Evaluation of the Usefulness of Guided Breathing for Dose-Rate-Modulated Tracking

Article Outline

• Purpose/Objective(s)
• Materials/Methods
• Results
• Conclusions
• Copyright

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Purpose/Objective(s) 

We are developing a new technique to compensate for tumor motion during radiotherapy treatment, called Dose-Rate-Modulated Tracking (DRMT).¹ We have already applied DRMT to simulate tumor tracking in free-breathing patients.² In this study, we evaluate the usefulness of guided breathing for DRMT.

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Materials/Methods 

DRMT uses preprogrammed MLC sequence that tracks the tumor motion derived from 4D CT and the corresponding breathing signals measured prior to treatment. Since MLC speed can be controlled by adjusting the dose rate, the MLC positions are adjusted in real time during the treatment by dose-rate modulation, thereby maintaining synchrony with the tumor motion. If the monitored breathing pattern is slower than the preprogrammed breathing signal, the dose rate is decreased to slow down the MLC, and vice versa. In this study, we simulate DRMT treatment using breathing signals acquired (with RPM/Varian), from 23 lung-cancer patients at VCU.³ Each patient participated in five sessions at intervals of about one week. In each session, data were obtained for free breathing, audio-guided breathing, and audio-visual-guided breathing. The breathing signals taken from the first session were used to generate the preprogrammed MLC sequences. The data sets for the other four sessions were used for the DRMT treatment simulations. The tracking error and the duty cycle for each breathing signal were determined as a function of the time delay–the response time of the tracking system for the dose-rate correction, ranging from 0 to 0.3 s.

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Results 

The tracking error and the duty cycle averaged over all four sessions and all 23 patients are 0.60 ± 0.25 mm and 76 ± 5% for free breathing, 0.62 ± 0.21 mm and 76 ± 7% for audio-guided breathing, and 0.54 ± 0.25 mm and 80 ± 7% for audio-visual-guided breathing, for no time delay. Although the duty cycle for audio-visual guidance shows a modest increase over that for free breathing and audio-guided breathing, this modest increase is not statistically significant. The tracking error improvement with guided breathing is not statistically significant, either. The tracking error increases as the time delay increases: 0.6, 1.0, 1.6, and 2.2 mm for time delays of 0, 0.1, 0.2, and 0.3 s, respectively, regardless of the presence of breathing guidance.

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Conclusions 

DRMT by nature adapts well to the breathing frequency variations, which are also what the breathing guidance intended to reduce. Because of this redundancy, breathing guidance does not add significant value to improve the tracking error or the duty cycle when DRMT is used for real-time tumor tracking.