Respiratory Motion Tracking for Robotic Radiosurgery
Wed, 08/05/2009 - 10:44am
Tumors in the thorax and abdomen move up to several centimeters during respiration. This intra-fraction motion impacts all forms of external beam radiation therapy and is an issue that is becoming increasingly important in the era of image-guided radiotherapy. This article presents the concepts and methods of a recently developed approach that is a type of real-time respiratory motion tracking.
Calvin R. Maurer, Jr., Ph.D. is the VP of research at Accuray Inc. He has led the research and development of several technologies at Accuray including image fusion, Monte Carlo dose calculation, treatment plan optimization, image-based tracking, and beam collimation using internal research efforts and collaborations with external partners. Dr. Maurer can be reached at 408-716-4600 or firstname.lastname@example.org.
Another set of approaches attempt to minimize the margin by delivering radiation when the tumor is at a relatively fixed and reproducible position. Breath holding has long been used in diagnostic radiology to reduce the blurring of images. For radiation therapy, the goal is to attain the same breath-hold position between beams delivered during treatment fractions. Treatment is delivered during the breath-hold period. Breath holding is physically demanding and uncomfortable, and breath-hold repeatability and patient compliance are challenges, especially for elderly patients or patients with compromised pulmonary capacity, common among patients with lung cancer or other pulmonary disease. Thus, breath-hold methods may not be applicable to a significant population of patients.
With respiratory gating approaches, the patient continues breathing normally. The radiation beam is turned on only within a specified portion of the patient's breathing cycle, which is commonly referred to as the "gate." The position and width of the gate are determined by monitoring the patient's respiratory motion using an external respiration signal. The delivery of radiation during a limited portion of the breathing cycle can substantially reduce the duty cycle (the ratio of the gate width to the respiratory cycle period) and thus, increase the treatment time. The duty cycle is typically about 25%. Lesion motion and gating model stability, which can adversely impact the planned dose distribution, are also challenges for gating methods.
Respiratory Motion Tracking
Real-time tracking requires a method to move or shape the radiation beam relative to the moving target. For photon beams, there are three main ways to achieve this: move the patient using the treatment couch, change the aperture of the collimator, and move the beam by physically repositioning the radiation source [e.g., linear accelerator (LINAC)]. Robotic couch-based motion tracking has been shown to be technically feasible for real-time compensation of intra-fraction respiratory motion. However, continuous couch motion associated with real-time respiratory motion tracking has the practical issues of patient comfort and treatment tolerance. Alternatively, the beam can be effectively moved by changing the aperture of the collimator. The technical feasibility of this approach has been demonstrated for a multi-leaf collimator (MLC). However, there are several potential technical limitations to this approach. For example, the MLC motion required for target tracking superimposes on that required for intensity modulation, increasing the chances of exceeding the physical speed limitations of the MLC. A third approach is to physically reposition the radiation source to follow the tumor's changing position. The Synchrony Respiratory Tracking System is a realization of this approach.
Synchrony Respiratory Tracking SystemThe CyberKnife Robotic Radiosurgery System (Figure 1) moves the radiation beam by physically repositioning the radiation source. A miniature lightweight 6 MV X-band LINAC is mounted to an industrial multi-jointed robotic manipulator that can move freely and accurately aim the radiation beam with six degrees of freedom. Two digital x-ray imaging systems are orthogonally configured. The x-ray generators and amorphous silicon x-ray detectors are rigidly fixed so that their projection camera geometry is calibrated and known with high accuracy. Computer algorithms automatically compare the projection images of the target region with the patient's treatment planning CT image.
The Synchrony Respiratory Tracking System is an integrated subsystem of the CyberKnife system that allows irradiation of extracranial tumors that move due to respiration. One advantage is that patients can breathe normally during continuous treatment, enabling no reduction in the duty cycle.
The Synchrony system uses external optical markers to provide a breathing signal. Three optical markers are attached to a snugly fitting vest the patient wears during treatment (Figure 3). The optical marker positions correspond to the chest wall position. Light-emitting diodes (LEDs) transmit light through optical fibers that terminate at the cylindrical optical marker. The optical markers are sequentially strobed and a stereo camera system, consisting of three linear charge-coupled device detector arrays, measures the 3D marker positions continuously at a frequency of approximately 30 Hz.
Inter- and intra-fraction changes in position and motion are common. A correlation model that addresses the issue of inter-fraction variability is generated at the beginning of every treatment. However, the target position and motion typically changes during the treatment. This could be caused by gradual patient relaxation throughout the treatment period. In the lung, this could be attributed to gravity action on compliant lung tissue. Thus, it is important to regularly check and update the correlation model during treatment. This is accomplished in the Synchrony System by acquiring additional x-ray images every one to two minutes. Each newly acquired data point is used to update the correlation model and thus, the model adapts to gradual changes in target position and motion during treatment.
ConclusionPatients are living, breathing beings, and this movement must be taken into account if ultimate treatment accuracy is to be achieved. The CyberKnife System with the integrated Synchrony Respiratory Tracking System offers real-time tracking for tumors that move with respiration. Patients lie comfortably and breathe freely as the treatment beam is moved in "synchrony" with respiration, allowing highly conformal delivery of radiation to tumors with the smallest possible margins.
OnlineFor additional information on the technologies and products discussed in this article, see MDT online at www.mdtmag.com or Accuray Inc. at www.accuray.com.