All About Intensity-Modulated Radiation Therapy (IMRT)

Cancer is caused by cells growing out of control. As the number of cells grow, they form a mass or tumor. Cancer cells, which make up the tumor, grow and reproduce quickly. Normal, healthy cells know to stop dividing and growing when they touch other cells. Cancer cells do not and keep on growing. Radiation therapy uses high-energy x-rays to damage the DNA of cells. This kills the cancer cells or stops them from reproducing. There are two main types of radiation therapy: External radiation and internal radiation.

In external radiation therapy, a beam of radiation is directed into the body. Intensity-modulated radiation therapy (IMRT) is a specific type of external beam radiation. IMRT is one of the most common ways to deliver radiation therapy for many types of cancer.

What is Intensity-Modulated Radiation Therapy?

Computed tomography (CT) scans have advanced radiation therapy practices. CT scanners help create 3D models of the body. This 3D image allows multiple radiation beams to target a tumor and to better shape the radiation to the tumor. It also reduces the amount of radiation delivered to nearby normal, healthy tissue. 

Before treatment, a radiation oncologist uses a CT scan, and sometimes an MRI or PET/CT, to create a 3D model of the body on a computer. With this 3D image, the radiation oncologist can plan treatment beams coming from different angles outside of the body so that they all come together inside the body at the target (tumor).  

Treatment field shaped by a multileaf collimator (MLC).

IMRT is an advanced form of 3D planning. IMRT allows your radiation team to shape each treatment beam using small blocks, called “leaves.” The device is called a multileaf collimator (MLC). The MLC allows the small leaves to move across the beam’s path at different speeds and patterns. In some cases, the leaves do not move while the beam is on. Therefore, parts of the radiation beam are selectively blocked for part of the treatment time. Certain parts of the beam deliver higher-intensity radiation that results in a higher dose. Other parts of the beam deliver lower-intensity radiation that results in a lower dose. In other words, the intensity across the beam can be changed, or “modulated”. This is how intensity-modulated radiation therapy got its name. 

Combining multiple beams around the patient, each aiming at the target from a different angle, can create precise delivery of radiation within the body. The result is a dose of radiation that can be higher in certain areas, lower in others, and even curve around nearby organs.  

Like most other radiation treatments, IMRT is given as fractionated radiation, meaning that the total dose of radiation is given in many small daily, or twice daily, doses. This is divided over the course of several weeks of therapy. 

When is IMRT used?

IMRT is used in the treatment of prostate cancer, head and neck cancers, gastrointestinal and gynecologic cancers, lung cancers, and brain tumors, among others. 

IMRT is most often used when a tumor partly surrounds or is very close to a healthy part of your body that cannot tolerate the full dose of radiation that is being given to the tumor. When the tumor is not near sensitive areas, IMRT may not be needed. For example, superficial cancers are sometimes not best treated with IMRT but may be treated by other types of radiation. Talk with your radiation team to see which type of treatment is best for you.

Is IMRT right for me?

IMRT can be beneficial but it is not always the best option for patients. Because the radiation beams are grouped into several angles around the patient, a low dose ‘bath’ of radiation is created just outside the tumor. It is not known if this low-dose region causes issues, but your radiation team always works to lower radiation exposure to normal tissue. Spreading out low doses of radiation may cause acute or late radiation side effects, depending on which organs are within this low-dose region. 

IMRT can also cause ‘hot spots’ or ‘cold spots’ of radiation. Hotspots in organs can put the patient at higher risk for side effects and cold spots can mean the tumor is not receiving enough radiation dose to control the cancer.  

Planning and delivering an IMRT treatment takes longer than some other radiation treatments. As with all radiation treatments, patients must be able to stay comfortably still (immobilized), during the entire time that the beam is on. Even small movements by the patient during treatment can change how well IMRT works.  

Sometimes treatment needs to be done quickly to treat the cancer or manage symptoms. Some patients may be unable to stay still during radiation because of pain or mobility issues. In these cases, IMRT would not be the best option.  

The benefits of IMRT must be weighed, some factors to consider are:  

• The greater amount of low dose radiation to nearby structures.
• Possible added toxicity.
• Longer treatment times.
• The cost.

Talk with your radiation care team about your diagnosis and whether IMRT should be a part of your treatment plan.