In IMRT, the beam intensity is varied across the treatment field. Rather than being treated with a single, large, uniform beam, the patient is treated instead with many very small beams; each can have a different intensity. By cross firing the tumor with these beams, the physician delivers a relatively uniform radiation dose to the tumor, but protects sensitive, surrounding tissue from high-dose radiation.

This process of cross firing has been compared to computer tomographic imaging or CT. CT delivers a spatially uniform radiation exposure to the patient to create an image. In contrast, IMRT delivers a spatially non-uniform radiation exposure to the patient to deliver a uniform dose to the tumor. The DYNAMIC MLC delivers this dose in a slice by slice fashion similar to CT imaging.

Previous applications of conformal radiation therapy were limited to the use of radiation beams of uniform intensity, whose contours corresponded to the "beam's eye view" of the tumor.

When the tumor is not well separated from the surrounding organs at risk- such as what occurs when a tumor wraps itself around an organ- there may be no combination of uniform intensity beams that will safely separate the tumor from the healthy organ. In such instances, adding intensity modulation allows more intense treatment of the tumor, while limiting the radiation dose to adjacent healthy tissue.

Because of the number of beams involved and the range of beam weights present, treatment planning for IMRT is computationally complex. As a result, treatment planning for IMRT usually requires an inverse process. Through an iterative or linear algebraic process, the beams and beam weights needed to achieve user-defined goals are generated by the HELIOS Inverse Treatment Planning System. This is in dramatic contrast to the experience-based, trial and error approach common to conventional treatment planning, where the planning software actually does not plan (it dose simulates a user-defined set of beams and beam weights).