PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 738  July 21, 2005  by Phillip F. Schewe, Ben Stein
        
TREATING LUNG CANCER WITH 4D PROTONS.  Compared to the x rays traditionally
used in radiation therapy, protons offer the potential to destroy lung
tumors just as competently while inflicting less damage to surrounding
healthy tissue.  At next week's meeting of the American Association of
Physicists in Medicine (AAPM) in Seattle, a research team (including
Martijn Engelsman, now at MAASTRO clinic, Netherlands,
martijn.engelsman{at}maastro.nl) will describe a method for increasing the
effectiveness of using protons to treat lung tumors in a small experimental
study of four patients.
In traditional radiation therapy, one must use multiple beams of x rays to
deliver a uniform dose to a lung tumor; often at least one of the x-ray
beams will exit from the healthy (non-tumor-containing) lung and
potentially damage it. On the other hand, positively charged, subatomic
protons only travel a limited distance through the body; they never make it
to the other lung, and they also are more likely to spare nearby organs
such as the esophagus and heart.  However, the protons' finite range makes
their trajectories particularly sensitive to density changes in the lung,
caused, for example, by the expansion of the lung during inhalation.  For
that reason, if the proton treatment is not carefully planned, there is the
chance of missing the tumor, thus decreasing the chance of curing the
patient.  So in planning the treatment of lung cancer patients, the
researchers adopted the 4D approach, which is already used in traditional
x-ray cancer therapy.  In the 4D approach, one takes into account how the
patient's breathing moves the lung back and forth over time (the fourth
dimension) so that the radiation hits the tumor precisely over all phases
of a patient's breathing cycle.
    In a study of four patients at Massachusetts General Hospital, the
researchers have found that planning and carrying out 4D proton therapy
delivers excellent dose levels to lung tumors in all cases.  The only thing
preventing this technique from wider use is the need to develop an
algorithm that cuts down the currently lengthy time it takes to calculate
and plan the proton beam's direction and intensity for each breathing
phase. The 4D approach is also applicable to radiation therapy using carbon
ions, which is currently being used to help defeat lung cancer in a couple
of centers in Japan. (Paper WE-E-J-6C-7; for more information on the
meeting; go to http://www.aapm.org/meetings/05AM/)
While currently small, the numbers of proton therapy centers are expected
to grow exponentially over the next 20 years; for example, a major proton
therapy center is scheduled to open at the University of Texas's M.D.
Anderson Cancer Center in Spring 2006.

ELECTRON PARAMAGNETIC RESONANCE IMAGING (EPRI) may become a useful tool for
determining crucial oxygen levels in tumors and other biological tissue. 
Oxygen is central to many diseases; for example, the absence of oxygen
makes a cancer cell more resistant to radiation and chemotherapy.  Taking
advantage of the properties of electrons in certain biochemical compounds,
Charles Pelizzari (c-pelizzari{at}uchicago.edu) and his colleagues use a novel
technique to form images of the oxygen distribution in small animals with
millimeter spatial resolution. In a talk at the AAPM meeting, Pelizzari's
group will present EPR oxygen images superimposed on MRI anatomical images
of animals.  Developing these tools at the Center for In-Vivo EPR Imaging
at the University of Chicago, the researchers create these important maps
of oxygen levels by magnetically manipulating the unpaired electrons in
certain oxygen-containing molecules, including free radicals. Most
electrons in atoms and molecules form pairs that mutually cancel out their
internal magnetic properties, but unpaired electrons can give the
atom/molecule "paramagnetic" properties that cause them to be
weakly attracted to an external magnetic field.
Electron paramagnetic resonance imaging (EPRI) obtains pictures of
molecules with unpaired electrons in a way that is similar to the way MRI
obtains images of atomic nuclei such as the hydrogen in water: an image is
formed when paramagnetic molecules, lined up in a magnetic field, absorb
and then re-emit electromagnetic waves in or near the microwave portion of
the spectrum. Using a series of magnetic fields that vary in strength over
a given region of space, these emissions can be reconstructed into a 3D
image.
Where EPRI is advantageous over MRI is in providing quantitative images of
the distribution of oxygen in living tissues. Pelizzari expects that one
day this EPR methodology will obtain submillimeter-resolution maps and also
be scaled up to human dimensions. A potential long-term benefit of EPR
imaging should be in obtaining both maps of radiation-resistant tumor
regions before treatment and in providing quick feedback on the results of
cancer therapy in days or even hours, without the use of PET scans which
require radioactivity. (Meeting talk WE-D-I-609-8)

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