Three-Dimensional Conformal Radiotherapy
- What is 3DCRT?
- How does 3DCRT work?
- What is the 3DCRT treatment process?
- What are the potential side effects of 3DCRT treatment?
- What results are possible with 3DCRT?
What is 3DCRT?
Three-dimensional conformal radiotherapy (3DCRT) is a complex process that begins with the creation of individualized, 3D digital data sets of patient tumors and normal adjacent anatomy. These data sets are then used to generate 3D computer images and to develop complex plans to deliver highly "conformed" (focused) radiation while sparing normal adjacent tissue. Because higher doses of radiation can be delivered to cancer cells while significantly reducing the amount of radiation received by surrounding healthy tissues, the technique should increase the rate of tumor control while decreasing side effects.
3DCRT is used to treat tumors that in the past might have been considered too close to vital organs and structures for radiation therapy. For example, 3DCRT allows radiation to be delivered to head and neck tumors in a way that minimizes exposure of the spinal cord, optic nerve, salivary glands and other important structures.
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How does 3DCRT work?
3DCRT begins with a "virtual simulation" in which computed tomography (CT) scans of the region of interest are obtained. The virtual simulation creates a permanent digital file that can be accessed by the entire treatment planning group to develop multiple, individualized courses of therapy. PAMF's Radiation Oncology Department has recently installed a dedicated GE Lightspeed CT simulator.
Scanned images are then linked into treatment planning software that allows physicians to visualize the treatment area in three dimensions. With this capability, radiation beam direction and intensity can be selected to more precisely target the tumor while sparing surrounding tissue. Clinicians input these selections into computer systems that control treatment delivery.
Radiation therapy is delivered by medical linear accelerators, which use microwave energy to accelerate electrons to nearly the speed of light in a short distance. As they reach maximum speed, the electrons collide with a tungsten target, which in turn releases x-rays focused to the area of interest. As the radiation enters human tissue, it produces highly energized ions that are lethal to both normal and cancerous cells. Healthy cells can adapt over time, but cancer cells do not. Further, since tumor cells divide and reproduce more rapidly than normal cells, they become more sensitive. As a result, radiation therapy is given in multiple treatments rather than a single blockbuster dose.
PAMF uses two state-of-the-art Varian 23EX Linear Accelerators that can be rotated around the patient with great precision to send radiation beams from the most favorable angles. Accelerators are located within specially constructed concrete rooms that provide x-ray shielding. Click here for a view of one of PAMF's linear accelerators.
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What is the 3DCRT treatment process?
After conducting a physical exam and a medical history review, the radiation oncologist determines an individualized course of treatment for each patient. The radiation oncologist works closely with other doctors and heads a team that generally includes nurses and technical staff such as radiation physicists and dosimetrists. The latter group is integral in designing how the beam arrangement and radiation dose is to be delivered. (Visit the Physicians and Staff page for more information about team members.)
Most 3DCRT cases begin with a "virtual simulation" session that lasts between 30 and 90 minutes. This session might include a treatment planning CT scan; the development of special molded devices that help the patient maintain the same position for each treatment; and the placement of colored, 3-millimeter permanent ink tattoos on the patient's skin to help align radiation equipment with the target area. Following this session, it usually takes three to seven days for the team to create a treatment plan, after which the patient is given an appointment to begin radiation treatment.
A typical treatment session lasts about 15-30 minutes, although the first appointment – which often involves additional scans and checks – may be longer. In the treatment room, a radiation therapist (the person who operates the equipment) uses the marks on the patient's skin to locate the target area. The patient is positioned on a table, sometimes using the molded devices created at the preparation session. The radiation therapist then leaves the treatment room and the machines are turned on. The therapist operates the machines from a nearby control room, where he or she can see patients on a television screen or through a window and can talk with them through an intercom.
Radiation delivery is painless, just like having an x-ray taken. Patients do not see or hear the radiation and usually do not feel anything. If a patient becomes uncomfortable for any reason, however, therapists can stop the machine at any time.
Radiation therapy usually is given in short sessions, five days a week for six or seven weeks. The small doses and weekend breaks allow normal cells in the treatment area to recover. The total dose of radiation given and the number of sessions a patient needs depend on the size and location of the tumor, the type of tumor, the patient's general health and other factors.
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What are the potential side effects of 3DCRT treatment?
With 3DCRT, some patients have no side effects at all. When side effects do occur, they are generally related to the area being treated. Although unpleasant, side effects are usually not serious, can be controlled with medication or diet, and usually go away within four to six weeks after treatment ends. Doctors will discuss with patients in detail what the potential side effects will be for particular treatments, and nurses will help patients manage any problems.
It is important to note that external radiation therapy does not cause a person's body to become radioactive. Patients need not worry about exposing others to radiation, even during close interactions like hugging or kissing.
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What results are possible with 3DCRT?
A study presented in October 2003 by PAMF radiation oncologist Pauling Chang demonstrates how three-dimensional treatment planning can improve radiation treatment. The study found that 3DCRT could improve the delivery of radiation beams to breast cancer tumors while reducing burns to the surrounding skin.
Breast cancer patients treated with standard radiation therapy may develop skin burns, usually in predicted areas. The traditional technique uses two unadjusted beams to target the breast from opposing sides. Because of the breast's geometry, areas such as the armpit and the skin under the breast fold tend to be "hot," receiving more radiation than the rest of the breast. The hot areas are more likely to develop skin burns.
At the same time, the breast's location near the heart and lungs makes it difficult to treat with intensity modulated radiation therapy, an advanced form of 3DCRT. IMRT is a complex treatment process in which a computer is needed to plot the direction, adjustment and intensity of the radiation beams. As a result, the therapy requires absolute precision in the placement of the patient. Even small movements from breathing or cardiac pumping can throw off the radiation pattern.
Dr. Chang and his colleagues adjusted the radiation beams by creating "subfields" at certain locations -- modifications that were not so precise that tiny movements would displace them, but enough to even out the distribution of radiation across the breast. In effect, the subfields reduced the amount of radiation received by the hot areas, giving a more consistent dose of radiation throughout the entire breast.
Fifty-four women with intact breasts (meaning their cancer therapy did not involve breast removal) were treated with this technique. The study found that, compared to standard radiation therapy, the technique resulted in less skin burning, took no extra time to plan and did not prolong the length of treatment.
A few other institutions use radiation subfields to treat breast cancer, but most are large academic medical centers, Dr. Chang said. "It is unusual to find this kind of advanced radiation therapy in a health care setting not associated with a major university. From a technological perspective, PAMF is ‘keeping up with the Joneses.' But as a smaller, community-based outpatient center, we can also provide patients with more personal care," he said.
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