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There are three types of common
radiotherapies used as prostate cancer treatments: the
conventional electron beam radiation therapy (EBRT),
and the newer 3-dimensional conformal radiation therapy
(3D-CRT) and intensity modulated radiation (IMRT). Electron
beam radiation therapy was the first external radiation
therapy used as a prostate cancer treatment. The type
of particle used for these three therapies is called
photons which have no charge and no significant mass.
Photons are the same type of energy used in diagnostic
x-rays.
3D-CRT is an advanced form of EBRT,
and IMRT is an advanced form of 3D-CRT. The way radiation
oncologists prepare their patients for the procedure
therefore is similar. Patients receiving external radiation
therapy usually receive treatments Monday through Fridays
for 5 to 9 weeks depending on the doctor’s prescription.
Radiotherapy usually begins with a CT (computer tomography)
scan of the prostate gland and the surrounding organs.
The CT scan takes a 3D-picture of the prostate gland
and the surrounding organs and a team of radiation physicists
and dosimetrists plan a target area for the radiation
beam. The target area conforms to the shape of the patient’s
prostate gland and, to ensure correct positioning, the
patient will have his target area marked with permanent
ink. The team will further use alignment lasers to direct
the radiation beam. The patient, who is lying on his
back, receives therapy from the linear accelerator which
moves up, down, and around to hit the target area from
different angles.
3-Dimensional
Conformal Radiation Therapy
3-dimensional conformal radiation therapy introduces
the use of a body immobilization device. Since positioning
is everything for the radiation oncologist, an immobile
patient’s target area is much easier to hit than
the target area of a patient who might sneeze or inadvertently
change positions. The CT scan is performed on a computer
with specialized software that takes detailed cross-sectional
pictures of the patient’s internal anatomy. The
linear accelerator is then aimed precisely at each of
the targeted areas in these pictures. The prostate gland
therefore can receive a much stronger dose of radiation
while the surrounding organs receive much less.
Intensity Modulated Radiation
Beam Therapy
Intensity modulated radiation therapy takes 3D-CRT a
step further. The intensity of the beam is modified
so that much smaller beams of radiation are delivered
from multiple angles at smaller intervals. A patient
receives more beams of radiation, but since these beams
are thinner, they are capable of hitting the target
more precisely while not hitting the healthy tissue.
IMRT is currently not as common in the United States
as 3D-CRT, but is gaining popularity as a prostate cancer
treatment.
Proton Beam Radiation Therapy
The technique of conformal proton beam radiation therapy
is similar to that of 3D-conformal radiation therapy.
The proton is a subatomic particle, not a wave like
the photon. Protons deposit their energy into tissue
only as they begin to slow down after being produced
by the linear accelerator. A radiation oncologist can
therefore determine to which speed to accelerate the
protons resulting in maximum amount of energy being
deposited into the prostate tumor. The proton can be
aimed with extreme precision and is often used for delicate
surgeries such as those involving the eye or spine.
Conformal proton beam radiation therapy is the most
expensive type of radiotherapy, and is usually used
in conjunction with another type of EBRT.
How Radiotherapy Destroys
Prostate Cancer
Patients who opt for radiotherapy as their prostate
cancer will go to 5 times a week for 5 to 9 weeks depending
on the doctor’s prescription and the extent of
the disease. Radiotherapy can be inconvenient, but exposing
the cancerous cells to lowered doses of radiation damages
their DNA by creating free radicals. Normal cells are
able to quickly repair this damage before dividing but
cancerous cells cannot. Since prostate cancer grows
slowly, many weeks of therapy are necessary to continually
damage the DNA that a cancerous cell may otherwise have
time to repair.
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