Determination of target motion by using a respiration monitoring mask for 4-D radiotherapy

The normal tissue toxicity encountered in lung cancer radiotherapy is frequently due to large treatment margins. The organ motion due to respiration necessitates that large margins be added to the planning target volume (PTV) to ensure adequate dose coverage in the chest and abdomen. Many techniques have been developed to minimize the irradiated volume of normal tissue, including gated radiotherapy, breath-holding techniques, and motion adaptive radiotherapy. If these techniques are to be utilized effectively, the precise four-dimensional (4-D) localization of a moving tumor must be available in real time. The aim of this study is to develop a simple, useful respiration- monitoring mask (ReMM) that identifies respiration-induced organ motion during the clinical application of 4-D radiotherapy. A thermocouple was embedded in a dust-proof mask and connected to a thermocouple module, which amplified the signal from the thermocouple. The dust-proof mask and the thermocouple were used to measure the respiratory movements of the patient during radiation therapy. Three patients and three healthy volunteers were enrolled to investigate the utility of the ReMM. The thermocouple measured the temperature of the mask during respiration. The diaphragmatic motion along a craniocaudal direction was simultaneously monitored using a fluoroscope. The relationship between the respiratory pattern and the targeted motion along the craniocaudal direction was evaluated using a correlation coefficient. Respiration-induced target motion along a ventrodorsal direction was less than 5 mm for each patient. However, target motion along the craniocaudal direction was up to 3 cm in this study. The temperature in the mask does not drift more than 5 % of its maximum amplitude, even for a "held-time" of more than 10 seconds. The average correlation coefficients between the target motions during respiration along the craniocaudal direction and the ReMM signals were 0.91 for the eight cases. The ReMM signal showed strong correlation with the target motion, compared to the target motion in fluoroscopy, despite irregular breathing. ReMM is a noninvasive breathing surrogate apparatus. Our study shows that the target motion correlates with the ReMM signal. However, no baseline-drifting problem exists in this system. The spirometer, the skin motion, and the strain gauge have been reported to have gradual drifting problems. The thermocouple is accurate and can be practically applied for monitoring respiration for the determination of target organ motion. The authors hope that the ReMM can be used in 4-D radiotherapy, such as gated radiation therapy, motion-adaptive radiation therapy, retrospective CT, and prospective CT.