Abstract
PurposeA 3D printed geometric phantom was developed that can be scanned with computed tomography (CT) and magnetic resonance imaging (MRI) to measure the geometric distortion and determine the relevant dose changes.Materials and MethodsA self-designed 3D printed photosensitive resin phantom was used, which adopts grid-like structures and has 822 1 cm2 squares. The scanning plan was delivered by three MRI scanners: the Elekta Unity MR-Linac 1.5T, GE Signa HDe 1.5T, and GE Discovery-sim 750 3.0T. The geometric distortion comparison was concentrated on two 1.5T MRI systems, whereas the 3.0T MRI was used as a supplemental experiment. The most central transverse images in each dataset were selected to demonstrate the plane distortion. Some mark points were selected to analyze the distortion in the 3D direction based on the plane geometric distortion. A treatment plan was created with the off-line Monaco system.ResultsThe distortion increases gradually from the center to the outside. The distortion range is 0.79 ± 0.40 mm for the Unity, 1.31 ± 0.56 mm for the GE Signa HDe, and 2.82 ± 1.48 mm for the GE Discovery-sim 750. Additionally, the geometric distortion slightly affects the actual planning dose of the radiotherapy.ConclusionGeometric distortion increases gradually from the center to the outside. The distortion values of the Unity were smaller than those of the GE Signa HDe, and the Unity has the smallest geometric distortion. Finally, the Unity’s dose variation best matched with the standard treatment plan.
Highlights
Radiation therapy aims to maximize the delivered dose to tumor, while sparing normal tissue
The geometric distortion affects the original images, which affects the delineation of the gross tumor volume (GTV) and clinical target volume (CTV), affecting the treatment plans and dose delivery
By referencing the research of Dorsch et al [19], the mean distortion over the whole phantom was 0.60 ± 0.28 mm and 99.80% of the evaluated control points had distortions below 1.5 mm. This may prove that the precision requirements for the geometric distortion of magnetic resonance imaging (MRI)-Linac are stricter than for the ordinary MR, the scanning system and scanning sequence are similar to diagnostic MR scanners that are used in clinical applications [25]
Summary
Radiation therapy aims to maximize the delivered dose to tumor, while sparing normal tissue. Image-guided radiation therapy (IGRT) takes into account that changes in tumor size and shape during therapy may occur and allows for adapted treatment plans based on same-day tumor localization and volume measures. Compared with CT, magnetic resonance imaging (MRI) has the advantage of having a superior soft-tissue contrast. MRI has unique advantages, it has evident shortcomings, such as geometric distortions, signal dropout, and artifacts. Geometric distortion is an important factor in MRI-guided radiation therapy [9]. The geometric distortion affects the original images, which affects the delineation of the gross tumor volume (GTV) and clinical target volume (CTV), affecting the treatment plans and dose delivery
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