PHYSICS NEWS UPDATE -- Number 738 July 21, 2005 by Phillip F. Schewe, Ben Stein
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@xxxxxxxxxx) 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@xxxxxxxxxxxx) 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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