Assessment of dosimetric impact of system specific geometric distortion in an MRI only based radiotherapy workflow for prostate

Abstract

Dosimetric errors in a magnetic resonance imaging (MRI) only radiotherapy workflow may be caused by system specific geometric distortion from MRI. The aim of this study was to evaluate the impact on planned dose distribution and delineated structures for prostate patients, originating from this distortion. A method was developed, in which computer tomography (CT) images were distorted using the MRI distortion field. The displacement map for an optimized MRI treatment planning sequence was measured using a dedicated phantom in a 3 T MRI system. To simulate the distortion aspects of a synthetic CT (electron density derived from MR images), the displacement map was applied to CT images, referred to as distorted CT images. A volumetric modulated arc prostate treatment plan was applied to the original CT and the distorted CT, creating a reference and a distorted CT dose distribution. By applying the inverse of the displacement map to the distorted CT dose distribution, a dose distribution in the same geometry as the original CT images was created. For 10 prostate cancer patients, the dose difference between the reference dose distribution and inverse distorted CT dose distribution was analyzed in isodose level bins. The mean magnitude of the geometric distortion was 1.97 mm for the radial distance of 200-250 mm from isocenter. The mean percentage dose differences for all isodose level bins, were ⩽0.02% and the radiotherapy structure mean volume deviations were <0.2%. The method developed can quantify the dosimetric effects of MRI system specific distortion in a prostate MRI only radiotherapy workflow, separated from dosimetric effects originating from synthetic CT generation. No clinically relevant dose difference or structure deformation was found when 3D distortion correction and high acquisition bandwidth was used. The method could be used for any MRI sequence together with any anatomy of interest.

Intensity-based dual model method for generation of synthetic CT images from standard T2-weighted MR images – Generalized technique for four different MR scanners

“…Geometrical distortions stemming from system related effects such Table 3 Statistical analysis of the PTV mean dose differences between scanners with least significant difference (LSD) for the generalized model, p-values <0.05 are presented in bold. as gradient non-linearity or B 0 field inhomogeneities should be evaluated before introducing a clinical MRI-only workflow [37]. Additional artifacts can derive from patient induced distortions, such as susceptibility and chemical shift.…”

“…Previous studies have shown that with geometric accuracy better than 2 mm within the entire FOV it is possible to achieve clinically feasible dose calculation accuracy for MRI-only RTP [21,23,[38][39][40][41]. The geometric accuracy of the applied MR platforms was sufficient for MRI-only RT [10,11,21,23,37]. An MRI-only approach is feasible and utilized internationally in few clinics for radiotherapy of the prostate employing either commercial or in-house sCT techniques.…”

MRI-only based radiation therapy of prostate cancer: workflow and early clinical experience

“…The system-related distortion has been reduced significantly by recent development on MRI hardware and software and the patient-induced distortion can be minimized by appropriate sequence parameters. The geometric accuracy achieved by recently installed MR platforms is acceptable for RT planning [5][6][7][8][9][10][11].…”

Evaluation of dosimetric uncertainty caused by MR geometric distortion in MRI‐based liver SBRT treatment planning

“…detected up to 2 mm geometric distortion for a range of radial distances up to approximately 135 mm by performing rigid spatial co‐registration between MR and CT images, and investigated the dosimetric uncertainties caused by spatial offsets of 0.5 up to 3 mm to the target locations. Christian et al . measured a displacement map by detecting the difference of markers in the phantom between MR and CT images, and evaluated the dosimetric uncertainties for prostate patient by applying the displacement map to CT images.…”