Feasibility of gated IMRT

Recent years have seen the development of sophisticated delivery technologies designed to enhance the efficacy of clinical radiotherapy. In this study the authors evaluate the feasibility for combining two pivotal concepts, intensity modulated radiotherapy and respiratory gating, into a single effective therapeutic modality. Respiration is monitored in real-time through the use of cameras which track infrared-reflecting markers placed on the patient´s surface. With a sampling rate of 5 Hz, normal breathing produces a simple sinusoidal pattern in three spatial dimensions. A software interface enables gating about exhalation. Radiation is delivered using a Novalis^TM accelerator and integrated micro-multileaf collimator. The unit is capable of dynamic field shaping and IMRT. Other features making the Novalis ideally suited to gated IMRT include: high output (1200 mu/min) and the ability to start/stop the beam within 0.01 monitor unit. Beam triggering is facilitated through a clinical gating interface provided by the linac manufacturer. To evaluate the performance of the system, dose measurements were performed for dynamically wedged and IMRT fields. Gating frequency was varied from 0.2 to 1.0 Hz and monitor units were varied from 25 to 200. In this manner gating effects could be studied for a wide range of leaf speeds. Two-dimensional dose maps were obtained using an amorphous-silicon detector. Differences relative to gating frequency were observed only for low numbers of monitor units, Above 50 monitor units no variation exceeded 3.5%. In all cases, variations averaged over the entire field were less than 3.5% and less than 1% above 50 monitor units