Initial clinical evaluation of a respiratory gating radiotherapy system

We report on initial patient studies to evaluate the performance of a commercial respiratory gating radiotherapy system. The system uses a breathing monitor, consisting of an infrared video camera and passive reflective markers placed on the patient's abdomen, to synchronize radiation from a linear accelerator with the patient's breathing cycle. Patients receiving treatment for lung cancer participated in a study of system characteristics during treatment simulation with fluoroscopy, port film acquisition, and treatment. Breathing synchronized fluoroscopy was performed initially without instruction, followed by fluoroscopy with recorded verbal instruction (i.e., when to inhale and exhale) with the tempo matched to the patient's normal breathing period. Patients tended to inhale more deeply and consistently when given instruction, as assessed by external marker movement. This resulted in smaller variation in expiration and inspiration marker positions relative to total excursion, thereby permitting more precise gating tolerances at those parts of the breathing cycle. Verbal instruction also improved the correlation between diaphragm position and external marker position, as observed with fluoroscopy as well as in comparison of gated simulation and portal film; furthermore, a factor 2-4 reduction in diaphragm movement was achievable by gating on 30-50% of the breathing cycle at expiration. Patients expressed no difficulty with following instruction, and gated treatment procedures required little additional time. We conclude that verbal breathing instruction can improve reproducibility in breathing, and in turn improve the consistency of gated treatments with this system.