Rotational/helical/arc intensity-modulated radiation therapy (IMRT) is the most advanced form of IMRT is conceptually similar to helical or cone-beam CT 1. The radiation beam remains turned on throughout the treatment while the gantry is moved around the patient at variable speed and the multi-leaf collimator (MLC) simultaneously varies the shape of the beam.
Several competing implementations are available:
- tomotherapy (i.e. 'slice' therapy)
- modulated fan beam delivers radiation in a helical pattern
- several rotations of the beam are delivered while the patient moves through the machine
- volumetric arc therapy (VMAT)
- broad beam is shaped by an MLC as it goes around the patient in an 'arc'
- typically only a single arc is used, more complex treatments may use 2-3 arcs
- patient does not move during each arc but may be re-positioned between arcs
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