Commissioning of a respiratory gating system involving a pressure sensor in carbon-ion scanning radiotherapy

Abstract

This study reports the commissioning methodology and results of a respiratory gating system [AZ - 733 V/733 VI (Anzai Medical Co., Japan)] using a pressure sensor in carbon-ion scanning radiotherapy. Commissioning includes choosing a location and method for pressure sensor installation, delay time measurement of the system, and the final flow test. Additionally, we proposed a methodology for the determination of a threshold level of generating an on/off gate for the beam to the respiratory waveform, which is important for clinical application. Regarding the location and method for installation of the pressure sensor, the actual person's abdomen, back of the body position, and supine/prone positioning were checked. By comparing the motion between the pressure sensor output and the reference LED sensor motion, the chest rear surface was shown to be unsuitable for the sensor installation, due to noise in the signal caused by the cardiac beat. Regarding delay time measurement of the system, measurements were performed for the following four steps: (a). Actual motion to wave signal generation; (b). Wave signal to gate signal generation; (c). Gate signal to beam on/off signal generation; (d). Beam on/off signal to the beam irradiation. The total delay time measured was 46 ms (beam on)/33 ms (beam off); these were within the prescribed tolerance time (<100 ms). Regarding the final flow test, an end-to-end test was performed with a patient verification system using an actual carbon-ion beam; the respiratory gating irradiation was successfully performed, in accordance with the intended timing. Finally, regarding the method for determining the threshold level of the gate generation of the respiration waveform, the target motion obtained from 4D-CT was assumed to be correlated with the waveform obtained from the pressure sensor; it was used to determine the threshold value in amplitude direction.

Keywords: pressure sensor; quality assurance in radiotherapy; respiratory gating.

Figure 1

Left panel: Pressure sensors (two types with different responses). Right panel: Sensor port located beside the patient couch.

Figure 2

Left panel: An in‐house‐manufactured sensor holder. Right panel: The holder mounted to the patient immobilization shell.

Figure 3

Test signal acquisition using both a light‐emitting diode (LED) sensor and a pressure sensor. Both sensors were installed side by side.

Figure 4

Method for determining the threshold gate generation for beam on/off. The threshold level was derived from the tumor (clinical target volume;CTV) motion obtained from four‐dimensional computed tomography (4D‐CT) information.

Figure 5

(a) Respiratory signals acquired from the pressure sensor and the light‐emitting diode (LED) sensor in the abdominal region in the supine position. (b) Respiratory signals acquired from the pressure sensor and the light‐emitting diode (LED) sensor in the left thorax region in the supine position.

Figure 6

The delay times of each component of the respiratory gating system, and the methods used to measure them.