|
IMRT stands
for intensity modulated beam radiation therapy and uses
the electron beam implemented in both electron beam
radiation therapy (EBRT) and 3 dimensional conformal
radiation therapy (3D-CRT). Currently, there are only
a few centers in the United States who offer IMRT as
a prostate cancer radiation treatment; 3D-CRT is far
more popular. Many doctors predict however, that as
more long term survival rates measuring biochemical
recurrence become available, IMRT will become a more
widely available prostate cancer radiation treatment.
Intensity modulated means that the
strength of the radiation beam is modified according
to the extension of the disease throughout the organ.
Those who are familiar with the prostate biopsy, which
often returns negative results the first time, know
that prostate cancer does not affect the prostate gland
uniformly. IMRT takes the patient’s tumor extension
into account by using stronger doses of radiation on
more affected parts and weaker doses of radiation for
the less affected parts. The goal of Intensity Modulated
Beam Therapy is to deliver higher levels of irradiation
to the cancerous tissue while avoiding the healthy tissue.
The result is more effective prostate cancer treatment
and increased avoidance of severe radiation therapy
side effects.
What makes IMRT a more precise
method of prostate cancer radiation therapy?
Both 3D-CRT and IMRT use the CT scan to take 3-dimensional
pictures rather than the x-rays and fluoroscopy used
in EBRT. In the planning stages of IMRT, the CT scan
will take 30 to 40 pictures of the prostate gland and
the surrounding tissue. The radiation oncologist uses
these images to create a model and outlines the structures
that are to be irradiated. The radiation oncologists
set the dosage for the malignant tissue and dose limits
for the healthy tissue. Using these dosages, the software
programs an optimal mode of beam delivery. As treatment
starts, the software guides the collimator which divides
the beam into small precise slices with accuracy to
the millimeter and millisecond. The collimator itself
has 360 degrees of access around the patient. The mathematical
precision of IMRT allows the highest accuracy available
in beam radiation therapy.
What happens during IMRT?
During the planning stages, the patient undergoes a
CT scan which takes 30 to 40 pictures of his prostate
gland and the neighboring organs. These images are sent
to the planning computer where a radiation oncologist
outlines the treated area. The planning stage of radiation
therapy is the most important because planning allows
more precise delivery of radiation.
The patient may have his whole body
or just his pelvic region placed in a Styrofoam or foam
rubber “cradle” or immobilizing unit. Any
movement, even involuntary ones may skew the trajectory
of the radiation beam. Though movement is restricted,
most patients do not report discomfort. Some patients
say that receiving IMRT is like lying down on a table
and falling asleep or have x-rays taken. The patient
procedure for intensity modulated radiation is similar
to those for EBRT and IMRT;
click to read more on prostate
cancer radiation treatment procedures.
The collimator will rotate completely
around the body. The purpose of the rotation is to achieve
the best angle for treating a certain point on the prostate
gland. The rotation also allows the beam to avoid organs
for which the radiation oncologist has listed certain
radiation tolerances. The oncologist lists radiation
tolerances to give priority for avoidance.
How long does IMRT last?
The actual procedure may not last longer than 15 to
45 minutes. Prostate cancer radiation treatment however
will last 5 to 9 weeks and must be received Monday through
Friday over those weeks. The length and frequency of
this prostate cancer treatment are used to ensure that
the disease has been adequately treated. Unfortunately,
the extended therapy invariably will end up damaging
some healthy tissue which may cause radiation therapy
side effects.
|
