BOSTON--Advances in three fields--imaging, medical physics, and computer
technology--have led to the development of a radiation therapy modality
that may represent a significant breakthrough in cancer treatment.
"Intensity modulated radiation therapy (IMRT) is the most sophisticated
form of computer-delivered radiation therapy currently available,"
said David E. Wazer, MD, director of the Radiation Oncology Center of the
New England Medical Center, which has pioneered the technique.
Mark J. Engler, PhD, physicist-in-chief at the Radiation Oncology Center,
said that the technique is not just an improvement on state-of-the-art
radiation therapy technology, but rather offers a complete change in the
way in which radiation therapy is administered. Dr. Engler headed one of
the first physics teams responsible for making IMRT a reality.
IMRT is a three-dimensional conformal radiation treatment that uses
a powerful computer model to plan therapy and a multileaf intensity-modulated
collimator to deliver highly focused radiation doses with minimal damage
to surrounding tissue (see images ).
In an interview with Oncology News International, Dr. Engler said that,
in effect, all radiation therapy is conformal. "Obviously, from the
time the first physicians aimed an x-ray tube at a cancer, they wanted
the dose to conform to the target." But unlike conventional 3D-conformal
radiation therapy (3D-CRT), IMRT treatment decisions are based on complex
mathematical models, resulting in a level of precision previously unattainable,
Dr. Engler said.
A Brief History of IMRT
When Mark Carol, MD, first envisioned dose optimization using a computer-controlled
In 1989, London physicist Steve Webb, PhD, published a paper applying
He named the system Peacock, from the fan of peacock feathers that symbolize
In 1996, Dr. Mark Engler and his colleague Jen-San Tsai, PhD, defined
The Planning Phase
For treatment planning with IMRT, a series of 40 to 80 CT images are
obtained and sent to the planning computer where radiation oncology personnel
delineate targets and sensitive surrounding normal tissues. The clinician
then determines the optimal dosage for the tumor site and the maximum tolerated
dosages for the surrounding normal organs and tissues. This allows for
a radiation prescription that expresses the relative importance of sparing
different normal tissues.
"In a prostate cancer patient, for example, the physician can tell
the computer the maximum tolerated dose for the rectum, the bladder, the
heads of the femurs, and so forth," Dr. Engler said. "In a brain
tumor patient, the system forces the physician to quantify the relative
importance of, say, the auditory nerve versus the optic nerve."
Once all the data are entered, the IMRT software simulates the radiation
physics for the desired doses, using mathematical models to search for
the plan that best satisfies the physician's multifaceted prescription.
The plan typically includes "an astronomical number of beam patterns,
providing dynamic, optimized, intensity-modulated 3D radiation therapy,"
Dr. Engler said.
The data for the optimal plan are then transferred to a disk, which
is inserted into the MIMic collimator controller on the accelerator for
delivery of the treatment plan. (The MIMic--multileaf intensity-modulated
collimator--is part of the Peacock IMRT system, manufactured by NOMOS Corporation.)
As the MIMic rotates around the patient, it constantly measures the
beam angle and adjusts the small vanes that shape the beam. Thus, the field
shape and intensity of the beam are continuously varied so as to mold the
radiation beam to the target and modulate the intensity of the radiation
across the target.
In contrast, with conventional 3D-CRT, Dr. Engler said "you're
aiming at a silhouette of the target, and you're treating normal tissues
in front of and behind that silhouette in a somewhat arbitrary fashion.
IMRT technology is aimed at minimizing the dose to these tissues in front
of and behind the target in a very systematic manner with intensity modulatation."
The conformity of the dose distribution to the target using IMRT is
shown in the images , in which a set
of colored lines represents the actual radiation dose that was delivered
Typically, the prescription dose (shown by the innermost overlapping
red-yellow lines around the tumor) is about 85% of the maximum dose, falling
off to 55% of maximum at the outermost blue-black line. "The whole
point of this system is that it creates a very sharp fall off of dose right
around the target," Dr. Engler said.
While IMRT is currently being used primarily on head and neck tumors,
in the near future, refinements in immobilization and imaging techniques
should allow its use to treat cancers in almost any location, including
the breast and lung.
"We are already seeing dramatic outcomes in patients with head
and neck tumors," Dr. Wazer said.
"One patient with a brain tumor wrapped around the optical nerve would
have been blinded as a result of the tumor. Using IMRT, we were able to
dramatically reduce the size of the tumor and preserve the person's eyesight
without damaging the optical nerve with radiation."
Dr. Wazer has submitted the results of preliminary clinical trials using
IMRT for presentation at the annual ASTRO meeting this fall. Nine other
American sites are involved in IMRT clinical trials, but the Radiation
Oncology Center has treated about one quarter of the more than 360 IMRT
patients in the United States.
Dr. Engler noted that many of the initial brain cancer patients were
treated under investigational device exemptions and protocols with criteria
that resulted in "extremely sick patients who did not have other options."
Nonetheless, he said, one of the preliminary observations is that with
IMRT most of these patients have not needed repeat surgery for aggressive
He said that prostate cancers are now being treated using IMRT, and
"the system is allowing us to cut down the dos-ages to the rectum
and bladder by about 50%." Several years of follow-up will be necessary
to determine if the technique does indeed produce fewer complications.
National research groups, including RTOG, have drafted dose escalation
protocols to be used with IMRT in prostate cancer patients, he added.