Technical note: determination of the optimized image processing and template matching techniques for a patient intrafraction motion monitoring system.

PURPOSE: In this work, the authors determine the optimal template matching method and selection of pixel data for use in a system for monitoring patient intrafraction motion. METHODS: The motion monitoring system is based on optical tracking of a marker block placed on the patient. The temporal resolution of the system was evaluated with a respiratory motion phantom. The phantom moved the marker with a peak-to-peak amplitude of 0.6-4.0 cm and a period of 1, 3, and 6 s. Three template matching methods were applied: Sum of squared difference (SSD), sum of absolute difference (SAD), and normalized cross-correlation (NCC) using each of four pixel color data schemes (RGB and gray level modified by one of three image processing steps). An in-house algorithm called auto region-of-interest (AutoROI) automatically reset the marker detection region-of-interest to improve the calculation speed. RESULTS: RGB and gray level temporal resolutions were 54.22 ± 10.81 (1 SD) s and 12.70 ± 3.87 (1 SD) s, respectively. The temporal resolution when using SSD and SAD was higher than when using NCC. Positional accuracy was within 1 mm. Both values were within the tolerance specified by AAPM Task Group 142. To avoid misidentification of the marker, a threshold-based self-validation within the marker recognition system was implemented and was found to improve the tracking of motion with a high amplitude and short period. CONCLUSIONS: An intrafraction motion monitoring system using SSD or SAD and applied to gray pixel data can achieve high temporal resolution and positional accuracy.

Management of the baseline shift using a new and simple method for respiratory-gated radiation therapy: detectability and effectiveness of a flexible monitoring system.

PURPOSE: In respiratory-gated radiation therapy, a baseline shift decreases the accuracy of target coverage and organs at risk (OAR) sparing. The effectiveness of audio-feedback and audio-visual feedback in correcting the baseline shift in the breathing pattern of the patient has been demonstrated previously. However, the baseline shift derived from the intrafraction motion of the patient's body cannot be corrected by these methods. In the present study, the authors designed and developed a simple and flexible system. METHODS: The system consisted of a web camera and a computer running our in-house software. The in-house software was adapted to template matching and also to no preimage processing. The system was capable of monitoring the baseline shift in the intrafraction motion of the patient's body. Another marker box was used to monitor the baseline shift due to the flexible setups required of a marker box for gated signals. The system accuracy was evaluated by employing a respiratory motion phantom and was found to be within AAPM Task Group 142 tolerance (positional accuracy <2 mm and temporal accuracy <100 ms) for respiratory-gated radiation therapy. Additionally, the effectiveness of this flexible and independent system in gated treatment was investigated in healthy volunteers, in terms of the results from the differences in the baseline shift detectable between the marker positions, which the authors evaluated statistically. RESULTS: The movement of the marker on the sternum [1.599 +/- 0.622 mm (1 SD)] was substantially decreased as compared with the abdomen [6.547 +/- 0.962 mm (1 SD)]. Additionally, in all of the volunteers, the baseline shifts for the sternum [-0.136 +/- 0.868 (2 SD)] were in better agreement with the nominal baseline shifts than was the case for the abdomen [-0.722 +/- 1.56 mm (2 SD)]. The baseline shifts could be accurately measured and detected using the monitoring system, which could acquire the movement of the marker on the sternum. The baseline shift-monitoring system with the displacement-based methods for highly accurate respiratory-gated treatments should be used to make most of the displacement-based gating methods. CONCLUSIONS: The advent of intensity modulated radiation therapy and volumetric modulated radiation therapy facilitates margin reduction for the planning target volumes and the OARs, but highly accurate irradiation is needed to achieve target coverage and OAR sparing with a small margin. The baseline shifts can affect treatment not only with the respiratory gating system but also without the system. Our system can manage the baseline shift and also enables treatment irradiation to be undertaken with high accuracy.