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Dedicated to Clinical & Academic Exchange of Information |
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| Overview of Radiation Medicine |
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What
is Radiation Oncology? How does radiation work to control
cancer?
Radiation Oncology is the branch of medicine
that specializes in the use of high-energy radiation to attack
and kill cancer. Radiation is directed in a carefully controlled
fashion to a specific part of the body, with the goal of
destroying cancer cells without significant injury to the
surrounding normal tissue. Radiation works by damaging the
genetic material (DNA) of the tumor cell so that it loses
the ability to reproduce itself; upon attempted division,
the cell dies. Radiation is similar to surgery in that it
is a form of local therapy that only treats a specific
area of the body. In contrast, chemotherapy and hormonal
therapy, which by design spread throughout the entire body,
are referred to as systemic therapy.
How
is radiation treatment delivered?
The workhorse of the modern Radiation Oncology
department is the linear accelerator (linac), a
large X-ray machine that bears some resemblance to the devices
that produce chest and dental X-rays. The difference is that
the linac generates X-rays that are much higher in energy
than the more commonly known diagnostic X-rays. When the
linac is turned on, a well-defined beam of X-rays flows out
of the machine and passes through the targeted part of the
body. The instant the linac is turned off, there is no more
radiation in the machine, in the room, or in the patient.
The linac machine is not radioactive and the patient does
not become radioactive from treatment.  The process described above is called external
irradiation. Additionally, treatment can be administered
in the form of small radioactive sources that are actually
implanted inside a part of the body near the tumor. This
latter form of internal irradiation, known as brachytherapy,
is particularly important in the treatment of gynecologic
and prostate cancers. Brachytherapy also is increasingly
being used for the treatment of a wide variety of malignancies,
either as the sole form of radiation or in conjunction
with external irradiation. Regardless of how the radiation
is delivered (by external therapy, brachytherapy, or a
combination of the two), the goal of treatment is to kill
cancer cells and spare non-cancerous (normal) cells. Radiation
is sometimes used as the only treatment for cancer, but
most frequently, it is used as a supplement to open surgery.
In such cases, surgical removal of the tumor is the primary
therapy, and radiation therapy is used pre-operatively,
or more commonly post-operatively, to "mop up" any microscopic
cancer cells that escape "the surgeon's knife". Its specific
purpose is to improve the likelihood of local control of
the cancer, and ultimately, to increase survival. When
radiation is used to supplement surgery it is referred
to as adjuvant therapy.
What
are possible side effects from radiation treatment?
Patients often become nervous at the idea
of being treated with radiation. Typical concerns include
nausea and vomiting, diarrhea and hair loss (often patients
do not recognize the distinction between chemotherapy and
radiation side effects), worries about being “burned”,
fears that radiation can actually cause cancers, and numerous
other less common fears. These concerns sometimes may contain
a grain of truth, but more often are exaggerations or misinterpretations
of the facts. In the vast majority of cases, treating cancers
with radiation involves simply using X-rays. To be sure,
these X-rays are much more powerful than the diagnostic (“regular”)
X-rays used for a chest film or a mammogram, but the experience
need not be too different. The key difference is that because
of their high energy, the X-rays used to treat cancer are
able to penetrate deep inside the body and deposit their
energy within the target tissues.Radiation is a purely local form of treatment,
with the radiation being directed only to a particular part
of the body. By the manner in which it works, a radiation
beam does not distinguish between a cancer cell and a normal
cell. All tissues in the targeted field will receive the
same amount of radiation from a given beam. Radiation side
effects arise from damage, which is often transient (temporary),
to normal functioning tissue in the treatment field. Potential
complications depend on the specific location of the body
that is being irradiated. Thus, unlike the case with systemic
chemotherapy, radiation can potentially cause hair loss only
if the radiation beam is directed to the head, it can cause
nausea and vomiting only if the abdomen is treated, and it
can cause diarrhea only if the pelvis is treated.
Can
radiation treatment cause cancer?
Many years ago, before the use of
antibiotics, tuberculosis was routinely treated by puncturing
and deflating the lungs; because the bacteria require air
to survive, removing their oxygen supply kills them. This
required multiple daily fluoroscopic examinations, which
emit low dose radiation. The procedure was highly effective,
but unfortunately, females who underwent the procedure
in their teens and 20's experienced a greatly increased
risk of breast cancer. It was later determined that repetitive
exposure of young, and especially developing, breast tissue
to low dose radiation was the culprit. Along the same lines,
people who were close enough downwind from the nuclear
bomb blasts in Japan and the power plant disaster in Chernobyl
to receive significant doses of radiation have had a markedly
increased risk of many malignancies. However, the relatively
high doses of radiation used in Radiation Oncology translate
into only a minimally increased risk of cancer. Why is
there a difference?Simply stated, repetitive low dose radiation
may cause mutations in cells that are perpetuated - the cells
may be damaged, but not lethally, and when they divide, their
mutations are passed on. In contrast, repetitive high dose
irradiation causes lethal damage. The cells die when they
attempt to divide and cannot pass on any mutations to a new
generation of cells. Practically speaking, there is little
risk that a course of high dose irradiation will induce cancer,
certainly far less than 1%, and the potential benefits far
outweigh any long-term risks.
Will
radiation cause damage to normal tissue too?
The basic dilemma and challenge for the
radiation oncologist is the fact that cancer never exists
in a vacuum and is by definition always surrounded by normal
tissue. Obviously, if radiation only damaged tumor cells,
it would be simple to cure any cancer simply by giving a
sufficiently high dose of radiation. But this is never the
case, and instead radiation oncologists are constantly faced
with the trade-off of inflicting the maximal damage to the
tumor while keeping the risk of damage to normal tissue to
an absolute minimum. The art and science of Radiation Oncology
is to figure out ways to achieve local control at lower doses,
so that the chance of cure is maximized, while the risk of
complications is minimized. Radiation Oncology is really
in many ways the branch of medicine that deals with the irradiation
of normal tissue. While the objective is always to kill off
the cancer, a radiation oncologist must always be concerned
with minimizing risk to nearby normal tissue.
How
is risk of radiation damage to normal tissue minimized?
There are several strategies that
radiation oncologists routinely utilize to keep the risk
of damage to normal tissue as low as possible. These strategies
include.
- Use of high energy X-rays
- Precise delivery of radiation
- Use of multiple treatment
fields
- Fractionation
Use of high energy X-rays
The first important means of sparing normal tissue, which
was already touched upon earlier, is the use of high energy
X-rays. Whereas diagnostic X-rays lose most of their energy
at the skin surface, the linac-generated high energy X-rays
are able to penetrate deep within the body before depositing
their energy. Most modern radiation oncology departments
have several technologies with different X-ray energies
at their disposal. The higher the energy used, the deeper
the penetration into the tissue. Because the X-rays do
not lose most of their energy at the surface of the body,
the skin is spared. Obviously the surface does receive
some radiation, but unless the skin is specifically targeted
for treatment, radiation "skin burns" are now very uncommon.
Precise delivery
of radiation
A second key concept of modern Radiation Oncology is one
of precision. The region to be irradiated, termed the tumor
volume, should be as well-defined as possible such that it
receives the full brunt of the treatment, while as little
normal tissue as is feasible is irradiated. This may sound
rather obvious, but in practice the ideal balance is very
difficult to achieve. Unless the tumor volume is on or just
below the skin surface, rarely is any abnormality seen or
felt, particularly if the cancer has already been removed
and treatment is only for potential microscopic residual
disease. Instead, radiation oncologists must take advantage
of all available means to define the radiation site. This
involves looking at X-rays, CT scans and the like, studying
the surgeon's operative note and the pathologist's report
for details, and even going to the operating room to personally
visualize the tumor while it is still in place. Knowledge
of internal anatomy is also critical, not only so that normal,
non-cancerous structures can be spared, but also so that
seemingly normal tissues that are at risk for microscopic
tumor involvement will not be mistakenly left untreated. Precision also comes into play with
regard to the reproducibility of the daily treatments. Many
courses of therapy require 30 to 40 or more treatments, and
the radiation must be precisely delivered to the same target
site - within 1/2 to 1 centimeter daily variation - or else
there is a risk of under-dosing the tumor and/or overdosing
the surrounding normal tissue. Elaborate means are used to
ensure that the patient is lying in nearly the same position
every day. Use of multiple
treatment fields
A third major principle is the use of multiple fields. Except
in very rare circumstances, radiation oncologists treat from
at least two different directions (e.g. front-to-back and
side-to-side). For locations deep in the body, multiple angles
are routinely used. The idea here is that only the tumor
volume receives radiation from all the angles, and most of
the surrounding normal tissues are excluded from one or more
of the X-ray beams. The benefit is that normal regions receive
only a portion of the total dose.
Fractionation
Perhaps the most important basic principle of modern Radiation
Oncology is that of fractionation. Since the radiation
beam cannot distinguish between tumor and normal tissue,
radiation oncologists must take advantage of any inherent
differences between the two. Fortunately most types of
cancer cells are irreversibly damaged and killed by radiation
at much lower doses than are normal cells. In other words,
cancer cells tend to be more sensitive and normal tissues
more resistant to radiation. Therefore, a dose of radiation
is chosen for each individual treatment (or fraction) that
is high enough to cause some damage to the cancer, but
sufficiently low that most, if not all, of the normal tissue
will be spared permanent damage. By delivering multiple
such “fractions” in rapid succession (usually
daily), it is possible to administer a total dose of radiation
that is adequate to kill off the tumor, yet only minimally
damaging to the surrounding normal tissue.
How
are newer radiation technologies different?
Recently publicized innovations in
radiation therapy - "Conformal" or "3D" treatment and "Intensity
Modulated Radiation Therapy" (IMRT) - are all computer-controlled
technological variations on the multiple field technique.
They allow shaping of the radiation beam to better conform
to the shape of the tumor, thereby providing greater sparing
of normal tissue. Importantly, because of these new technologies
it is now possible to administer larger doses of radiation
than were ever previously possible.Stereotactic Radiosurgery (SRS) is potentially
one of the most important and exciting areas of all the recent
innovations in radiation treatment. Stereotactic radiosurgery
uses multiple precise and pencil-thin radiation beams to
literally ablate and eradicate a tumor, which most typically,
cannot be removed with conventional forms of surgery. Finally
radiosurgery provides physicians with a tool that enables
the ultimate goal of therapuetic irradiation--to perform "surgery
without the scalpel". The basic principle of radiosurgery has
actually been around for over 40 years in the guise of the
Gamma Knife, first conceived by neurosurgeon Dr. Lars Leksell
in Sweden. The Gamma Knife focuses up to 201 precise and
narrow beams of gamma rays (similar to x-rays) on a target
in the brain, and delivers massive doses of radiation while
largely sparing the surrounding normal brain tissue. The
results have been spectacular such that for many brain tumors--both
benign and malignant--where surgery is considered too risky,
SRS is now the treatment of choice.But the Gamma Knife and its derivatives,
which use a specially modified large linear accelerator,
still have major drawbacks. Most of them can be used only
for treating lesions in the brain, whereas tumors can arise
anywhere within the body. In contrast, the unique CyberKnife
system, which consists of a miniature linear accelerator
mounted on a flexible robotic arm, is capable of delivering
precise stereotactic radiosurgery to almost any area of the
body. The CyberKnife relies on image-guided tracking of bony
skull landmarks or small implanted markers (fiducials) near
the tumor for targeting the radiation. In contrast, the Gamma
Knife and other conventional radiosurgery technologies rely
on an external metal head frame to target the radiation and
hold the patient completely still during treatment. The CyberKnife
does not require the head frame, which limits treatment to
lesions in the head only, and is thus capable of performing
radiosurgery on lesions anywhere in the body. To learn more
about the CyberKnife and how it works, see CyberKnife
Overview
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