV covered by 100% of the prescription dose. All patients underwent a planning MRI and setup MRIs prior to each fraction, all acquisitions utilized breath-hold conditions. All SBRT fractions were delivered under MR guidance with cine-MR gating of the gross tumor volume (GTV) based on a modified breath-hold technique. Patients received visual feedback throughout treatment delivery. The planning target volume (PTV) was created by expanding the GTV by 3mm uniform margin. The initial treatment plan was generated using step-and-shoot IMRT and utilized 9-12 beam groups, each beam group consisted of 3 equally spaced Co-60 beams. The electron density (ED) map and contours from planning MR were propagated to daily setup MRI using deformable image registration and were reviewed/edited by the physician. Each adapted plan was obtained by re-optimizing based on the contours on daily setup MRI using the same beam parameters and optimization objectives from the initial MR plan. Non-adaptive plans for each patient were created by recalculating the dose from the initial plan on the daily setup MRI, daily ED map and edited contours. The non-adaptive and adaptive plans were compared using PTV coverage and dose-volume constraints for OARs: duodenum, bowel, stomach, kidneys, and spinal cord. Daily plan adaptation provided increased PTV coverage for 11 of 23 fractions. While 7 fractions required adaptations due to violation of maximum dose constraint for duodenum and 3 fractions required adaptation due to violation of V30Gy <5cc constraint for bowel. Up to 27% decrease in mean dose to liver-GTV was achieved when using daily adaption even though this constraint was satisfied for all fractions. Similarly, up to 28% reduction in maximum cord dose and up to 18% reduction in mean kidney dose was achieved with daily adaptation. Online ART MR-Guided SBRT of liver cancer using daily re-optimization resulted in better target conformity and OAR sparing compared with non-adaptive SBRT. In our experience, adaptation provides the largest benefit when two or more OARs are proximal to the target additionally the larger the intra-fractional motion of the target and OARs the greater the benefit. Significant OAR sparing was still achieved for fractions that did not exceed OAR tolerances. Daily adaptation may allow for PTV dose escalation without compromising OAR sparing as compared to non-adaptive treatments. Further studies are needed to determine the limits to any dose escalation strategy.