Topic: 22) Intrafraction motion management. Keyword: Gating. Title: Monitoring of surface motion for 4D CT and gated radiotherapy. Authors. Susanne Lise.
Topic: 22) Intrafraction motion management
Keyword: Gating
Title: Monitoring of surface motion for 4D CT and gated radiotherapy Authors Susanne Lise Bekke
Center for Nuclear Technologies, Technical University of Denmark, Risø Campus Build. 201, Roskilde, Denmark. Department of Oncology, Radiotherapy Research Unit (52AA), Herlev Hospital, University of Copenhagen, Herlev, Denmark.
Jakob HeltHansen
Center for Nuclear Technologies, Technical University of Denmark, Risø Campus Build. 201, Roskilde, Denmark.
Faisal Mahmood
Department of Oncology, Radiotherapy Research Unit (52AA), Herlev Hospital, University of Copenhagen, Herlev, Denmark. Department of Oncology, Radiotherapy Research Unit (52AA), Herlev Hospital, University of Copenhagen, Herlev, Denmark.
Claus F. Behrens
Purpose/Objective: Patient surface monitoring is used for 4D CT and for gated radiotherapy. In this study we measure the accuracy and precision of the amplitude and period of a surface tracking signal measured with two systems: An optical surface scanning system (OSS) and a marker-based system (MB). The OSS tracks the patient surface directly, while the MB tracks an object-marker placed on the patient’s skin. Materials/methods: A commercially available respiratory motion phantom (QUASAR, Modus Medical) was used to simulate sinusoidal breathing signals for amplitudes (half the peak-to-peak value) from 0.125 to 3.75 mm and periods from 0.75 to 8 s. The average free breathing amplitude is around 34 mm and the period 4 s for men and women. The signal was measured for at least 50 full periods using the MB (RPM with two-dot object-marker, Varian Medical Systems) and the OSS (Catalyst, C-rad Positioning AB). An encoded signal, which represents the phantom’s actual motion during run, was acquired from the Quasar software. The maximum and minimum peak values were estimated automatically in MATLAB and were visually verified. The amplitude errors were calculated as the absolute difference between each peak value of the signal from the MB or OSS and the corresponding peak value from the encoded signal. The time interval between maximum peaks and minimum peaks were found. The period errors were calculated as the absolute difference between each time interval from the MB or OSS and the corresponding time interval from the encoded signal. The mean (μ) and standard deviation (σ) of the amplitude errors and period errors were calculated as a measure of accuracy and precession, respectively. Results: Overall, the accuracies of the MB and OSS in estimating the amplitude and period are similar. For the OSS there is in general a higher standard deviation, i.e. lower precision, which can be explained
by the lower sampling frequency. The sampling frequency of the OSS and MB for all signals in this study was calculated to 12.2 ± 4.5 Hz and 25.0 ± 0.2 Hz, respectively. For amplitude values above 0.25 mm and a fixed period of 4 s, the mean amplitude error is slightly increased for the MB compared to the OSS (Figure 1A). The same is valid for period values above 1 s with amplitude of 3.75 mm (Figure 1B). The maximum amplitude error was 1.85 and 0.24 mm for the OSS and MB signal, respectively. The mean error of the period is reduced for the MB in comparison to the OSS for all combinations (Figure 1C and 1D). Conclusion: The two systems have similar performance with an amplitude and frequency corresponding to free breathing. With the OSS you avoid the object-marker, which can introduce errors due to tilting of the marker during breathing. The MB can handle breathing patterns with lower periods and hence with steeper increase of the breathing signal, which is explained by the higher sampling frequency.
Figure 1: The mean (μ) and standard deviation (σ) of the amplitude error (A and B) and period error (C and D) for different sinusoidal breathing patterns. In A and C the period is fixed to 4 s while the amplitude is varied. In B and D the period is varied while the amplitude is set to 3.75 mm.
