Capacitive monitoring system for real-time respiratory motion monitoring during radiation therapy

This work introduces a novel capacitive-sensing technology capable of detecting respiratory motion with high temporal frequency (200 Hz). The system does not require contact with the patient and has the capacity to sense motion through clothing or plastic immobilization devices.

PURPOSE: This work presents and evaluates a novel capacitive monitoring system (CMS) technology for continuous detection of respiratory motion during radiation therapy. This modular system provides real-time motion monitoring without any contact with the patient, ionizing radiation, or surrogates such as reflective markers on the skin.

The novel prototype features an array of capacitive detectors that are sensitive to the position of the body and capable of high temporal frequency readout. Performance of this system was investigated in comparison to the RPM infrared (IR) monitoring system (Varian Medical Systems). The prototype included three (5 cm × 10 cm) capacitive copper sensors in one plane, located at a distance of 8-10 cm from the volunteer. Capacitive measurements were acquired for central and lateral-to-central locations during chest free-breathing and abdominal breathing. The RPM IR data were acquired with the reflector block at corresponding positions simultaneously. The system was also tested during deep inspiration and expiration breath-hold maneuvers.

Capacitive monitoring system data demonstrate close agreement with the RPM status quo at all locations examined. Cross-correlation analysis on RPM and CMS data showed an average absolute lag of 0.07 s (range: 0.03-0.23 s) for DIBH and DEBH data and 0.15 s (range: 0-0.43 s) for free-breathing. Amplitude difference between the normalized CMS and RPM signal during chest and abdominal breathing was within 0.15 for 94.3% of the data points after synchronization. CMS performance was not affected when the subject was clothed.

This novel technology permits sensing of both free-breathing and breath-hold respiratory motion. It provides data comparable to the RPM system but without the need for an IR tracking camera in the treatment room or use of reflective markers on the patient.