Helical tomotherapy (HT) is a novel treatment approach where the ring gantry irradiation geometry of a helical CT scanner is combined with an intensity-modulated megavoltage x-ray fan beam. An inverse treatment planning system (TomoTherapy Inc., Madison) was used to optimize the treatment plans for ten randomly selected prostate patients. Five different sets of margins (2, 5, 7.5 and 10 mm uniform 3D margins and a non-uniform margin of 5 to 10 mm) were employed for the prostate (GTV2) and seminal vesicles (GTV1). The dose distribution was evaluated in targets, rectum, bladder and femoral heads. HT plans are characterized by a rapid dose fall off around the target in all directions resulting in low doses (less than 30% of the dose at ICRU reference point) to the femurs in all cases. Up to a margin of 5 mm for target structures, it was always possible to satisfy the requirements for dose delivery set by RTOG protocol P-0126. Using a 'class solution', HT plans require minimal operator interaction and result in excellent sparing of normal structures in prostate radiotherapy.
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