Intensity Modulated Radiation Therapy (IMRT)
The next major advance in external beam radiotherapy was the development of intensity-modulated radiation therapy (IMRT). This new powerful computer-driven method followed soon after the adoption of three-dimensional conformal radiotherapy and is currently being rapidly adopted as the new standard treatment method by American centers of excellence, such as the Memorial Sloan Kettering Cancer Center in New York (3). Radiation Medical Group adopted this treatment method in February 2002, only becoming possible after a two-year intensive capital, computer and software investment, and extensive personnel training and hiring.
Simply stated, IMRT is three-dimensional conformal radiotherapy, but represents a much more sophisticated and robust method of three-dimensional radiation planning and delivery. With standard three-dimensional conformal radiation therapy (3DCRT), each beam typically has a uniform intensity across its entire area. This is not the case with IMRT.
Intensity Modulated Radiation Therapy (IMRT) allows a much greater degree of control of the radiation volume, by varying the strength of a beam across its entire area. This is known as modulating the intensity of the beam (FIGURE 8)while the radiation is on. Effectively, this converts one large beam into many tiny “beamlets,” each with its own customized intensity, designed by the computer. These beamlets may assume complex patterns (FIGURE 9), effectively allowing control of a new radiation physics dimension in the planning process. By cross firing through the tumor volume with a number of these intensity modulated beams, the physician may deliver a much more highly conformal volume of radiation to a target volume, while simultaneously better protecting protect sensitive, adjacent organs.
The superiority if IMRT becomes particularly prominent when the target volume has a complicated shape or curves around nearby normal organs. The IMRT method allows the high-dose radiation volume to be effectively curved to fit virtually any volume requirement. In the case of prostate cancer, the prostate target volume usually curves to some degree around the rectum, and thus typically represents an area in need of improved radiation dose distribution. In response to this need, the IMRT high-dose radiation volume may be made to conform to the prostate +/- seminal vesicles plus a specified margin, while curving inward where the rectum encroaches (FIGURE 10), to better protect it. This is a major advance compared with standard three-dimensional conformal radiation therapy, though there are still many situations where standard 3DCRT is used.
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