The medical linear accelerator, first used in the 1950s, revolutionized the way radiation was delivered to treat cancer. However, until the more recent ability of digital diagnostic imaging, powerful computer work stations, and specialized software tools, it was virtually impossible to sculpt the radiation beam to the shape of the tumor.  Beam shaping has been a manual process making heavy lead-like blocks that are cut and positioned for each exposure.  The introduction of the multi-leaf collimator (MLC) has significantly contributed to recent advances in radiation therapy.  The MLC is a specialized, computer-controlled device with as many 120 tungsten fingers, or leaves, inside the linear accelerator.  This device enables radiation beams to be shaped automatically to conform to the shape of the tumor.  And by automatically adjusting the shape, and moving the linear accelerator to treat the tumor from several different angles, it is possible to deliver a prescribed dose across all three dimensions of the tumor.