Abstract
Online adaptive radiotherapy (ART) with frequent imaging has the potential to improve dosimetric accuracy by accounting for anatomical and functional changes during the course of radiotherapy. Presented are three interesting cases that provide an assessment of online adaptive magnetic resonance-guided radiotherapy (MRgRT) for lung stereotactic body radiotherapy (SBRT).The study includes three lung SBRT cases, treated on an MRgRT system where MR images were acquired for planning and prior to each treatment fraction. Prescription dose ranged from 48 to 50 Gy in four to five fractions, normalized to where 95% of the planning target volume (PTV) was covered by 100% of the prescription dose. The process begins with the gross tumor volume (GTV), PTV, spinal cord, lungs, heart, and esophagus being delineated on the planning MRI. The treatment plan was then generated using a step-and-shoot intensity modulated radiotherapy (IMRT) technique, which utilized a Monte Carlo dose calculation. Next, the target and organs at risk (OAR) contours from the planning MRI were deformably propagated to the daily setup MRIs. These deformed contours were reviewed and modified by the physician. To determine the efficacy of ART, two different strategies were explored: 1) Calculating the plan created for the planning MR on each fraction setup MR dataset (Non-Adapt) and 2) creating a new optimized IMRT plan on the fraction setup MR dataset (FxAdapt). The treatment plans from both strategies were compared using the clinical dose-volume constraints.PTV coverage constraints were not met for 33% Non-Adapt fractions; all FxAdapt fractions met this constraint. Eighty-eight percent of all OAR constraints studied were better on FxAdapt plans, while 12% of OAR constraints were superior on Non-Adapt fractions. The OAR that garnered the largest benefit would be the uninvolved lung, with superior sparing in 92% of the FxAdapt studied. Similar, but less pronounced, benefits from adaptive planning were experienced for the spinal cord, chest wall, and esophagus.Online adaptive MR-guided lung SBRT can provide better target conformality and homogeneity and OAR sparing compared with non-adaptive SBRT in selected cases. Conversely, if the PTV isn’t adjacent to multiple OARs, then the benefit from ART may be limited. Further studies, which incorporate a larger cohort of patients with uniform prescriptions, are needed to thoroughly evaluate the benefits of daily online ART during MRgRT.
Highlights
Lung stereotactic body radiotherapy (SBRT) has proven to be effective in the local control of lung tumors [1]
Utilizing MR-guided adaptive radiotherapy (ART) has the potential to improve target coverage and to provide better organs at risk (OAR) sparing by using the daily 3D magnetic resonance imaging (MRI) to account for anatomical and functional changes and adapt the plan to deliver the optimal treatment for that day [2,3,4]
For certain lung SBRT patients, ART strategies have significant benefits when using MR-guided radiotherapy (MRgRT) systems while others have more subdued or negligible benefits. From these few examples, having several OARs adjacent to the PTV is the strongest indicator of deriving a benefit from ART
Summary
Lung stereotactic body radiotherapy (SBRT) has proven to be effective in the local control of lung tumors [1]. Presented are three interesting cases that provide an assessment of online adaptive MR-guided radiotherapy (MRgRT) for lung SBRT with the purpose of beginning to determine under what circumstances are there significant advantages to ART and, just as crucial, under what circumstances it does not provide a measurable benefit. Case #1 is a 66-year-old male with a T3N1M1a adenocarcinoma of the lung; M1a disease was designated due to a solitary tumor in the opposite lung He received 50 Gy in four fractions to the central lesion in the right lung. Case #2 is an 87-year-old female with a cT1bN0M0 lung adenocarcinoma She received 50 Gy in five fractions to the peripheral lesion in the right lung. Case #3 is a 70-year-old male with an adenocarcinoma of the pancreas metastatic to the lung He received 48 Gy in four fractions to the peripheral lesion in the left lung.
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