Comparison Of Two Different Respiratory Monitoring Systems Using External Surrogates For Respiratory-Guided Radiation Therapy

Two different respiratory monitoring systems using external surrogates are used in our institution for four-dimensional computed tomography (4DCT). The RPM system tracks two infrared reflecting markers placed on the abdomen to measure the abdominal surface movement as the respiratory signal. The Anzai belt employs a pressure sensor on the abdomen to measure the pressure change during breathing. The objective of this study was to compare these two systems in the respiratory signal and the 4DCT images reconstructed based on both systems and to assess the effect of mixed use of different systems in 4DCT imaging for treatment planning and in respiratory gated radiotherapy treatment. The respiratory signals were acquired simultaneously using both surrogates for the same patient during the 4DCT scan on a Siemens SOMATOM Definition AS (Open 20 RT). Ten patients with regular breathing traces were included for data analysis. Comparisons were made between the two breathing traces to evaluate the phase shift in signal acquisition and the difference in breathing patterns. Two sets of 4DCT images with the same raw sinogram data for the same patient were reconstructed using the respiratory signals acquired by both systems. An amplitude-based sorting algorithm was applied to obtain the images of ten respiratory phases. The images reconstructed from the two different respiratory signals were compared for each individual phase. Image segmentation was performed in the treatment planning system to assess the differences in lung contours and the position of the diaphragm. The phase shifts calculated by the time latency at the end inhalation between the two breathing traces were consistent for all ten patients and were measured as 0.2±0.1 seconds. The breathing trace measured by the Anzai pressure sensor had a steeper falloff curve during exhalation than the RPM trace measured by the abdominal surface movement. In the 4DCT images reconstructed separately, changes in lung contouring and shifts in position of diaphragm were observed in each individual phase. Largest shifts of more than 5mm in the diaphragm location were found in phases between 40% exhalation and 20% inhalation. Phase shifts between breathing traces and differences in breathing patterns as measured by the abdominal pressure change and surface displacement were observed in this study. The internal thoracic motion during 4DCT scan can have different correlation relationships with different respiratory monitoring systems using external surrogates. This implicates that mixed use of different respiratory monitoring systems in 4DCT imaging for treatment planning and in respiratory gated treatment delivery may result in errors in the dose distribution received by the patient.