Photodynamic therapy (PDT) is a treatment that uses a drug, called a photosensitizer
or photosensitizing agent, and a particular type of light. When photosensitizers
are exposed to a specific wavelength of light, they produce a form of oxygen that
kills nearby cells.
Each photosensitizer is activated by light of a specific wavelength. This wavelength
determines how far the light can travel into the body. Thus, doctors use specific
photosensitizers and wavelengths of light to treat different areas of the body with
In the first step of PDT for cancer treatment, a photosensitizing agent is injected
into the bloodstream. The agent is absorbed by cells all over the body but stays
in cancer cells longer than it does in normal cells. Approximately 24 to 72 hours
after injection, when most of the agent has left normal cells but remains in cancer
cells, the tumor is exposed to light. The photosensitizer in the tumor absorbs the
light and produces an active form of oxygen that destroys nearby cancer cells.
In addition to directly killing cancer cells, PDT appears to shrink or destroy tumors
in two other ways. The photosensitizer can damage blood vessels in the tumor, thereby
preventing the cancer from receiving necessary nutrients. In addition, PDT may activate
the immune system to attack the tumor cells.
The light used for PDT can come from a laser or other sources of light. Laser light
can be directed through fiber optic cables (thin fibers that transmit light) to
deliver light to areas inside the body. For example, a fiber optic cable can be
inserted through an endoscope (a thin, lighted tube used to look at tissues inside
the body) into the lungs or esophagus to treat cancer in these organs. Other light
sources include light-emitting diodes (LEDs), which may be used for surface tumors,
such as skin cancer.
PDT is usually performed as an outpatient procedure. PDT may also be repeated and
may be used with other therapies, such as surgery, radiation, or chemotherapy.
To date, the U.S. Food and Drug Administration (FDA) has approved the photosensitizing
agent called porfimer sodium, or Photofrin®, for use in PDT to treat or relieve
the symptoms of esophageal cancer and non-small cell lung cancer. Porfimer sodium
is approved to relieve symptoms of esophageal cancer when the cancer obstructs the
esophagus or when the cancer cannot be satisfactorily treated with laser therapy
alone. Porfimer sodium is used to treat non-small cell lung cancer in patients for
whom the usual treatments are not appropriate, and to relieve symptoms in patients
with non-small cell lung cancer that obstructs the airways. In 2003, the FDA approved
porfimer sodium for the treatment of precancerous lesions in patients with Barrett
esophagus (a condition that can lead to esophageal cancer).
The light needed to activate most photosensitizers cannot pass through more than
about one-third of an inch of tissue (1 centimeter). For this reason, PDT is usually
used to treat tumors on or just under the skin or on the lining of internal organs
or cavities. PDT is also less effective in treating large tumors, because the light
cannot pass far into these tumors. PDT is a local treatment and generally cannot
be used to treat cancer that has spread (metastasized).
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References in this section:
Adapted from the National Cancer Institute's PDQ Database: http://www.cancer.gov/cancertopics/factsheet/Therapy/photodynamic.
(Accessed August 2013)