Intensity-modulated radiation therapy (IMRT)

This page was reviewed under our medical and editorial policy by

Maurie Markman, MD, President, Medicine & Science.

This page was reviewed on March 24, 2022.

Intensity-modulated radiation therapy, or IMRT, is a type of external radiation delivered in different amounts from varying angles during treatment. This allows higher doses of radiation therapy to target cancerous cells and reduces the effect of radiation on healthy tissues surrounding the tumor.

Real-time three-dimensional (3D) images guide the IMRT process, providing precise information on the location of cancerous cells and healthy cells in the path of the radiation beam. IMRT is a type of 3D conformal radiotherapy, meaning it uses computer images from computed tomography (CT), magnetic resonance imaging (MRI) and positron emission tomography (PET) scans.

These images show your care team the exact size and shape of the tumor, as well as the healthy tissue nearby. The scans help plan the direction, angles and dose strength of radiation treatment.

Unlike traditional 3D conformal radiation, IMRT uses more and smaller radiation beams. The strength of each radiation beam can be rapidly changed during treatment to give higher amounts to cancerous areas and lower amounts to healthy areas during the treatment as the machine moves around the patient.

Why is IMRT done?

IMRT is a common type of radiation therapy. Because the treatment can be planned so precisely, with different levels of radiation given at different angles depending on the target tissue (cancerous or healthy), it’s especially used to treat head and neck cancers—such as oropharyngeal cancer, nasopharyngeal cancer or laryngeal cancer—where key organs and tissues are more condensed. It’s also commonly used as the radiation treatment for prostate cancer.

How to prepare for IMRT

A key part of IMRT is having image scans so your care team can learn as much as possible about the size and shape of your tumor.

Before treatment, you may hear your care team refer to inverse planning, which is how IMRT treatment is planned. First, your care team determines the ideal dose of radiation. From there, they use your scans to plan for the cancerous cells to get the ideal dose, while healthy tissue nearby will get lower doses. Because they first determine the ideal dose and then outline the different strengths at different angles—which is the opposite of more traditional radiation treatment planning—it’s called inverse planning.

While you and your care team are in the treatment planning process, be sure to ask any questions you may have about your treatment or what to expect. You may consider asking:

  • How long will each treatment take?
  • What are the side effects of this treatment?
  • How will treatment affect my day-to-day life?
  • Does this treatment have any long-term effects?

What to expect during IMRT

During the time you’re getting IMRT, you’ll work with a radiation oncologist who will plan and oversee your treatment. This specialist meets with you before your treatment starts to review your medical history and any recent test results.

A radiation therapist is the one who delivers the treatment and ensures you’re in the right position each time. Because radiation treatments need to be very precise, the therapist may mark your body with dots of ink that remain through all the IMRT sessions.

Though simulation scans are typically done to help plan treatment, your care team may also use real-time images during the IMRT procedure to guide the treatment and intensity.

Your care team can tell you how often you’ll need to undergo IMRT and for how long. Most people receive treatment once a day, five times per week, for about five to eight weeks. Your treatment schedule will depend on the size, shape and type of cancer, as well as your general health and any other cancer treatments you’re getting. IMRT is usually done at an outpatient treatment center or as an outpatient treatment at a hospital.

Risks of IMRT

Any radiation procedure comes with possible side effects, which tend to depend on the area of the body getting treatment. Some common side effects include fatigue and skin changes such as:

Ask your care team about possible side effects or risks associated with your IMRT treatment and how to manage them.

Benefits of IMRT

IMRT is designed to reduce damage to healthy tissue during radiation treatment by using higher doses directed at the tumor itself and lower doses when the beam is closer to healthy tissue. This is an especially important benefit for patients with head and neck cancers in avoiding tissue of important nearby structures such as the eyes, optic nerve, tongue or brain.

IMRT is also associated with a higher quality of life after treatment—again, especially when it comes to head and neck cancers, where the side effects of radiation treatment often affect the patient’s ability to eat and drink. Patients with head and neck cancers who undergo IMRT have reported fewer instances of dry mouth and better experiences with swallowing, allowing them to return to more normal eating and drinking habits after treatment.

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