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 very quickly. Normal, healthy cells know to stop reproducing 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. Below, we will discuss IMRT and its role in cancer treatment.
History of IMRT
The development of the CT scan in the 1970’s-1980’s paved the way for advances in radiation therapy. With a CT scanner, simple 2D pictures of the body from x-rays could be expanded into a complex 3D model. 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.
Prior to starting radiation treatment, a radiation oncologist uses a CT scan, sometimes combined with other imaging such as MRI or PET/CT, to create a computerized 3D model of the body. With this 3D image, the radiation oncologist can learn how to line up treatment beams around the body so that they all intersect at the target (tumor).
IMRT is a sophisticated form of 3D planning. IMRT allows your radiation team to divide each treatment beam by small blocks, called “leaves.” These small leaves move across the beam’s path at different speeds and patterns while the beam is on. Therefore, parts of the radiation beam are selectively blocked for a portion 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 adjusted, or “modulated”–hence the term “intensity-modulated radiation therapy.”
Combining up to seven to nine such beams around the patient, each aiming at the target from a different angle, can create precise delivery of radiation within the body. The end result is a dose of radiation that can be higher in certain regions, lower in others, and even curve around nearby organs.
Like most other radiation treatments, IMRT is delivered as fractionated radiation, meaning that the total dose of radiation is delivered in many small daily, or twice daily, doses. This is divided over the course of several weeks of therapy.
What is IMRT used for today?
IMRT is used routinely 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 partially 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 necessary. Talk with your radiation team to see which type of treatment is best for you.
Risks of IMRT
While IMRT offers many advantages, it is not always the best option for patients. Because the radiation beams are grouped into 9 angles surrounding the patient, a low dose ‘bath’ of radiation is created just outside the main target. It is unclear whether this low dose region causes any consequences, but your radiation team is always mindful of lowering radiation exposure to normal organs. Spreading out low doses of radiation may cause acute or late radiation side effects, depending on which organs or which parts of the body are within this low dose region.
Another source of toxicity from IMRT relates to the unevenness of the dose. Sometimes, the complex levels of radiation that occur with IMRT can cause unwanted ‘hot spots’ or ‘cold spots’ of radiation. Hotspots located in important organs can put the patient at higher risk for side effects and cold spots within the target could mean the tumor is not receiving enough radiation dose to control the cancer.
Additionally, superficial cancers are sometimes not best served by IMRT, but may actually be treated by other modalities. An example of this is in patients who have breast cancer. In fact, the American Society of Radiation Oncology has recommended as part of the American Board of Internal Medicine’s Choosing Wisely campaign that IMRT not routinely be offered for whole breast radiation for breast cancer; it is warranted, however, under certain clinical circumstances.
In addition, planning and delivering an IMRT treatment takes longer than other radiation treatments. As with all radiation treatments, patients must be able to stay comfortably still, or immobilized, during the entire time that the beam is on. Even small movements by the patient during treatment can change the accuracy of IMRT.
Sometimes, delivering a plan quickly is more important than delivering one with a highly conformal dose distribution, such as when pain or disability limits a patient’s ability to maintain the treatment position. In these situations, IMRT would not be the best option.
The benefits of IMRT must be weighed against the greater amount of low dose radiation to nearby structures, possible added toxicity, longer treatment times, and cost. Talk with your radiation care team about the specifics of your diagnosis and whether IMRT should be a part of your treatment plan.
American Board of Internal Medicine. Choosing Wisely: An initiative of the American Board of Internal Medicine. 2018. Available at www.choosingwisely.org/, Accessed January 30th 2018.
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