The goal of all radiation therapy is to irradiate the tumor with a lethal dose while limiting the radiation received by the normal tissue that surrounds the tumor. IMRT uses a sophisticated dose calculation to design and deliver precisely targeted radiation. This is accomplished by using computer controlled small radiation shields in a linear accelerator. Dozens of uniquely shaped radiation fields are delivered to the patient using various angles and approaches.
The difference between 3-D conformal radiation and IMRT is that IMRT can create high dose volumes that are concave in shape, sparing critical normal issues that are extremely close to and surrounded by the tumor. During each field of treatment, the dose delivered is being modulated or adjusted by the multi-leaf collimator (a device that consists of a number of "fingers" or "leaves" which project into the primary beam to create the required shape). However, in order to spare some areas, other areas will receive more radiation. It is the job of the radiation oncologist and the radiation physicist to critically evaluate the trade-offs between avoiding normal tissues and adjusting radiation doses to the tumor.
IMRT is useful in treating small, fairly stationary targets surrounded by a large volume of normal tissue and/or critical structures that are especially close to the targeted tumor. Types of tumors that may be treated with IMRT include brain, head & neck cancer, prostate, spinal cord or tumors very close to radiosensitive normal tissues, such as the optic nerve (e.g., pituitary or nasopharyngeal cancer). Many tumors are too large or too mobile to be treated with IMRT.