BOSTONA new, intensity modulated radiation therapy delivery
system allows patients to lie down rather than stand up for total body
irradiation (TBI) and avoid the discomfort of heavy lead shields to block the
lungs, John T. Keane, PhD, reported at the American Society for Therapeutic
Radiology and Oncology (ASTRO) annual meeting.
Dr. Keane, computer systems manager in the Department of
Radiation Oncology, Montefiore Medical Center, Bronx, NY, said that the
modified technology, known as intensity modulated arc therapy (IMAT), was
initially developed to save space. Conventional total body irradiation with a
linear accelerator requires a very large room, he said, with enough distance
between the patient and the accelerator to project radiation beams uniformly
over the entire body.
Instead of using a gravity-oriented compensator to account for
differences in dose distribution, IMAT uses an intensity modulated continuous
arc beam pattern.
The patient lies on the floor, part of the time prone and part
of the time supine, as a gantry moves the radiation beam above the patient in a
pendulum-like swing, or arc beam pattern (see Figure
1). A high-power computer changes the
multileaf collimator aperture field sizes dynamically during the treatment.
Dr. Keane and his colleagues concluded that the uniform dose
profiles required for total body irradiation treatment could be delivered
accurately (see Figure 2) and in a more comfortable way using this method.
In an interview with ONI, Bhadrasain Vikram, MD, chairman of
the Department of Radiation Oncology, Montefiore, described the technique as a
boon to patients, who often find the lung-shielding lead blocks heavy to wear
and sometimes become nauseated during treatment.
"Necessity is the mother of invention," he said.
"The traditional technique requires a very large room to treat adult
patients. I couldn’t spare such a large room, so I asked the physicists to
come up with something we could use in a small room."
The hospital had not been able to do away with the blocks
because of the risk to the lungs. "The linear accelerator has a built-in
computer, but the computer was not powerful enough," Dr. Vikram said. He
expressed confidence that the computer will now be able to reduce radiation
when passing over the lung.
Dr. Vikram said the researchers are working with Varian Medical
Systems to make the technique commercially available. "Many more hospitals
can use this technique," he said, crediting Dr. Keane with developing the
software that makes it possible.
IMAT to Specific Organs
In another ASTRO presentation, researchers from the University
of Maryland Medical Center, Baltimore, discussed their use of IMAT to deliver
radiation therapy to specific organs.
"Other intensity modulated rotational methods require
patients to move during treatment and use thinner beams to deliver the
radiation," said Cedric Yu, DSc, director of Medical Physics. "IMAT,
however, does not require patient movement and uses open beams, which allow
more radiation photons to pass through."
In the Maryland study, 25 patients with various types of
cancer, including prostate, head and neck, and nasal/pharynx cancer, received
therapy for an average of 30 days. Before each procedure, a practice treatment
was performed to ensure accuracy. "The results showed that the IMAT method
delivers a more uniform, higher concentration of radiation for different sites
in the body. Also, patients spend less time, about 7 minutes, on the treatment
table." Dr. Yu said.