Physics Contribution
Magnetic Resonance Imaging Assessment of Spinal Cord and Cauda Equina Motion in Supine Patients With Spinal Metastases Planned for Spine Stereotactic Body Radiation Therapy

https://doi.org/10.1016/j.ijrobp.2014.12.037Get rights and content

Purpose

To assess motion of the spinal cord and cauda equina, which are critical neural tissues (CNT), which is important when evaluating the planning organ-at-risk margin required for stereotactic body radiation therapy.

Methods and Materials

We analyzed CNT motion in 65 patients with spinal metastases (11 cervical, 39 thoracic, and 24 lumbar spinal segments) in the supine position using dynamic axial and sagittal magnetic resonance imaging (dMRI, 3T Verio, Siemens) over a 137-second interval. Motion was segregated according to physiologic cardiorespiratory oscillatory motion (characterized by the average root mean square deviation) and random bulk shifts associated with gross patient motion (characterized by the range). Displacement was evaluated in the anteroposterior (AP), lateral (LR), and superior-inferior (SI) directions by use of a correlation coefficient template matching algorithm, with quantification of random motion measure error over 3 separate trials. Statistical significance was defined according to P<.05.

Results

In the AP, LR, and SI directions, significant oscillatory motion was observed in 39.2%, 35.1%, and 10.8% of spinal segments, respectively, and significant bulk motions in all cases. The median oscillatory CNT motions in the AP, LR, and SI directions were 0.16 mm, 0.17 mm, and 0.44 mm, respectively, and the maximal statistically significant oscillatory motions were 0.39 mm, 0.41 mm, and 0.77 mm, respectively. The median bulk displacements in the AP, LR, and SI directions were 0.51 mm, 0.59 mm, and 0.66 mm, and the maximal statistically significant displacements were 2.21 mm, 2.87 mm, and 3.90 mm, respectively. In the AP, LR, and SI directions, bulk displacements were greater than 1.5 mm in 5.4%, 9.0%, and 14.9% of spinal segments, respectively. No significant differences in axial motion were observed according to cord level or cauda equina.

Conclusions

Oscillatory CNT motion was observed to be relatively minor. Our results support the importance of controlling bulk patient motion and the practice of applying a planning organ-at-risk margin.

Introduction

The application of stereotactic body radiation therapy (SBRT) to spinal metastases has steadily increased in recent years as the therapeutic intent has shifted from control of symptoms alone to durable local control (1). The challenge with spine SBRT lies in keeping the dose to the spinal cord and thecal sac, critical neural tissues (CNT), within tolerance levels while satisfying the competing objective of delivering adequate dose to the adjacent tumor. Excessive sparing of the CNT risks local progression (epidural disease progression has been recognized as the most common pattern of failure) (2) while allowing higher than safe doses to the CNT increases the risk of radiation myelopathy (3). Therefore, safe spine SBRT planning requires accurate and precise delineation of the CNT and the application of an appropriate maximum point dose volume constraint. Rather than applying the constraint to the CNT itself, some advocate for a planning organ-at-risk volume (PRV). In essence, the PRV margin is analogous to the planning target volume margin, which is routinely defined beyond the clinical target volume to account for sources of uncertainty in contouring, delivery, patient positioning, and organ motion. The known evidence-based dose limits for safe spine SBRT practice with respect to the spinal cord have been conservatively derived on the basis of the thecal sac as a surrogate for the spinal cord (3), and the thecal sac is typically equivalent to a 1.5-mm PRV beyond the delineated spinal cord.

One of the main components considered in the PRV margin is organ motion. Spinal cord motion secondary to periodic cerebrospinal fluid (CSF) movement was first described in the 1940s, when transmission of arterial pulsations in the skull were thought to generate pressure waves along the outflow path into the spinal canal 4, 5. The potential for spinal cord motion has since been studied with the use of various imaging techniques that include intraoperative ultrasonography, phase-contrast magnetic resonance imaging (MRI), functional magnetic resonance imaging (fMRI) in conjunction with diffusion-tensor imaging, and dynamic MRI (dMRI) 6, 7, 8, 9, 10, 11, 12. However, these investigations have been limited with respect to sample size, axis of motion, and the evaluation of healthy volunteers as opposed to patients with spinal metastases.

The aim of the present study was to determine whether motion of the CNT is in fact an issue to consider, in particular when the vertebral segment is affected by metastatic disease with or without epidural disease extension. We report our CNT motion analysis, using non–cardiac-gated dMRI, as a composite of physiologic oscillatory motion and random patient bulk motion in both the axial and the sagittal planes for 74 spinal metastases planned for spine SBRT. This is the largest and most comprehensive analysis on the subject to date.

Section snippets

Methods and Materials

Sixty-five consecutive patients, comprising a total of 74 spinal metastases, underwent treatment planning for spine SBRT at the Princess Margaret Cancer Centre between March 2011 and July 2013. With the patient in the supine position, dynamic axial and sagittal MRI (3 Tesla Verio, Siemens) sequences were obtained at the midaxial vertebral level and the midsagittal plane, respectively, for each vertebral segment to be treated with SBRT as part of routine MRI simulation for treatment planning.

Results

Figure 2A illustrates a typical example of automatic and manual tracking. Statistically, there were no significant differences (95% confidence level) in calculated CNT motion between the 2 methods at more than 95% of the time points. However, it should be noted that the precision (ie standard deviation over multiple trials) of manual tracking (=0.5 ± 0.5 mm) was approximately 4 to 5 times worse than that of the automated method (=0.1 ± 0.05 mm) (Fig. 2A). To provide further insight into the

Discussion

We report the largest and most comprehensive analysis of CNT motion, as a composite of physiologic oscillatory motion and random bulk motion, at the level of vertebrae involved with cancer in both the axial and sagittal planes. On the basis of an imaging acquisition time of 137 seconds and 2.3 seconds per image, the median magnitude of significant physiologic oscillatory motion in all directions was between 0.16 mm and 0.44 mm (range, 0.12-0.77 mm). The median random bulk motion magnitude was

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    Conflict of interest: A. Sahgal has received honoraria for educational seminars from Medtronic Kyphoplasty division and Elekta AB, and research grants from Elekta AB. The authors report no other conflict of interest.

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