The information which follows is the opinion of the named author(s).
It does not necessarily constitute the opinion of The Prostate Cancer InfoLink or of
CoMed Communications, Inc.
Three-Dimensional Conformal Radiation Therapy (3DCRT) for Prostate Cancer
Part 1 of 2
[divided only to help load time - go to Part 2]
Jeff M. Michalski, MD,
Carlos A. Perez, MD, and James A. Purdy, PhD
Radiation Oncology Center, Mallinckrodt Institute of Radiology,
Washington University Medical Center, St. Louis, Missouri
Originally Received July 6, 1996; Last Revised July 8, 1996
Introduction |
What is 3DCRT? |
How is 3DCRT planned?
The following are found in Part 2
How is 3DRCT delivered? |
What are the side effects of radiation therapy? |
Is 3DCRT better than conventional radiation therapy? |
What studies are currently ongoing investigating 3DCRT for prostate cancer? |
What will the future hold for 3DCRT and the treatment of prostate cancer? |
References |
Editorial comment
Introduction
This article is arranged as a mini-"FAQ" -- a short list of
answers to frequently asked questions for patients with prostate cancer
and their families. It certainly is not an exhaustive
scientific literature review. Comments and suggestions are
welcome, and can be sent to the authors by using the e-mail address links above.
Radiation therapy has been used to treat prostate cancers
for decades. It plays an important role in the curative
management of patients with early stage disease and also for
the palliation of symptoms from advanced or metastatic
disease.
Technical developments have made incremental improvements
in the delivery of high dose radiation therapy for the
curative treatment of prostate cancer. In the 1970s,
medical linear accelerators were designed and developed that
could deliver high doses of radiation therapy to deep parts
of the anatomy, while sparing the superficial tissues.
Despite the high energy X-ray beams that spare superficial
normal tissues, organs adjacent to the cancer often received
large volumes of high dose radiation therapy. This large
treatment volume was required to assure that the cancer was
always treated. If too small radiation fields were used to
avoid treating the normal tissues, there was a risk that the
radiation would not adequately treat the cancer [1, 2]. Many
radiation oncologists, and their patients, would accept the
extra radiation dose to the normal adjacent organs in order
to assure adequate coverage of the prostate target volume.
This "extra" radiation is partly responsible for the side
effects and complications that accompany high dose curative
radiation therapy. In order to keep the complication rates
at an acceptable level, most physicians would keep the total
radiation dose to a moderate and safe range. There is
evidence that higher radiation doses may cure more prostate
cancers but this would be at too great a risk with
conventionally fractionated and planned radiation therapy
[3, 4].
Figure 1. A two-dimensional pelvic
X-ray demonstrates the bony
anatomy but not pelvic organs.
Over the past decade there has been a significant
increase in the availability of new and powerful computers
that are making better targeting of radiation therapy
possible. In the past, physicians had to rely on low
contrast, two-dimensional X-ray images to target radiation
therapy for the treatment of prostate and other cancers.
These X-rays films illustrated the bony pelvis but neither
the prostate nor normal tissues. Now cross-sectional imaging
modalities such as computed tomography (CT) or even magnetic
resonance imaging (MRI) allow clinicians to see inside a
patient's body. Using these high quality imaging techniques and
powerful image workstation computers, doctors are now able to
visualize radiation therapy targets and direct radiation
therapy beams directly at the tumor (and tissues at risk of having
cancer) while sparing adjacent normal critical structures.
Figure 2. The pelvic CT scan can demonstrate human anatomy in
cross section. The organs of the pelvis can be readily
identified on these CT slices.
What is 3D conformal radiation therapy?
The goal of three-dimensional conformal radiation therapy (3DCRT)
is to have the
prescribed radiation dose distribution be shaped like or "conform" to
a target volume. In the past, radiation therapy plans were
reviewed in a single plane. The distribution of radiation
dose above and below the plane was assumed to be adequate.
This assumption was valid if there was not significant change
in the shape or contour of an organ or tumor outside of that
single plane. Unfortunately, human beings are not regularly
configured for simple radiation therapy techniques. Three-dimensional
conformal radiation therapy allows a three-dimensional and volumetric
appreciation of a target volume
and normal tissues that are not dependent on arbitrary and
regular geometric shapes.
A volumetric CT scan acquired with the patient in the
position identical to the one he will be in for his radiation therapy
treatments is used for 3DCRT
planning. On each slice of a CT scan, normal and target
volume contours are defined by a radiation oncologist or a
medical dosimetry assistant. These contours are then
reconstructed and displayed (rendered in 3D) on a computer
video monitor. This 3D visualization
allows the radiation oncologist to choose various radiation
therapy beam arrangements to maximally cover the tumor with
radiation and minimize exposure to the normal structures.
Once the beams are selected, the fields are shaped to conform
to the shape of the target volume in that projection. This
shaping can be done by the physician using computer drawing
tools, like a computer mouse, or by automatic generation of a
block contour by the computer. This process is repeated
until a volumetric radiation dose distribution is conformed
to the shape of the target volume. The dose distribution is
then analyzed using graphical tools, such as dose volume
histograms (DVHs), or 3D dose displays. DVHs are analytical
tools that depict the volume of each organ or target tissue
and the dose that it receives. They help the radiation
oncologist know if the target volume is adequately covered
and that the volume of normal tissues are not receiving
excessive radiation doses. 3D dose displays
allow a visual appreciation of how the radiation doses are
spatially distributed.
The process of implementing 3DCRT is slightly different
from standard radiation therapy. Patient positioning is
critical, and for this reason immobilization devices are
routinely used. These devices are aids to the radiation
therapist for repositioning the patient on a daily basis for
seven or more weeks of outpatient therapy. 3DCRT often employs
multiple treatment fields -- as many as six or eight. Treating this
many uniquely shaped fields is done most efficiently and
accurately with a beam shaping device called a multi-leaf
collimator (MLC). An MLC is made up of many small leaves (1
cm projection at the radiation therapy linear accelerator
isocenter) that are driven by independent motors to create
the desired shape of the radiation field. These devices
avoid the use of the heavy lead alloy blocks that were used in
the past. (Note: These blocks are still used today in most
radiation therapy clinics and even in some clinics using 3DCRT. However, the
use of lead alloy blocks for 3DCRT requires increased time
with less efficiency in treatment delivery.)
How is 3D conformal radiation therapy planned?
After your radiation oncologist decides that it is
appropriate for you to be treated with 3DCRT, he or she will
schedule a CT scan for 3DCRT planning.
Step 1: Immobilization
As described above, patient positioning is a critical
part of active 3DCRT. A device is made that
will allow the radiation therapist to reposition you
accurately on a day to day basis. These devices come in
a variety of different forms and may be made of a foam that
hardens as the result of a chemical reaction or from a
thermally sensitive plastic that hardens as it cools to room
temperature. Once the device is made, a treatment planning CT
scan is acquired with the patient in his immobilization
device.
Figure 3. A patient with prostate cancer lying in an
immobilization device known as an alpha cradleTM. This
device helps improve the accuracy with which radiation
therapy treatments can be delivered.
Step 2: CT acquisition
Although physically similar to a diagnostic CT scan, the
treatment planning CT is acquired with the patient in his treatment
position. Sometimes the use of contrast media is necessary to help
visualize
the prostate. The CT scan is acquired with "slices" taken at
2-5 mm intervals. After the scan is acquired, the patient's skin and
immobilization cast may be marked with ink to assist in
subsequent treatment setup.
Figure 4. Patient is undergoing a CT scan for treatment
planning.
Figure 5. A large number of CT slices are acquired for 3DCRT.
This data will be used to
develop the radiation therapy treatment plan.
Step 3: Target volume and normal tissue definition
This is a critical and time-consuming task for the
radiation oncologist. He or she will identify normal and
tumor-containing tissues on relevant slices of your treatment
planning CT scan. A medical dosimetrist will often assist
the doctor in defining some of the more obvious tissues, such
as the skin and bones.
After the doctor defines the gross tumor (usually the
whole prostate because of the risk of multi-focal disease)
he/she will sometime add a safety margin to account for
microscopic spread of disease. We know from surgical data
that cancer cells can spread outside the prostate or to the
seminal vesicles even when the tumor is felt to be confined
to the gland. The safety margin for cancer spread (called
the clinical target volume or CTV) helps the radiation
oncologist be sure that the areas where cancer cells may have
spread will not be underdosed. In some cases the seminal
vesicles will not be treated if the calculated risk of cancer
spread to this tissue is very low. If there is a high risk
of extracapsular disease or seminal vesicle invasion because
of a high PSA or Gleason score (or because there is definite
extension of disease into the seminal vesicle), the seminal
vesicles will be included in the clinical target volume.
Figure 6. On each slice of the treatment planning CT scan,
the physician and his assistant, a medical dosimetrist,
will identify the prostate and seminal vesicles as well
as the normal tissues at risk of radiation injury,
including the bladder, rectum, and femoral heads.
Another margin needs to be added to the clinical target
volume to account for setup variation and internal organ
motion. Because patients are living and breathing human
beings, we cannot control their movements down to the exact
millimeter. We don't want to risk being "too conformal" and
miss treating the prostate gland or any cancer within it.
The position of the prostate and seminal vesicles changes
daily with the degree of bladder and rectum filling.
Likewise, even with assistance of an immobilization device,
there is still some variation with daily treatment setup. A
margin of 5-10 mm is generally added to the CTV to
accommodate for the variation in positioning of the target
volume and patient. After the target volumes and
normal tissues are defined and contoured on the CT scan, the
next step is virtual simulation.
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