Physics contribution
Patterns of patient movement during frameless image-guided radiosurgery

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Abstract

Purpose

Image-guided radiosurgery aligns the treatment beam to the target site by using a radiographic imaging system to locate anatomic landmarks associated with the treatment target. Because the procedure is performed without a rigid frame, the precision of dose alignment can be affected by patient movement. Movement is limited by noninvasive restraints and compensated by remeasuring the target position at short intervals throughout treatment and then realigning the beam. Frameless image-guided radiosurgery has been used at our institution to treat 250 cranial, 23 spinal, 9 lung, and 3 pancreas cases involving malignant and benign tumors as well as vascular malformations. We have analyzed the target position records for all of these cases to assess the frequency, magnitude, and case-by-case patterns of patient movement.

Methods and materials

The position of the treatment site during image-guided radiosurgery was measured at approximately 1–2-min intervals, on average, using orthogonal amorphous silicon X-ray cameras and an image registration process that determined all six degrees of freedom in the target’s position. The change in position from one measurement to the next was indicative of patient movement.

Results

The treatment site position along each axis of translation was observed to vary by an average of 0.45 mm for the cranium, 0.53 mm for the cervical spine, 0.53 mm for the lumbar and thoracic spine, 1.06 mm for the lung, and 1.50 mm for the pancreas. Half of all cranial cases showed systematic drifting of the target away from the initial setup position.

Conclusions

Using noninvasive restraints and supports, short-term movement of the head and spine during image-guided radiosurgery was limited to a radius of 0.8 mm, which satisfies the prevailing standard for radiosurgical dose alignment precision, but maintaining this margin of error throughout a treatment fraction requires regular monitoring of the target site’s position.

Introduction

Radiosurgery, developed by Lars Leksell (1), delivers an ablative dose of radiation to the treatment site in one fraction, using a three-dimensional configuration of small-diameter radiation beams. It can be performed using a fixed array of radiation sources or a movable linear accelerator. In the past 50 years radiosurgery has been used with considerable success to treat a wide range of lesions in the brain, including benign and malignant tumors and vascular malformations. It has also been used in a functional capacity to treat trigeminal neuralgia and epilepsy.

Radiosurgery requires high accuracy in dose alignment, both to provide adequate dose coverage of the target and to avoid injuring healthy normal tissue. The required accuracy standard is customarily recognized to be 1–2 mm overall uncertainty in dose placement (2). At this level of accuracy, even small patient movements can compromise the treatment.

The necessary accuracy has traditionally been obtained by rigidly immobilizing the patient in a frame that defines a coordinate system for configuring the treatment beams and simultaneously positions the target site at the isocenter of the radiation delivery device. When tested with phantoms, this approach has demonstrated 1–2 mm precision in the alignment of dose distributions, but frame-based treatment has three shortcomings: the frame is invasive, fractionation is inhibited, and treatment is restricted to sites amenable to frame fixation. Frameless image-guided radiosurgery has been developed in an effort to overcome these limitations. However, when the treatment site is not immobilized with a rigid frame, patient movement can potentially affect dose alignment. Therefore, it is important to characterize the movement that is likely to occur during frameless radiosurgery.

Some “frameless” radiosurgery concepts, such as the optical tracking system described by Ryken et al. (3), allow the stereotactic frame’s function as a coordinate reference to be transferred to a less invasive optical tracking fixture. Such a system provides passive monitoring of position during treatment but cannot adapt beam alignment in response to patient motion. Furthermore, these tracking devices are designed for use with gantry-mounted linear accelerators (linacs) or other isocentric beam delivery systems. Consequently, it still remains necessary to control the position of the treatment site at the beam isocenter, either with an immobilization device or by repositioning the patient each time movement occurs.

The CyberKnife is a frameless, image-guided, stereotactic radiosurgery system that uses X-ray radiographic imaging in the treatment room to locate and track the treatment site while controlling the alignment of radiation beams via a robot-mounted linear accelerator (4). This system is unique in the way it eliminates all three functions of the stereotactic frame: (1) the target reference coordinates provided by the external frame are replaced by reference coordinates in the patient’s anatomy; (2) the radiation delivery system has no isocenter, but instead moves the beam to the position of the tumor; and (3) a real-time control loop between the imaging and beam delivery systems allows the beam to follow a moving target, eliminating the need for rigid fixation.

The design of the CyberKnife makes it intrinsically capable of treating sites anywhere in the body in either a single-fraction or multifraction manner, without the use of an invasive frame or immobilization device. This system has been in use at our institution for 7 years. During this period, it has been used to treat both cranial and extracranial lesions in more than 300 patients. During each treatment, the imaging system captured a sequence of measurements of the target position that provides a valuable record of patient movement.

We have reviewed the records of target position for 285 CyberKnife treatments to analyze patient movement in several different treatment scenarios. From these data, we are able to characterize the patient movement allowed by the patient support and restraint procedures used during treatment, and from those characteristics to assess the adequacy with which automatic image-guided beam alignment techniques can compensate for the observed patterns of movement. These data will allow the physician to estimate, for a specific treatment scenario, the overall accuracy of radiosurgical dose delivery for any given schedule of intraoperative target position measurements.

Section snippets

Methods and materials

The CyberKnife radiosurgery system is illustrated in Fig. 1. The system utilizes a commercial robotic arm to direct the X-ray beam generated by an X-band 6-MV linac. A pair of orthogonally positioned amorphous silicon diagnostic X-ray cameras is used to image the patient during treatment. The acquired radiographs are automatically registered to digitally reconstructed radiographs (DRRs) derived from the treatment planning computed tomography (CT) study to determine the patient’s position. A

Cranial radiosurgery

The target position records for 250 cranial treatment cases were analyzed for this study. All six degrees of freedom (translation and rotation) in the skull position were measured for 202 of the cases; for the remaining 48, only the three degrees of translational position were measured. The patients were treated in one to five fractions. For each patient, the position measurements over all fractions were combined to characterize that patient’s movement. The position record for each fraction

Discussion

The CyberKnife allows radiosurgical treatment with minimal or no patient restraint. The face mask used during cranial treatments, while restrictive, does not completely immobilize the head. The spine support cradle minimizes motion without confining the patient. Any residual movement is compensated by controlling the beam direction rather than the patient’s position. Consequently, the adaptive delivery system can complete an entire radiosurgery fraction without ever adjusting the patient’s

Summary

With the head restrained by a conformable plastic mask, or with the spine supported supine by a conformable cradle support, the typical image-guided stereotactic radiosurgery patient stayed within a radius of 0.85 mm, on average, of the intended target position over short (1–5 min) periods of time. This meets the generally accepted targeting standard for radiosurgery and indicates that the current methods of restraint provide satisfactory stabilization. However, the accumulation of

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