Intensity-modulated radiation therapy (IMRT) uses multiple small radiation beams to precisely irradiate a tumor. In addition to conform to the tumor volume like 3D conformal radiotherapy, the radiation intensity of each IMRT beam is divided into small segments and modulated throughout the treatment by the multi-leaf collimator (MLC) attached to the linear accelerator. With multiple modulated radiation segments, the radiation dose is tuned to focus on tumor and spare surrounding healthy tissue as much as possible, therefore the possibility of radiation-induce side effects is significantly lowered compared with conventional non-modulated 3D conformal radiotherapy techniques. Due to its complexity, IMRT needs slightly longer treatment times than conventional radiotherapy. Longer planning time and additional safety checks are also required before treatment starts to make sure all planning data are properly transferred from the treatment planning system to the treatment machine and all modulated beam can be accurately delivered.