Helical tomotherapy radiation leakage and shielding considerations.

Leakage radiation and room shielding considerations increase significantly for intensity-modulated radiation therapy (IMRT) treatments due to the increased beam-on time to deliver modulated fields. Tomotherapy, with its slice by slice approach to IMRT, further exacerbates this increase. Accordingly, additional shielding is used in tomotherapy machines to reduce unwanted radiation. The competing effects of the high modulation and the enhanced shielding were studied. The overall room leakage radiation levels are presented for the continuous gantry rotations, which are always used during treatments. The measured leakage at 4 m from the isocenter is less than 3 x 10(-4) relative to calibration output. Primary radiation exposure levels were investigated as well. The effect of forward-directed leakage through the beam-collimation system was studied, as this is the leakage dose the patient would receive in the course of a treatment. A 12-min treatment was calculated to produce only 1% patient leakage dose to the periphery region. Longer treatment times might yield less patient dose if the field width selected is correspondingly narrower. A method for estimating the worst-case leakage dose a patient would receive is presented.