Intensity-modulated radiation therapy
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Intensity-modulated radiation therapy (IMRT) is a type of conformal radiation therapy where not only the shape but also the intensity profile (i.e. fluence) of each beam is varied, therefore, superior to the 3D-CRT.
Early implementations involved building up a fluence profile by summing smaller beam 'segments' until the desired profile was achieved. The beam was turned off while the multi-leaf collimator (MLC) leaves were in motion and only turned on for a specific period to deliver each segment 1.
Modern 'sliding-window' techniques use carefully calibrated MCLs which slide across the beam path while the beam remains on. The variable speed of each MLC leaf allows the creation of hot and cold spots in the beam profile 1.