Dosimetric Assessment for Real-Time Anatomical Motion during MR-Guided Radiation Therapy by MR-LINAC

Abstract

<h3>Purpose/Objective(s)</h3> To develop a method to assess the dosimetric impact of real-time anatomical motion during MR-guided LINAC radiotherapy, and to validate adaptive treatment plans. <h3>Materials/Methods</h3> In MR-guided adaptive RT, the treatment plan is adapted to account for changes in the patient's anatomy before each fraction. This adaptive plan is presumably effective if the target motion throughout the beam-on duration is within the PTV margin. However, the amount of dosimetric impact from the internal motion during dose delivery is undetermined. It is important to precisely assess this dosimetric impact so that the current adaptive plan can be validated or be possibly improved by better margin definition. To reconstruct the 3D anatomical motion, three steps were conducted: (1) the real-time two-dimensional anatomical motion during dose delivery was captured by 2D monitoring MR images (mMR) continuously. The mMR images were time stamped and grouped into ten phases in a respiration cycle, which will then be synchronized to each segment (or control point) individually according to the log file. (2) A set of pre-acquired 4DCT images were separated into ten phases of CT volumes. All phases of CT volume were then registered to the 3D planning MR images (pMR) of the patient in the treatment position. (3) The registered CT volumes were correlated to the processed mMR images in a phase-to-phase basis and correlated to the segments individually. An in-house Monte Carlo method developed was used to simulate the dose distribution for each segment individually in the related CT volumes. The accuracy of the in-house MC tool is confirmed by benchmarking to the commissioning data in water phantom with difference <1% and standard uncertainty < 0.8 %. The delivered dose was then estimated by accumulating the dose from all segments in the plan. On the basis of this information, dose-volume histograms and 3D dose distributions were calculated to assess the discrepancy from the planned dose caused by anatomical motion. In this project, three prostate, pancreas, and liver cancer patients were retrospectively studied. The hypothesis is that the margin defined during adaptive planning is dosimetrically effective after taking into account the internal motions during dose delivery. <h3>Results</h3> The characteristic dose indices such as the minimum dose to the gross tumor volume (D2cc) and the maximum dose to the surrounding OARs are slightly different (both < 2%) for individual fractions but this effect was reduced after multiple fractions and no significant difference observed (<1%) in DVH. <h3>Conclusion</h3> The margin definition of adaptive planning was dosimetrically valid among three cases. The same setup and methodology of this study can be employed to test the patients with greater respiratory motion (such as lung cancers) or those treated with stereotactic body radiation therapy, which would benefit more from this assessment because the dose variation will be less faded over multiple fractions.