Intensity-Modulated Radiation Therapy (IMRT): The Basics
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 commonsource 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 three main strategies physicians use to treat cancer–surgery, chemotherapy, and radiation–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 technologiesthat 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. Withthis 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 planning. With a CT scanner, simple 2D pictures of the body from x-rays could be expanded into a complex 3-dimensional 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 avoiding nearby normal 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 3-dimensional model of the body. By understanding the 3-dimentional 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. This approach causes the dose at the target to escalate, while ensuring the dose to the normal structures that surround the target 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 inthe exposed area instead of all black, or all white.
To do so, in IMRT, each individual beam is further subdivided by small ‘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 theradiation beam deliver a high dose, while other regions deliver a low dose. In other words, the intensityacross the beam can be modulated–hence 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 our most sophisticated radiation delivery technique 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.
Toxicities of IMRT
While IMRT offers many technical advantages over prior technologies, it is not always the best technique for our patients. As discussed above, IMRT’s strength is to create highly conformal doses of radiation–again, meaning that the high dose region of radiation within the body easily takes on the shape of the target structure. 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 ofradiation 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, just last year, 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 blur away 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 costof 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.
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.
- American Board of Internal Medicine. Choosing Wisely: An initiative of the American Board of Internal Medicine. 2015. Available at www.choosingwisely.org/, Accessed March 8th2015.
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