Intensity-modulated radiation therapy (IMRT) is the most technologically advanced, most precise method of external beam radiation therapy available. It delivers high doses of radiation directly to the tumor while sparing the surrounding healthy tissue. To understand how IMRT works, it helps to understand how radiation therapy in general works. Radiation therapy is a treatment technique for cancer in which radiation (a stream of energy carried by waves or particles) is administered to the tumor site. Radiation therapy works by damaging or destroying cells that are actively reproducing. Tumor cells reproduce more quickly and more often than normal cells, and are therefore more likely to be affected by the radiation. Normal cells that are reproducing at the time of treatment may also be affected by the radiation, possibly leading to side effects. Unlike tumor cells, however, most normal cells can recover with little to no permanent damage.

IMRT allows doctors to customize the radiation dose my modulating, or varying, the amount of radiation dose given to different parts of the treatment area. This modulation is done in highly accurate, three-dimensional detail, according to the shape, size, and location of the tumor.

IMRT treatments are planned using three-dimensional, computer –generated images of the treatment area. The actual treatments are delivered using a linear accelerator (a highly specialized radiation-producing machine) and a multileaf collimator (MLC). The MLC is a modulating device that comprises as many as 120 computer-controlled, tungsten “leaves,” or finely shaped plates. It attaches to the head of the linear accelerator at the point where the radiation beam comes out. During treatment, the MLC leaves move across the beam, blocking it in different places for differing amounts of time, according to the treatment plan. The result is that some parts of the treatment area are given higher doses than other parts.

A useful analogy for how IMRT works is the exposure of film to light: if the light is blocked more in some places and less in others, the more exposed places will be darker (higher doses), while the less exposed places will be whiter (lower doses). With careful blocking, many different shades of gray can appear, representing many different gradations of dose.

IMRT offers new hope to cancer patients. However, it may not be the most appropriate treatment option for your situation. Your doctor will work with you to determine which radiation therapy method is best for you.