All About Intensity-Modulated Radiation Therapy (IMRT)

David Guttmann, MD and Melissa Frick
The Abramson Cancer Center of the University of Pennsylvania
Last Modified: January 31, 2018

For many patients, radiation treatment can be confusing; answers to questions such as what is radiation, how does it work, why is it needed, and how is it delivered are foreign. The terminology is new, the machines mysterious, and abbreviations unfamiliar. In the following, I hope to shed light on a common source of such confusion in radiation oncology: Intensity-modulated radiation therapy (IMRT). IMRT represents one of the most common modern technologies used to deliver radiation therapy, yet patients are often asking what, exactly, is it? To begin to answer this question, it is first important to understand the scientific and historical contexts from which it arose.

An Overview of Radiation Treatment in Cancer

Cancers develop when a population of cells in the body begins to divide in an unregulated way. The goal of cancer treatment, therefore, is to kill off these cells while minimizing ‘collateral damage’ to normal healthy tissue. The main strategies physicians use to treat cancer–surgery, chemotherapy, radiation, and now most recently, immunotherapy –all work on this principle. Surgeons, for example, use fine robotic instruments to dissect the cancer away from entangled blood vessels and nerves. Similarly, medical oncologists prescribe drugs with the goal of targeting only cancerous cells, and minimizing the effect on normal cells. Likewise, radiation oncologists use various radiation techniques–3D conformal radiation, IMRT, and proton radiation, for example–in an effort to expose only the dividing cancer cells to the high energy beams while shielding away the rest of the body.

History of IMRT

The modern techniques of radiation delivery, such as IMRT, are a natural outgrowth of the technologies that preceded them. Radiation has been used to treat cancer since the start of the 20th century. In that era, treatment was often limited to superficial cancers that could be easily treated without sophisticated imaging–skin cancers, for instance. As technology progressed over the coming decades, radiation oncologists would refine their ability both to locate, and to treat, cancer cells deep within the body.

A major advance in radiotherapy occurred with the emergence of 2-dimensional, or 2-D, radiation. With this technique, simple x-ray images were used to help define a region of treatment relative to bony landmarks. For instance, radiation could be delivered accurately to the pelvis by lining up the treatment beams relative to an x-ray of the pelvic bones.

Soon after, the development of the CT scan in the 1970’s-1980’s paved the way for a shift from 2-dimensional to 3-dimensional (3D) planning. With a CT scanner, simple 2D pictures of the body from x-rays could be expanded into a complex 3D model. Such models would help us to better understand how to incorporate multiple radiation beams oriented around a target to achieve better conformality; that is, to better shape the radiation to the tumor, while reducing the amount of radiation delivered to nearby normal, healthy tissue. It was in this era that IMRT was born.

The Difference between 3D radiation and IMRT

With 3D planning, 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. By understanding the 3D relationships between tumor and surrounding structures, the radiation oncologist can identify how to orient up to 4-5 treatment beams around the body so that they all intersect at the target. Where these beams overlap at the tumor site is where the greatest radiation dose is delivered, while the dose of non-overlapping beams passing through normal structures remains low. Therefore, the normal tissues of the body are receiving less radiation than the tumor.

IMRT can be understood as a sophisticated form of 3D planning. The main difference between 3D and IMRT lies in what is occurring on the level of each individual radiation beam. In 3D planning, each beam is given a shape that attempts to match the outline of the tumor, or target, as seen from the beam’s perspective (or ‘beam’s eye view’). The beam is then turned on and all of the radiation from that beam is delivered in that ‘shape’ toward its target. When the treatment is complete, the beam is turned off. If a piece of film were placed in the beam’s path, a solid white shape in the exact shape of the beam would be exposed on the film, surrounded by black. IMRT, by contrast, allows us to create shades of gray in the exposed area instead of all black, or all white.

To do so, in IMRT, each individual beam is further subdivided by small blocks - which we call ‘leaves’ - that 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. As a result, certain regions of the radiation beam deliver a high dose, while other regions deliver a low dose. In other words, the intensity across the beam can be modulated–hence the term “intensity modulated radiation therapy.” Combining up to seven to nine such beams oriented around the patient, each aiming at the target from a different angle, can create even more sophisticated distributions 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, typically on the order of 5-9 weeks, depending on the specific treatment. From the patient’s perspective, the process of immobilization, simulation, and daily treatment delivery are largely similar to 3D radiation.

What is IMRT used for today?

IMRT represents one of our most sophisticated radiation delivery techniques available for conventional photon, or x-ray, radiation. It is used routinely in the treatment of prostate cancer, head and neck cancers, gastrointestinal and gynecologic cancers, lung cancers, brain tumors, among others, at cancer centers worldwide.

As IMRT has the ability to deliver highly conformal doses of radiation – meaning that the high dose region of radiation within the body easily takes on the shape of the target structure – you could image that this feature is best leveraged when a tumor partially surrounds or is very close to a normal structure that cannot tolerate the full dose of radiation that is being given to the tumor. When the tumor is not near sensitive areas, the clinical benefit of IMRT is less obvious. Radiation oncologists are trained to balance the advantages of using IMRT against its weaknesses, as discussed below, on a case-by-case basis when deciding if it is the appropriate treatment approach.

Toxicities of IMRT

While IMRT offers many technical advantages over prior technologies, as discussed above, it is not always the best technique for our patients. Because the radiation beams are oriented in up to 9 angles surrounding the patient, however, a low dose ‘bath’ of radiation is created just outside the main target. While this effect can occur in 3D planning as well, in general, only 2-4 beam angles are used with 3D radiation and so the low dose region is not spread out as widely. It is unclear whether this low dose region necessarily results in any real consequences for an individual patient, but radiation oncologists are always mindful of reducing radiation exposure to normal organs. This concern becomes particularly true for our younger patients, in whom we fear the possibility that the low dose radiation could actually predispose the patient to developing a second cancer. For our adult patients as well, spreading out low doses of radiation may pose other risks of acute or late radiation side effects, depending on which organs or which parts of the body lay within this low dose region.

Another source of toxicity from IMRT relates to the dose inhomogeneity, or unevenness. Sometimes, the complex shapes of radiation dose that can be achieved with IMRT result in unwanted ‘hot spots’ or ‘cold spots’ of radiation within the patient. 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 our breast cancer patients. 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 substantially longer than a 3D conformal treatment. As with all radiation treatments, our patients must be able to stay comfortably immobilized during the entire time that the beam is on; small patient movements may offset the conformality 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 necessarily be the best option.

Lastly, from a healthcare system’s perspective, the use of IMRT must also be balanced against the cost of such treatments. Due to their technical complexity, IMRT plans are more expensive and labor-intensive to plan and deliver and may not always justify the expense of finite economic, technological, and human resources.

Concluding Thoughts

In summary, IMRT represents the outgrowth of increasingly more sophisticated radiation delivery systems that have evolved over the past century. It is useful in creating highly conformal radiation dose distributions and has become a workhorse for radiation oncologists across many disease sites. 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.

References

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.

Bortfeld T. IMRT: a review and preview. Phys. Med. Biol. 51 (2006) R363—R379.

Gunderson L and Tepper J. Clinical Radiation Oncology, 4th ed. Elsevier Saunders, 2016.

Thariat J et al. Past, present, and future of radiotherapy for the benefits of patients. Nat. Rev. Clin. Oncol. 10, 52—60 (2013).

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