Optical surface scanning for respiratory motion monitoring in radiotherapy: a feasibility study

Purpose. We evaluated the feasibility of a surface scanning system (Catalyst) for respiratory motion monitoring of breast cancer patients treated with radiotherapy in deep inspiration breath-hold (DIBH). DIBH is used to reduce the radiation dose to the heart and lung. In contrast to RPM, a competing marker-based system, Catalyst does not require an objectmarker on the patient's skin.

Materials and Methods. Experiment 1: a manikin was used to simulate sinusoidal breathing. The amplitude, period and baseline (signal value at end-expiration) were estimated with RPM and Catalyst. Experiment 2 and 3: the Quasar phantom was used to study if the angle of the monitored surface affects the amplitude of the recorded signal.

Results. Experiment 1: we observed comparable period estimates for both systems. The amplitudes were 8 ± 0.1 mm (Catalyst) and 4.9 ± 0.1 mm (RPM). Independent check with in-room lasers showed an amplitude of approximately 8 mm, supporting Catalyst measurements. Large baseline errors were seen with RPM.

Experiment 2: RPM underestimated the amplitude if the object-marker was angled during vertical motion. This result explains the amplitude underestimation by RPM seen in Experiment 1. Experiment 3: an increased (fixed) surface angle during breathing motion resulted in an overestimated amplitude with RPM, while the amplitude estimated by Catalyst was unaffected.

Conclusion. Our study showed that Catalyst can be used as a better alternative to the RPM. With Catalyst, the amplitude estimates are more accurate and do not depend on the angle of the tracked surface, and the baseline errors are smaller.