Sarah is ready. The therapist leaves the room, closes the door, and moves
to a computer workstation to administer the treatment.
Inside the room, the Clinac rotates
and locks into a fixed position at the first of the planned beam angles.
The beam goes on and the leaves of the MLC begin moving the aperture across
the treatment field, changing its size and shape in order to deliver the
prescribed dose. This "sliding window" approach to IMRT puts
the leaves of the MLC in continuous motion while the beam is on and yet
maintains a pattern that conforms to the 3D shape and size of the tumor.
Sarah hears a low humming noise but feels nothing. The Clinac rotates and
delivers beams from several angles until the treatment is completed.
During treatment, Sarah’s medical team uses Varian’s
PortalVision™ device on the linac to instantly capture X-ray images of Sarah’s
anatomy as viewed through the beam aperture. By using Varian’s image-processing
software to compare the PortalVision views on a computer monitor with diagnostic
images and the treatment plan, the team is able to verify treatment accuracy
and make any adjustments that might be needed in her position or the plan
for future sessions.
It is 10:00 a.m. when Sarah enters the IMRT treatment room. Ten minutes
later, her placement and immobilization on the positioning couch are completed
and her treatment begins. Five minutes later, the session is complete.
Sarah is free to return home to her family and resume her daily activities.
Sarah's Outcome
Sarah will have to undergo multiple treatment sessions, on a Monday through
Friday schedule, over a period of weeks. Otherwise her life should not
be disrupted. What is the outcome of her IMRT treatment? Hopefully, follow-up
tests will show that Sarah’s tumor has been functionally eliminated.
She will have to be re-scanned, perhaps six months after treatment has
been completed, for the possible appearance of new lesions and to be sure
that all of the original tumor was destroyed. If either situation should
prove to be the case, she will have to undergo another round of treatment
and the process will continue until all the lesions are gone. In the end,
however, the likelihood is good that she will be cured of her lung cancer.
Is this too rosy a scenario to project for Sarah? No, nor would it have
been had our patient been George, 57, diagnosed with prostate cancer, or
Robin, 54, diagnosed with breast cancer, or Bill, 63, who had cancer of
the head and neck. IMRT is being used to treat all of these major cancers.
According to the early clinical results and the testimony of oncologists
who are at the forefront in the fight against cancer, it can be highly
effective. For example, in a study conducted by researchers at Memorial
Sloan-Kettering Cancer Center in New York between April 1996 and January
2001, 772 patients with prostate cancer were treated with IMRT at fairly
high doses made possible by IMRT’s precision. The 3-year relapse-free
survival rates for favorable, intermediate, and unfavorable risk group
patients were 92 percent, 86 percent, and 81 percent, respectively. Compare
that success rate with comparable rates of only 75 percent, 55 percent,
and 35 percent in an earlier study in which prostate cancer patients were
given a more conventional treatment at a lower dose.
A Treatment That is Becoming More Common
Sloan-Kettering’s chief of radiation oncology, Steven Leibel, MD,
has said, “IMRT is revolutionary in its ability to modulate the radiation
beam. It can do what standard conformal therapy can't. IMRT has become
the standard mode of conformal treatment delivery for localized prostate
cancer treatment at our institution.”
Leibel says the Sloan-Kettering Cancer Center, which treated its first
patient with IMRT in 1995, now treats roughly a quarter of their patients
with the technology, approximately 1,000 patients a year. George T.Y. Chen,
Ph.D., head of radiation physics, Department of Radiation Oncology at Massachusetts
General Hospital, and professor at Harvard Medical School, says his department,
which began using IMRT a couple of years ago, is now using IMRT to treat
between 10 and 15 percent of their patients.
“IMRT is in its infancy and so
we don’t know, for example, what the
10-year success rate will be,” Chen has said. “In some cases,
such as cancer of the head and neck, the impact is obvious. It provides
the opportunity to spare critical structures such as the parotid gland
and this enables us to reduce the side effects of radiation. It’s
a technological revolution that’s really changing radiotherapy. The
oncology community is very excited about it.”
The opinions of Leibel and Chen have been echoed by other leading oncologists
across the nation (see “What They’re Saying from the Front”).
Although less than ten percent of the world’s nearly 5,500 radiotherapy
centers for cancer treatment are currently offering IMRT to their patients,
the numbers using Varian’s SmartBeam IMRT climbed from one in 1995,
to 40 in 2000, to 98 in 2001, to an estimated 200 by the end of 2002. Expectations
are for continued expansion around the world.
The Future: Dynamic Targeting Approaches
The IMRT procedures being implemented at radiotherapy centers now are a
first-generation technology. Already in the works as a next evolutionary
step is a Dynamic Targeting™ initiative that will eventually
equip Clinac linear accelerators with an X-ray-based on-board imaging system.
The aim of this research initiative is to attach Varian’s latest
amorphous-silicon flat-panel image detector directly to the Clinac on a
pair of robotic arms that move relative to one another. The goal is to
provide oncology teams with images and motion tracking capabilities that
can help them guide the beam during a treatment session.
Varian unveiled a research prototype of this next step in Dynamic Targeting
at the 2002 American Society of Therapeutic Radiology and Oncology (ASTRO)
meeting.
As we move into the new millennium, humans for the first time ever
have the technology at hand with which they can confront their ancient enemy
and bring it under manageable control. The idea of cancer being transformed
from a life-threatening condition to a manageable disease is no longer
wildly speculative science fiction but is tantalizingly close to becoming
scientific fact.
Related Articles
An Ancient and Persistent Foe
What They're Saying from the Front
How a Linac Works
Viewpoint
The Radiation Oncology Department of the Future |
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