Intensity-Modulated Radiation Therapy (IMRT)

What is Intensity-Modulated Radiation Therapy and how is it used?

Intensity-modulated radiation therapy (IMRT) is an advanced mode of high-precision radiotherapy that utilizes computer-controlled x-ray accelerators to deliver precise radiation doses to a malignant tumor or specific areas within the tumor. The radiation dose is designed to conform to the three-dimensional (3-D) shape of the tumor by modulating—or controlling—the intensity of the radiation beam to focus a higher radiation dose to the tumor while minimizing radiation exposure to surrounding normal tissues. Treatment is carefully planned by using 3-D computed tomography (CT) images of the patient in conjunction with computerized dose calculations to determine the dose intensity pattern that will best conform to the tumor shape. Typically, combinations of several intensity-modulated fields coming from different beam directions produce a custom tailored radiation dose that maximizes tumor dose while also protecting adjacent normal tissues.

Because the ratio of normal tissue dose to tumor dose is reduced to a minimum with the IMRT approach, higher and more effective radiation doses can safely be delivered to tumors with fewer side effects compared with conventional radiotherapy techniques. IMRT also has the potential to reduce treatment toxicity, even when doses are not increased.

Currently, IMRT is being used to treat cancers of the prostate, head and neck, breast, thyroid and lung, as well as in gynecologic, liver and brain tumors and lymphomas and sarcomas. IMRT is also beneficial for treating pediatric malignancies.

Radiation therapy, including IMRT, stops cancer cells from dividing and growing, thus slowing tumor growth. In many cases, radiation therapy is capable of killing cancer cells, thus shrinking or eliminating tumors.

Who will be involved in this procedure?

Most facilities rely on a specially trained team for IMRT delivery. This team includes the radiation oncologist, medical radiation physicist, dosimetrist, radiation therapist and radiation therapy nurse.

The radiation oncologist, a specially trained physician who heads the treatment team, sets an individualized course of treatment with the help of the radiation physicist, who ensures the linear accelerator delivers the precise radiation dose and that computerized dose calculations are accurate. A dosimetrist, under the supervision of the medical radiation physicist, calculates the IMRT exposures and beam configurations necessary to deliver the dose prescribed by the radiation oncologist. A highly trained radiation therapist positions the patient on the treatment table and operates the machine. The radiation therapy nurse provides the patient with information about the treatment and possible adverse reactions.

What equipment is used?

A medical linear accelerator generates the photons, or x-rays, used in IMRT. The machine is the size of a small room—approximately 10 feet high and 15 feet long. The patient lies on the treatment table, while the linear accelerator delivers beams of radiation to the tumor from various directions. The intensity of each beam's radiation dose is dynamically varied according to treatment plan.

See the Linear Accelerator page for more information.

Who operates the equipment?

The radiation therapist operates the equipment from a radiation-protected area nearby. The therapist is able to communicate with the patient throughout the procedure. The therapist can observe the patient through a window or on closed circuit television.

Is there any special preparation needed for the procedure?

Before planning treatment, a physical examination and medical history review will be conducted. Next, there is a treatment simulation session, which includes CT scanning, from which the radiation oncologist specifies the three-dimensional shape of the tumor and normal tissues. The dosimetrist and medical radiation physicist use this information to design the IMRT beams used for treatment. Several additional scanning procedures, including positron emission tomography (PET) and magnetic resonance imaging (MRI), might also be required for IMRT planning. These diagnostic images help the radiation oncologist determine the precise location of the tumor target. Typically, IMRT sessions begin about a week after simulation. In some cases, a treatment preparation session may be necessary to mold a special device that will help the patient maintain an exact treatment position. Prior to treatment, the patient's skin may be marked or tattooed with colored ink to help align and target the equipment.

How is the procedure performed?

IMRT is an aggressive therapy that requires multiple or fractionated treatment sessions. Several factors come into play when determining the total number of IMRT sessions and radiation dose. The oncologist considers the type, location and size of the malignant tumor, as well as the patient's health. Typically, patients are scheduled for IMRT sessions five days a week for six to ten weeks.

At the beginning of the treatment session, the therapist positions the patient on the treatment table, guided by the marks on the skin defining the treatment area. If molded devices were made, they will be used to help the patient maintain the proper position. The patient may be repositioned during the procedure. Treatment sessions usually take between 15 and 30 minutes.

What will I feel during this procedure?

IMRT is painless. You will not feel or sense anything out of the ordinary during treatment. However, the machine can be stopped if you become uncomfortable. As treatment progresses, some patients may experience treatment-related side effects. The nature of the side effects depend on the normal tissue structures being irradiated. The radiation oncologist will discuss and try to help you with any side effects.