Intensity Modulated Radiation Therapy (IMRT)
A state-of-the-art external radiation delivery system which uses advanced computer technology to create a three-dimensional model of a tumor, and direct precisely-focused beams of radiation at the tumor with laser accuracy. By precisely modulating (controlling) the intensity of the radiation beams to conform to the tumor shape, IMRT can treat difficult-to-reach tumors with new levels of accuracy.
With IMRT, we’re able to deliver higher radiation doses to tumors virtually anywhere in the body.
IMRT addresses the shortcomings of 3DCRT and improves dose distributions
- IMRT is a computer-generated plan which uses multiple small fields called beamlets to generate complex field shapes to avoid critical normal structures.
- IMRT modulates a number of fields as well as intensity within each field, thereby ensures greater control in delivering dose to the tumor while minimizing dose to normal structures.
- IMRT also helps in dose escalation to tumor there by helping in improving treatment outcomes.
- The decrease in toxicity also helps to improve the quality of life.
Treatment is carefully planned by using 3-D computed tomography (CT) images of the patient in conjunction with computerized dose calculations to determine the dose intensity pattern that will best conform to the tumor shape. Typically, combinations of several intensity-modulated fields coming from different beam directions produce a custom tailored radiation dose that maximizes tumor dose while also minimizing the dose to adjacent normal tissues.
Because the ratio of normal tissue dose to tumor dose is reduced to a minimum with the IMRT approach, higher and more effective radiation doses can safely be delivered to tumors with fewer side effects compared with conventional radiotherapy techniques. IMRT also has the potential to reduce treatment toxicity, even when doses are not increased. IMRT does require longer daily treatment times and delivers a low dose to larger volumes of normal tissue than conventional radiotherapy.
Multi-dimensional planning allows us to simultaneously treat multiple tumors with different doses of radiation. Thus, if the patient have previously had conventional radiation therapy and are experiencing recurrent tumors in the treated area, IMRT may be a treatment option.
Currently, IMRT is being used most extensively to treat cancers of the prostate, head and neck and central nervous system. IMRT has also been used in limited situations to treat breast, thyroid, lung, as well as in gastrointestinal, gynecologic malignancies and certain types of sarcomas. IMRT may also be beneficial for treating pediatric malignancies.
Radiation therapy, including IMRT, stops cancer cells from dividing and growing, thus slowing or stopping tumor growth. In many cases, radiation therapy is capable of killing all of the cancer cells, thus shrinking or eliminating tumors.
There are several ways IMRT differs from conventional radiation therapy:
Employs powerful, advanced software to plan a precise dose of radiation, based on tumor size, shape and location.
Delivers radiation in sculpted doses that match the exact 3D geometrical shape of the tumor, including concave and complex shapes.
Adjusts the intensity of radiation beams across the treatment area as needed with laser accuracy.
Most facilities rely on a specially trained team for IMRT delivery. This team includes the radiation oncologist, medical radiation physicist, dosimetrist, radiation therapist and radiation therapy nurse.
The radiation oncologist, a specially trained physician who heads the treatment team, sets an individualized course of treatment with the help of the radiation physicist, who ensures the linear accelerator delivers the precise radiation dose and that computerized dose calculations are accurate.
A dosimetrist, under the supervision of the medical radiation physicist, calculates the IMRT exposures and beam configurations necessary to deliver the dose prescribed by the radiation oncologist. A highly trained radiation therapist positions the patient on the treatment table and operates the machine. The radiation therapy nurse provides the patient with information about the treatment and possible adverse reactions as well as help in managing any reaction.
A medical linear accelerator generates the photons, or x-rays, used in IMRT. The machine is the size of a small car—approximately 10 feet high and 15 feet long. The patient lies on the treatment table, while the linear accelerator delivers multiple beams of radiation to the tumor from various directions. The intensity of each beam’s radiation dose is dynamically varied according to treatment plan.
The radiation therapist operates the equipment from a radiation-protected area nearby. The therapist is able to communicate with the patient throughout the procedure. The therapist observes the patient on closed circuit television.
Before planning treatment, a physical examination and medical history review will be conducted. Next, there is a treatment simulation session, which includes CT scanning, from which the radiation oncologist specifies the three-dimensional shape of the tumor and normal tissues. In some cases, a treatment preparation session may be necessary to mold a special device that will help the patient maintain an exact treatment position. The dosimetrist and medical radiation physicist use the CT information to design the IMRT beams used for treatment. Several additional scanning procedures, including positron emission tomography (PET) and magnetic resonance imaging (MRI), might also be required for IMRT planning. These diagnostic images can be merged with the planning CT and help the radiation oncologist determine the precise location of the tumor target. In some cases it is necessary to insert radio dense markers into the target for more accurate positioning. Typically, IMRT sessions begin about a week after simulation. Prior to treatment, the patient’s skin may be marked or tattooed with colored ink to help align and target the equipment.
IMRT often requires multiple or fractionated treatment sessions. Several factors come into play when determining the total number of IMRT sessions and radiation dose. The oncologist considers the type, location and size of the malignant tumor, doses to critical normal structures, as well as the patient’s health. Typically, patients are scheduled for IMRT sessions five days a week for five to ten weeks.
At the beginning of the treatment session, the therapist positions the patient on the treatment table, guided by the marks on the skin defining the treatment area. If molded devices were made, they will be used to help the patient maintain the proper position. The patient may be repositioned during the procedure. Imaging systems on the treatment machine may be used to check positioning and marker location. Treatment sessions usually take between 10 and 30 minutes.
IMRT is painless. You will not feel or sense anything out of the ordinary during treatment. However, the machine can be stopped if you become uncomfortable. As treatment progresses, some patients may experience treatment-related side effects. The nature of the side effects depend on the normal tissue structures being irradiated. The radiation oncologist and the nurse will discuss and try to help you with any side effects.
Side effects of radiation treatment include problems that occur as a result of the treatment itself as well as from radiation damage to healthy cells in the treatment area. The number and severity of side effects you experience will depend on the type of radiation and dosage you receive and the part of your body being treated. You should talk to your doctor and nurse about any side effects you experience so they can help you manage them.
Radiation therapy can cause early and late side effects. Early side effects occur during or immediately after treatment and are typically gone within a few weeks. Common early side effects of radiation therapy include tiredness or fatigue and skin problems. Skin in the treatment area may become more sensitive, red, irritated, or swollen. Other skin changes include dryness, itching, peeling and blistering.
- Hair loss in the treatment area
- Mouth problems and difficulty swallowing
- Eating and digestion problems
- Nausea and vomiting
- Soreness and swelling in the treatment area
- Urinary and bladder changes
- Brain changes
- Spinal cord changes
- Lung changes
- Kidney changes
- Colon and rectal changes
- Joint changes
- Mouth changes
- Secondary cancer
There is a slight risk of developing cancer from radiation therapy. Following radiation treatment for cancer, you should be checked on a regular basis by your radiation oncologist for recurring and new cancers.
Using techniques such as IMRT, imaging specialists are maximizing the cancer-destroying capabilities of radiation treatment while minimizing its effect on healthy tissues and organs and the side effects of the treatment itself.