MRI-Related Geometric Distortions in Stereotactic Radiotherapy Treatment Planning: Evaluation and Dosimetric Impact

Abstract

In view of their superior soft tissue contrast compared to computed tomography, magnetic resonance images are commonly involved in stereotactic radiosurgery/radiotherapy applications for target delineation purposes. It is known, however, that magnetic resonance images are geometrically distorted, thus deteriorating dose delivery accuracy. The present work focuses on the assessment of geometric distortion inherent in magnetic resonance images used in stereotactic radiosurgery/radiotherapy treatment planning and attempts to quantitively evaluate the consequent impact on dose delivery. The geometric distortions for 3 clinical magnetic resonance protocols (at both 1.5 and 3.0 T) used for stereotactic radiosurgery/radiotherapy treatment planning were evaluated using a recently proposed phantom and methodology. Areas of increased distortion were identified at the edges of the imaged volume which was comparable to a brain scan. Although mean absolute distortion did not exceed 0.5 mm on any spatial axis, maximum detected control point disposition reached 2 mm. In an effort to establish what could be considered as acceptable geometric uncertainty, highly conformal plans were utilized to irradiate targets of different diameters (5-50 mm). The targets were mispositioned by 0.5 up to 3 mm, and dose–volume histograms and plan quality indices clinically used for plan evaluation and acceptance were derived and used to investigate the effect of geometrical uncertainty (distortion) on dose delivery accuracy and plan quality. The latter was found to be strongly dependent on target size. For targets less than 20 mm in diameter, a spatial disposition of the order of 1 mm could significantly affect (>5%) plan acceptance/quality indices. For targets with diameter greater than 2 cm, the corresponding disposition was found greater than 1.5 mm. Overall results of this work suggest that efficacy of stereotactic radiosurgery/radiotherapy applications could be compromised in case of very small targets lying distant from the scanner’s isocenter (eg, the periphery of the brain).

Large field of view distortion assessment in a low‐field MR‐linac

“…For most radiation therapy applications, an accuracy of 1 mm or higher is necessary . In cases where the target size is smaller, geometric accuracy becomes a more critical issue, for example, for a target size of 3 cm, geometric distortions of 1.5 mm may impact the dose to 95% of the volume . According to a recent consensus paper from seven clinical institutes, in the case of MRI simulation for external beam radiation therapy, geometric distortion of <2 mm in the central region of treatment is deemed to be acceptable for sites such as brain, head and neck, and cervix .…”

“…Subject‐specific distortions are mainly due to the chemical shift and magnetic susceptibility while GNL and field inhomogeneity are system‐specific causes of distortion. Several factors such as type and field strength of the MR scanner, imaging sequence, and receiver bandwidth can influence the magnitude of distortions from one or more of these sources . It has been shown that at low magnetic field strengths, effects of susceptibility and chemical shift are negligible .…”

“…Several factors such as type and field strength of the MR scanner, imaging sequence, and receiver bandwidth can influence the magnitude of distortions from one or more of these sources. [15][16][17] It has been shown that at low magnetic field strengths, effects of susceptibility and chemical shift are negligible. 11 GNL is one of the major contributors to spatial distortion, which is independent of subject and imaging sequence.…”

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

“…Previous studies evaluated the geometric distortion and dosimetric effect using different approaches. Eleftherios et al . 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.…”

Dosimetric impact of rotational errors on the quality of VMAT‐SRS for multiple brain metastases: Comparison between single‐ and two‐isocenter treatment planning techniques

“…4,5 However, increased conformity and presence of steep dose gradients in SRS treatment plans demand increased spatial accuracy in order to ensure effective treatment delivery, as spatial errors of just a few millimeters can induce considerable target underdosage, especially in tiny brain lesions. [6][7][8] Volumetric modulated arc therapy (VMAT) is commonly employed for SRS treatment delivery. Several studies have demonstrated that multiarc noncoplanar VMAT can deliver highly conformal plans to the target(s) and spare adjacent critical structures.…”