Out-of-field dosimetry measurements for a helical tomotherapy system.

Helical tomotherapy is a rotational delivery technique that uses intensity-modulated fan beams to deliver highly conformal intensity-modulated radiation therapy (IMRT). The beam-on time needed to deliver a given prescribed dose can be up to 15 times longer than that needed using conventional treatment delivery. As such, there is concern that this delivery technique has the potential to increase the whole body dose due to increased leakage. The purpose of this work is to directly measure out-of-field doses for a clinical tomotherapy system. Peripheral doses were measured in-phantom using static fields and rotational intensity-modulated delivery. In-air scatter and leakage doses were also measured at multiple locations around the treatment room. At 20 cm, the tomotherapy peripheral dose dropped to 0.4% of the prescribed dose. Leakage accounted for 94% of the in-air dose at distances greater than 60 cm from the machine's isocenter. The largest measured dose equivalent rate was 1 x 10(-4) Sv/s in the plane of gantry rotation due to head leakage and primary beam transmission through the system's beam stopper. The dose equivalent rate dropped to 1 x 10(-6) Sv/s at the end of the treatment couch. Even though helical tomotherapy treatment delivery requires beam-on times that are 5 to 15 times longer than those used by conventional accelerators, the delivery system was designed to maximize shielding for radiation leakage. As such, the peripheral doses are equal to or less than the published peripheral doses for IMRT delivery on other linear accelerators. In addition, the shielding requirements are also similar to conventional linear accelerators.