Santa Clara, CA--There's a new weapon in the war against cancer, and
it may be coming to your local hospital soon. Called Mobetron, it's an electron
linear accelerator that delivers high doses of radiation directly to an exposed
tumor site during surgery.
Interoperative Radiation Therapy (IORT)--as this treatment is known--is a
proven technique that more than doubles the survival rates of patients with some
types of advanced cancers. Studies, for example, show the five-year survival
rate for IORT patients with advanced primary colorectal cancer is 46%, as
compared to 24% without IORT.
Radiotherapy's laptop. What makes Mobetron unique is the
fact that it's the first-ever, truly portable electron linear accelerator
designed for use in radiation therapy in unshielded facilities.
Conventional radiotherapy machines are behemoths:Consisting of a
variable-energy electron linear accelerator, collimators (beam shapers),
shielding, microwave source, beamstopper, and supporting structure, they
typically weigh on the order of 8 tons or more. Size notwithstanding, these
machines must be housed in a special room (usually located in the hospital
basement) that provides the 50 tons of concrete shielding needed to protect
against undesirable radiation exposure.
Two compelling arguments can be made for a radiotherapy machine that travels
to the operating room. The first is better patient health. By obviating the need
to trundle patients up and down hospital corridors to the oncology department
midway through their surgery, there is less risk of accidents or infection
(which currently precludes patients with lung and other cancers from IORT
treatment altogether). The second is lower cost. With a portable unit,
cash-strapped hospitals will have more flexibility in scheduling and be able to
maximize the use of capital-intensive resources.
At 1/8th the size of conventional machines and weighing in at
2,750 lb, Mobetron is a laptop in a world of mainframes. What enabled
engineers to achieve Mobetron's small mass and proportions is a patented,
miniature linear accelerator specifically designed for portable field use. The
accelerator incorporates proprietary x-band technology, which operates at 8.20
to 12.4 GHz. Conventional radio-therapy equipment uses s-band frequencies
operating at 2.60 to 3.95 GHz, about one-third that of x-band.
Simple math, says Mechanical Engineer David Fadness, accounts in part for the
overall size reduction. "The diameter of our accelerator and related components
is approximately one-third that of comparable s-band hardware. Also, owing to
their physical dimensions alone, these smaller components require less
shielding."
The way in which energy is controlled within the linear accelerator also
impacts the overall physical dimensions of such machines. Multiple energy output
is desirable, as different types and sizes of tumors are treated with different
energy levels.
Bending the rules. Conventional radiotherapy machines
control output energy downstream of their linear accelerators by using a bending
magnet with a series of slits that allow passage of a narrow band of energy and
by varying the magnetic field. Radiation shielding requirements are high because
resulting e-beam collisions produce high-energy x-rays at the slits. In
contrast, Mobetron varies the energy and phase within two collinear,
back-to-back accelerators using a patented electronic control system.
"We
divide the RF-power and inject 1/3 of it into the first accelerator guide,
resulting in a constant energy of 4 MeV," says Fadness. "By adjusting the power
and phase of the RF-microwaves that are injected into the second accelerator
guide, we have selectable output energies each having a desirable, narrow energy
spectrum. And no bending magnet means less shielding."
To minimize concentrated exposure to healthy tissues, conventional
radiotherapy requires that the accelerator head (weighing 1.5 tons) rotate
through 360 degrees of motion around the patient during treatment. As a
consequence, conventional machines require massive gantry structures and
counterweights. Since Mobetron is significantly lighter and remains stationary
during treatment, designers were able to use thin-wall (3/16-inch) tubular
aluminum structural members with high stiffness and low weight.
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Mobetron consists of a self-shielded, 4- to 12- MeV electron beam
accelerator mounted on a motor-driven gantry. To achieve a more compact
configuration for storage and transport, the head rotates 90 degrees to a
horizontal orientation. |
But they did have to contend with rotation of the 900-lb accelerator head to
allow alignment of its e-beam with the patient's tumor. The main gantry rotates
±45 degrees about a transverse axis (for transport) and rotation of the 900-lb
accelerator head is ±30 degrees along the top curved beams of the gantry.
Fadness first thought about using the beamstopper, a 5-inch-thick lead plate
designed to intercept patient-generated radiation, as a counterbalance.
But it was a Catch-22: "To keep the overall weight of the machine down, we
had to keep the beamstopper as small as possible. Also, the heavier the
beamstopper, the bigger the drive required to move it, as it is step-motor
slaved to follow the electron beam when the accelerator head moves," says
Fadness.
He also wanted to position the beamstopper close to the center of gantry
rotation because the e-beam is a cone, and the frontal area of the beamstopper
is a function of the distance between the two.
After seeing a baby bouncing up and down in a jump-up seat, Fadness got the
ingenious idea of using compression springs in the extension mode as a draw-bar
counterbalance. He used two sets of heavy-duty die springs in series (one set on
either side of center) to obtain the required travel. When the machine is in its
centered orientation, both sets are engaged with a slight amount of pretension
to avoid any over-center slumping. As the gantry rotates one way or the other,
it engages one set of springs and disengages the other, increasing spring
tension as a function of the angle.
A prototype is currently in use treating patients at the University of
California at San Francisco. FDA approval was granted on July 24, 1998, and the
first three production units will ship in early 1999.
Additional details…Contact Intraop Medical Inc., 3170
De La Cruz Blvd., Suite 108, Santa Clara, CA 95054; (408) 986-6020 or:
David Fadness received his B.S. from Stanford
University in 1972 and is the holder of five patents for medical and industrial
equipment used in electron beam and x-ray technology. As owner of Mechanical
Design and Development Co., he spearheaded the mechanical design of Mobetron.
Two other key members of the development team were: George Spalek, a senior
principal engineer at General Atomics at Los Alamos National Laboratory and
Russell G. Schonberg, chief executive officer/technical director of Schonberg
Research Corp.
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