The Impact of System-Related Magnetic Resonance Imaging Geometric Distortion in Stereotactic Radiotherapy: A Case Report

Abstract

Magnetic resonance imaging (MRI) is now essential in stereotactic radiotherapy (SRT) planning for brain tumors because of its excellence in soft-tissue contrast and high spatial resolution. However, MRI distortion is sometimes difficult to recognize, and it may cause large misalignments in radiotherapy planning. In this case report, we will show how much difference in the dose distribution of SRT can be made by using MRI without distortion correction.

Keywords: brain metastases; distortion; distortion correction; magnetic resonance imaging; radiotherapy treatment planning; stereotactic radiotherapy.

Figure 1. Dose distribution comparison between tumor delineation with and without distortion correction (DC)

(a) GTV on MRI without DC; (b) How GTV contoured on MRI without DC (pink line) looks like on MRI with 3D-DC. GTV is not covering some parts, especially the outer part, of the contrast-enhanced tumor; (c) Geometric difference between (a) and (b). The gratitude of the distortion depends on the distance from the central axis; (d) A stereotactic radiotherapy plan optimized based on MRI without DC showing beautiful coverage of the GTV and PTV; (e) Re-contouring performed on MRI with DC (blue). 2-3 mm difference is captured; (f) Actual dose distribution the patient will receive if the plan was made on MRI without DC. PTV coverage and dose to GTV are both inadequate.

Figure 2. Dose-volume histogram (DVH) and dose coverage of PTV and GTV

(a) Dotted line showing the DVH of PTV and GTV of plan optimized based on MRI without DC (Figure 1d) and solid line showing DVH of PTV and GTV the patient will receive (Figure 1f). (b) V23.4Gy of GTV and V14.4Gy of PTV are shown. If the treatment plan was optimized on MRI without DC (blue), actual GTV and PTV (re-contoured on MRI with DC) will not be covered as planned (orange).