WHAT IS IMRT?
Intensity Modulated Radiation Therapy (IMRT) is a brand new way of treating patients with radiation therapy. IMRT is the most technologically advanced treatment available in external beam radiation therapy. By allowing your physician to treat cancers with more accuracy than ever before possible, the cure rates of certain cancers can be increased while decreasing the side effects of treatment at the same time. It is the single biggest improvement in radiation therapy in the last 30 years. It is very much like going from the propeller age to the jet age in aviation.
One out of every three people will be diagnosed with cancer in his or her lifetime. More than half will be treated with radiation therapy. Radiation has been used for treating cancer for the last 100 years, and it remains the single best weapon that is available for treating cancer.
IMRT OFFERS HIGHER CURE RATES WITH LESS SIDE EFFECTS
In the early days, simple low-energy x-rays were found to be very effective for treating superficial skin cancers. Higher energy machines and then Cobalt were developed that could treat deeper cancers. However, these early machines often gave a higher dose to the skin than the tumor, limiting cure rates. Thirty years ago, linear accelerators, which accelerate electrons to near the speed of light to generate even higher energy x-rays, were developed. These have allowed treatment to far higher doses while staying below the tolerance of surrounding normal tissues. This has resulted in the higher cure rates for many cancers that we see today. In 1970 the overall cure rate for cancer was 25%; today it is 60%.
In order for us to continue to improve in our constant struggle to cure more people who develop cancer, we have to give even higher doses of radiation in order to kill every remaining cancer cell. At the same time, it is imperative to stay within the tolerance of the surrounding normal tissues, or else unacceptable side effects may result. IMRT allows us to accomplish exactly that, resulting in higher cure rates, decreased side effects, or both. Thus it allows us to push forward towards our ultimate goal of curing every person who develops cancer while giving them no side effects from the treatment.
IMRT is a highly specialized conformal radiation treatment technique that conforms a high dose radiation beam to a defined target, such as a tumor, while restricting dose to the surrounding sensitive structures. Conventional radiation therapy techniques use radiation beams of the same intensity, or strength, to kill tumor cells in the body. While some tissues outside the immediate area of interest can be spared using lead blocks, some tissues from the beam’s entrance to its exit are indiscriminately exposed to radiation. IMRT improves upon conventional radiation therapy in two ways.
IMRT TREATMENT PROCESS
First, the treatment planning is performed in an entirely new way called inverse planning. Conventional radiation treatment planning involves placing beams to treat a tumor, looking at the result, and than adjusting the beams by a trial and error process in order to get the best fit. IMRT uses inverse planning, whereby the computer is told “Give enough dose to completely kill the tumor while staying within tolerance of the spinal cord and kidney.” The computer will then create thousands of field combinations and produce the one that does the job best. While it takes the computer 8-40 hours to do each plan, it would take a human ten times that amount to do a job which would still not be as good as what the computer can do!
Second, but just as important, adjusting or modulating the strength or intensity of the beam while the radiation is on minimizes dose outside the tumor. In other words, the beam intensity is varied across the treatment field. An analogy would be to think conventional radiation therapy like a shower head, which puts out a uniform amount of water from each opening. IMRT allows us to vary the strength of the stream (actually radiation) coming out of each opening. Rather than being treated with a single, large, uniform beam from several angles, the patient is treated instead with many very small beams from many more angles, each one of which can have a different intensity. The result is that instead of being treated with 2 -6 broad beams of radiation, the patient is treated with 80-200 small, precise doses delivered precisely to the tumor each day. This allows us to effectively wrap the radiation around a normal structure, sparing it from injury. At the same time, since the normal structures have been more effectively excluded from the treatment beam, a higher dose can be given to the tumor. Over the years, it has been found that increasing the dose of radiation is the single best way to increase cure rates from cancer.
The process of treating a patient with IMRT is a very elaborate one that takes far more time and intensity of both people and equipment than conventional treatment. First, the patient is immobilized in a position which is comfortable. Sometimes, this can involve manufacture of a custom device using a plastic mask. The patient is then scanned in the treatment position on a dedicated high-speed CT scanner. The images are then downloaded to a very sophisticated and fast treatment-planning computer. The attending physician then sits down at the computer screen and draws, outlining the tumor, its possible microscopic extensions, any lymph nodes in the area which may be involved, and all normal structures in the area to which dose must be monitored. In the head and neck area, this can involve outlining 12-14 separate structures on each CT slice, which are spaced at 3 to 5 mm (1/12th to 1/5th of an inch) intervals. When necessary, other imaging studies, such as MRI scans or Positron Emission Tomography (PET) scans can be “fused” with the planning CT images in order to more accurately define the tumor, its extensions, and normal tissues.
Dedicated medical physicists then begin their work to produce a plan to actually treat the patient. The physicist works with the radiation oncologist much as an anesthesiologist works with a surgeon – just as it’s not possible to perform great surgery without a top-notch anesthesiologist, it’s not possible to do great radiation therapy without a top-notch physicist, and the physicists at Genesis are as good as they come. The physicist directs the computer to begin the planning process, and then monitors progress. Many adjustments are often necessary in order to produce a plan which will be the best to treat a patient. The physician and physicist then meet and review the plan together, reviewing carefully the dose to the tumor and each surrounding organ CT slice by CT slice, to ensure that the plan is the absolute best one for the individual patient.
Before the patient is actually given their first dose of radiation, two more important steps must be completed. First, a mannequin which has the same density as the human body is placed on the treatment table and treated in the exact same manner as the patient will be. By use of complex dosimetric analysis using both film and diodes, the dose to each area is calculated precisely as a second check. Then, the patient is brought in and films are taken which are compared to digitally-reconstructed radiographs that the computer generates. Only after both of these second checks have been completed is the ok given to actually treat the patient. The total time for this entire process is often over 40 hours of dedicated staff time per patient.
IMRT has been shown to be particularly useful for cancers of the prostate and head and neck region. Other sites which show potential benefit include the breast, pelvis, and any tumor wrapping itself around critical organs or tissues.
Learn more about our Genesis Radiation Oncology Services