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. Therefore, the
way radiation oncologists prepare their patients for
the procedure is similar. Patients receiving external
radiation therapy usually receive treatments Monday
through Friday 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
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 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
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
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
Patients who opt for radiotherapy as their prostate
cancer treatment will go 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.