Tomotherapy is intensity-modulated rotational radiotherapy utilizing a photon fan beam [1-6]. In helical tomotherapy the gantry and couch are in continuous motion and, from the patient's point of view, the source describes a helical trajectory. Helical tomotherapy was designed around a ring gantry similar to a helical CT scanner. The constraints of a ring gantry are minimal since few patients are treated with non-coplanar radiation fields and IMRT diminishes the need for these types of field arrangements. Most importantly, a ring gantry is a very stable platform for CT scanning. The original helical tomotherapy concept had kV and MV beams for imaging and treatment respectively . On the current version megavoltage photons from the treatment linac are used to generate the CT scans [7,8]. The same detector canalsobeused todetect the exiting treatmentbeam in order to provide the data for dose reconstruction .
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