Prog Med Phys.  2022 Dec;33(4):142-149. 10.14316/pmp.2022.33.4.142.

Dosimetric Evaluation of Synthetic Computed Tomography Technique on Position Variation of Air Cavity in Magnetic Resonance-Guided Radiotherapy

• Jin H
• An HJ
• Chie EK
• Park JM
• Kim Ji

• KMID: 2537883
• DOI: http://doi.org/10.14316/pmp.2022.33.4.142

Abstract

Purpose
This study seeks to compare the dosimetric parameters of the bulk electron density (ED) approach and synthetic computed tomography (CT) image in terms of position variation of the air cavity in magnetic resonance-guided radiotherapy (MRgRT) for patients with pancreatic cancer.
Methods
This study included nine patients that previously received MRgRT and their simulation CT and magnetic resonance (MR) images were collected. Air cavities were manually delineated on simulation CT and MR images in the treatment planning system for each patient. The synthetic CT images were generated using the deep learning model trained in a prior study. Two more plans with identical beam parameters were recalculated with ED maps that were either manually overridden by the cavities or derived from the synthetic CT. Dose calculation accuracy was explored in terms of dose-volume histogram parameters and gamma analysis.
Results
The D _95% averages were 48.80 Gy, 48.50 Gy, and 48.23 Gy for the original, manually assigned, and synthetic CT-based dose distributions, respectively. The greatest deviation was observed for one patient, whose D _95% to synthetic CT was 1.84 Gy higher than the original plan.
Conclusions
The variation of the air cavity position in the gastrointestinal area affects the treatment dose calculation. Synthetic CT-based ED modification would be a significant option for shortening the time-consuming process and improving MRgRT treatment accuracy.

Keyword

Magnetic resonance-guided radiotherapy; Adaptive radiotherapy; Pancreatic cancer; Air cavity; Synthetic computed tomography

Figure

• 

  Fig. 1 Schematic diagram of the overall procedure. CT, computed tomography; MR, magnetic resonance.

• 

  Fig. 2 Example of simulation MR (left) and CT (center) images with air cavities located near the organ at risk. The synthetic CT image generated from the simulation MR is displayed at the right. MR, magnetic resonance; CT, computed tomography.

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  Fig. 3 Dose-volume histograms of representative cases: case surpassing the threshold of the modified Z-score (a), and relatively lower score case (b). The solid, dash and dot lines represent the Original, bulk density assign, and sCT dose distributions, respectively. PTV, planning target volume; sCT, synthetic computed tomography-driven electron density.

• 

  Fig. 4 Example of simulation MR (left) and CT (center) slices with air cavities located near the target volume. The synthetic CT image generated from the simulation MR is displayed at the right. MRI, magnetic resonance; CT, computed tomography.

Reference

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