Detecting skin cancer by sound waves
30 October 2006
Researchers at the University of Missouri-Columbia have developed a
technique to detect the spread of skin cancer cells through the blood by
listening to their sound. The minimally invasive technique uses a laser to
make the melanoma cells emit ultrasound noise. The technique could enable
oncologists to spot the early signs of metastases from as few as ten cancer
cells in a blood sample β before they even settle in other organs.
The results of the successful experimental tests appear in the October 15
issue of the journal Optics Letters (1), published
by the Optical Society of America.
The team's method, called photoacoustic detection, combines laser
techniques from optics and ultrasound techniques from acoustics, using a
laser to make cells vibrate and then picking up the characteristic sound of
melanoma cells. In a clinical test, doctors would take a patient's blood
sample and separate the red blood cells and the plasma. In a healthy person,
the remaining cells would be white blood cells, but in a melanoma patient
the sample may contain cancer cells. To find out, doctors would put the
sample in saline solution and expose it to rapid-fire sequences of brief but
intense blue-laser pulses, each lasting just five billionths of a second.
In lab tests, the Missouri-Columbia team was able to detect melanoma
cells obtained from actual patients, showing that the method can spot as few
as ten cells in saline solution. The dark, microscopic granules of melanin
contained in the cancer cells absorb the energy bursts from the blue laser
light, going through rapid cycles of expanding as they heat up and shrinking
as they cool down. These sudden changes generate loud cracks β relative to
the granules' size.
The sound waves produced by melanin are high-frequency ultrasounds,
meaning that they cannot be heard by the human ear, even if amplified.
However, researchers can pick them up with special microphones and analyze
them with a computer. Other human cells do not contain pigments with the
same colour as melanin, so the melanin signature is easy to tell apart from
other noises, said John Viator, a biomedical engineer at Missouri-Columbia
and a coauthor of the Optics Letters paper. And the presence of melanin
granules in the blood is an unmistakable sign. "The only reason there could
be melanin in the human blood is that there would be melanoma cells," he
said.
This new blood test would allow for a much more timely diagnosis of
metastasis and with early diagnosis comes early treatment and increased
likelihood for survival. As one of the most aggressive forms of cancer, if a
melanoma is not removed at its earliest stages, it will penetrate into the
deep layers of the skin. From there its cells can break off and pass into
the circulatory and lymphatic systems, spreading to other organs and
creating metastases even after the original melanoma has been surgically
removed.
An earlier metastasis warning, as this blood test provides, could alert
oncologists to the cancer when itβs at its earliest stages in other parts of
the body and help them to begin a quicker counterattack, for example by
administering chemotherapy, said Viator. "Our method can help doctors plan
treatment to battle the spread of the disease," he said.
Current techniques to monitor the disease spread and recurrence have
proven to be inaccurate, time-consuming and painful, according to the
researchers. This new blood test would enable physicians to have a more
accurate method of monitoring for metastasis.
In fact, the blood-test procedure could be performed regularly such as in
screenings for high-risk patients, requiring just a small sample of blood,
and its results would be almost immediate. "It could take just 30 minutes to
find out if there are any circulating cancer cells," Viator said.
Other labs have used photoacoustic detection for scanning mouse brains
and for mapping port-wine stains (birthmarks), but this would be its first
application to oncology, Viator said. The team is now planning a pilot study
on actual blood samples from patients, and larger clinical studies will need
to be done, but the test shows great promise for early detection of the
spread of this disease, according to Viator.
The team is also working with other Missouri-Columbia scientists in the
veterinary college and the department of surgery to extend the reach of its
technique to other types of cancer. Because of melanin, melanoma is the only
type of cancer whose cells will strongly absorb all wavelengths of light,
emitting ultrasounds that stand out from those of other cells. But
artificial materials could also be introduced, to act as light absorbers β
and as noise makers. "We're looking for methods to attach other kinds of
absorbers to cancer cells," Viator said. For example, he said, gold
nanoparticles (particles only a few millionths of a millimeter wide) could
be attached to the cells using proteins that bind to special receptors on
the cells' membranes. With their own photoacoustic signature, the gold
particles would then signal the presence of cancer cells.
1. "Photoacoustic detection of metastatic melanoma cells
in the human circulatory system," by Ryan M. Weight, John A. Viator, Paul S.
Dale, Charles W. Caldwell, and Allison E. Lisle, Optics Letters, Vol.
31, Issue 20, pp. 2998-3000; abstract at
http://ol.osa.org/abstract.cfm?id=111347
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