Investigation of the location effect of external markers in respiratory-gated radiotherapy

In 7 lung and breast cancer patients, we investigated the location effect of external markers on the correlation between the motions of external markers and of an internal target under various breathing patterns. Our department developed a tumor tracking system consisting of two infrared cameras and a medical simulator. Using the system, we monitored the simultaneous motions of tumor and external markers placed at various locations on a patient's skin and saved the results for offline analysis. We then used a cross-covariance approach to analyze the correlation between the motions of individual markers and of the tumor. Based on the additive model, we evaluated the predictability of tumor motion from the motions of the external markers. The effect of marker location on the correlation between the motions of the tumor and of the external markers varied widely from patient to patient. At no specific marker location did the surrogate signal consistently present superior correlation with tumor motion in 3 breathing sessions with 7 patients. When the composite external signal generated from multiple external motion signals was correlated with tumor motion, the quality of the correlation improved significantly. In most cases, the composite signal provided the best surrogate signal for correlating with tumor motion. Correlation between the motions of external markers and of a tumor may be affected by several factors, including patient characteristics, marker locations, and breathing pattern. A single external marker cannot provide sufficient and reliable tracking information for tumor motion. A composite signal generated from the motions of multiple external makers provides an excellent surrogate signal, which in this study demonstrated superior correlation with tumor motion as compared with the signal provided by an individual marker. A composite signal would be a more reliable way to track tumor motion during respiratory-gated radiotherapy.