Adaptive filtering to predict lung tumor motion during free breathing
Breathing-induced tumor motion during radiation therapy can be compensated either by gating or correcting the pointing of the radiation beam, but these techniques involve time delays in the corrective response. We have analyzed the accuracy of adaptive fi
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Adaptive filtering to predict lung tumor motion during free breathing Martin J. Murphy", Marcus Isaakson b, Joakim Jalden b aOepartment of Radiation Oncology, Stanford University bOepartment of Electrical Engineering, Stanford University
Abstract Breathing-induced tumor motion during radiation therapy can be compensated either by gating or correcting the pointing of the radiation beam, but these techniques involve time delays in the corrective response. We have analyzed the accuracy of adaptive filter algorithms in predicting tumor positions with sufficient lead time to compensate for these systematic delays. Tumor and chest motion during respiration has been recorded fluoroscopically for lung cancer patients, using gold fiducials implanted in the tumors to enhance visibility. The motions been analyzed for predictability up to 1.0 second in advance using tapped delay line, Kalman filter, and neural network filter algorithms. Breathing patterns are not stationary in time. Both internal tumor and external chest movement can show amplitude and period modulations during a 30 second interval. Tapped delay line and other stationary filters cannot compensate for the changes and consequently have poor predictability. The predictive accuracy of adaptive filters has little dependence on the type of algorithm, but depends mainly on the frequency of updating and deteriorates rapidly when predicting more than 0.2 seconds in advance of the breathing signal. Longer-period (e.g., 30 seconds) variability in breathing requires fTequent adaptation of the filter parameters. Keywords: Breathing motion, breathing compensation
1. Introduction Many internal tumor sites move with breathing. Lung tumors have been observed to move by 5 mm to 25 mm during regular breathing; the pancreas is known to move by up to 35 mm. If breathing motion is not restricted or compensated, dose margins must be enlarged to ensure complete irradiation of the target volume. This increases the exposure of healthy tissue and limits the total dose to the tumor. The approximately regular pattern of normal breathing suggests it may be possible to devise tracking schemes to monitor the tumor position during free breathing and make continuous adjustments in the alignment of the radiation beam. This could be done by shifting the aperture of a multi leaf collimator or moving a robotically mounted linear accelerator in synchrony with the breathing cycle. However, any adaptive response in beam delivery will be delayed with respect to the signal of the tumor's position. The delay can range from 50 ms for a signal to interrupt the beam cycle [I] to as much as 0.8
CARS 2002 - H.u. Lemke, M. W. Vannier; K. fnamura, A.G. Farman, K. Doi & JH.c. Reiber (Editors)
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seconds to mechanically reposition a linear accelerator. [2] This represents a familiar problem in adaptive signal processing for closed-loop control systems. If a tumor's breathing-induced cycle of movement were perfectly uniform and regular, so that its time average ov
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