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External beam radiation therapy
is the common therapy for the treatment of prostate
cancer. External beam radiation has been in use since
the mid 20th century. A carefully focused beam delivers
concentrated levels of radioactive energy to the prostate
cancer. Radiation damages the genetic material of the
cells, so when cells try to divide, they die. Because
cancer cells divide more quickly than healthy cells,
cancerous tissue is more susceptible to damage from
radiation. External beam radiation is non-invasive and
may be appropriate for patients who do not wish to undergo
a more invasive procedure.
External beam radiation can also
be called fractionated radiation, meaning that small
doses are given over a long period of time. Patients
receive radiation therapy once a day, Monday through
Friday over 5-9 weeks, depending on the patient. Normal
prostate cells can repair the damage of a small amount
of radiation fairly quickly. Cancerous cells cannot.
Receiving a small dose everyday helps to minimize the
damage sustained by the healthy cells of the surrounding
organs. Giving patients the weekend off helps their
body to recover enough to withstand the next five days
of treatment.
Photons,
Protons, and Neutrons
There are three times of radiation used in external
beam therapy: photons, protons, and neutrons. Photons
are particles that have no charge and no significant
mass. Protons and neutrons are subatomic particles that
compose the nuclei of atoms.
Photons are x-rays. The x-rays used
to take images of bones are much weaker than the ones
used for external beam radiation therapy for prostate
cancer. The photons are generated in waves by a linear
accelerator. As waves enter the body of the patient,
the photons deposit their energy, increasing in power
exponentially as they pass through tissue. 3D-CRT or
3-Dimensional-conformal Radiation Therapy and IMRT or
Intensity Modulated Radiation Therapy both use photons.
Protons deposit their energy into
tissue only as they begin to slow down. Radiologists
can also calculate how far a proton of a certain energy
can travel before it begins to slow down. A radiologist
therefore can position the beam depth to deposit energy
only in the cancerous tissue.
Protons are rarely used to treat
prostate cancer alone. Usually, weeks of proton therapy
are used with weeks of photon therapy. A patient may
begin with a standard treatment for 5 weeks then switch
to protons aimed specifically at the prostate for the
next 2-3 weeks. Neutrons are not used in therapy very
often. Recent studies suggest that those who have undergone
therapy with neutrons have a greater incidence of side
effects.
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