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Principles of radiation therapy

Principles of radiation therapy

This chapter provides a brief overview of the principles of radiation therapy.
The topics to be discussed include the physical aspects of how radiation works
(ionization, radiation interactions) and how it is delivered (treatment machines,
treatment planning, and brachytherapy). Recent relevant techniques of radiation
oncology, such as conformal and stereotactic radiation therapy, also will
be presented. These topics are not covered in great technical detail, and no
attempt is made to discuss the radiobiological effects of radiation therapy. It is
hoped that a basic understanding of radiation treatment will benefit those practicing
in other disciplines of cancer management. This chapter does not address
principles of radiobiology, which guide radiation oncologists in determining
issues of treatment time, dose, and fractionation or in combining radiation
with sensitizers, protectors, and chemotherapy or hormones.

How radiation worksIONIZING RADIATION
Ionizing radiation is energy sufficiently strong to remove an orbital electron
from an atom. This radiation can have an electromagnetic form, such as a highenergy
photon, or a particulate form, such as an electron, proton, neutron, or
alpha particle.

High-energy photons By far, the most common form of radiation used in practice
today is the high-energy photon. Photons that are released from the nucleus
of a radioactive atom are known as gamma rays. When photons are created electronically,
such as in a clinical linear accelerator, they are known as x-rays. Thus,
the only difference between the two terms is the origin of the photon.

Inverse square law The intensity of an x-ray beam is governed by the inverse
square law. This law states that the radiation intensity from a point source is
inversely proportional to the square of the distance away from the radiation
source. In other words, the dose at 2 cm will be one-fourth of the dose at 1 cm.

Electron volt Photon absorption in human tissue is determined by the
energy of the radiation, as well as the atomic structure of the tissue in
question. The basic unit of energy used in radiation oncology is the electron
volt (eV); 103 eV = 1 keV, 106 eV = 1 MeV.

PHOTON-TISSUE INTERACTIONS
Three interactions describe photon absorption in tissue: the photoelectric effect,
Compton effect, and pair production.

Photoelectric effect In this process, an incoming photon undergoes a collision
with a tightly bound electron. The photon transfers practically all of its
energy to the electron and ceases to exist. The electron departs with most of the
energy from the photon and begins to ionize surrounding molecules. This interaction
depends on the energy of the incoming photon, as well as the atomic
number of the tissue; the lower the energy and the higher the atomic number,
the more likely that a photoelectric effect will take place.

An example of this interaction in practice can be seen on a diagnostic x-ray
film. Since the atomic number of bone is 60% higher than that of soft tissue,
bone is seen with much more contrast and detail than is soft tissue. The energy
range in which the photoelectric effect predominates in tissue is about 10-25 keV.

Compton effect The Compton effect is the most important photon-tissue interaction
for the treatment of cancer. In this case, a photon collides with a "free
electron," ie, one that is not tightly bound to the atom. Unlike the photoelectric
effect, in the Compton interaction both the photon and electron are scattered.
The photon can then continue to undergo additional interactions, albeit
with a lower energy. The electron begins to ionize with the energy given
to it by the photon.

The probability of a Compton interaction is inversely proportional to the energy
of the incoming photon and is independent of the atomic number of the
material. When one takes an image of tissue using photons in the energy range
in which the Compton effect dominates (~25 keV-25 MeV), bone and softtissue
interfaces are barely distinguishable. This is a result of the atomic number
independence.

The Compton effect is the most common interaction occurring clinically, as
most radiation treatments are performed at energy levels of about 6-20 MeV.
Port films are films taken with such high-energy photons on the treatment machine
and are used to check the precision and accuracy of the beam; because
they do not distinguish tissue densities well, however, they are not equal to
diagnostic films in terms of resolution.

Pair production In this process, a photon interacts with the nucleus of an
atom, not an orbital electron. The photon gives up its energy to the nucleus
and, in the process, creates a pair of positively and negatively charged electrons.
The positive electron (positron) ionizes until it combines with a free electron.
This generates two photons that scatter in opposite directions.

The probability of pair production is proportional to the logarithm of the energy
of the incoming photon and is dependent on the atomic number of the
material. The energy range in which pair production dominates is ≥ 25 MeV.
This interaction occurs to some extent in routine radiation treatment with highenergy
photon beams.

ELECTRON BEAMS
With the advent of high-energy linear accelerators, electrons have become a
viable option in treating superficial tumors up to a depth of about 5 cm. Electron
depth dose characteristics are unique in that they produce a high skin dose
but exhibit a falloff after only a few centimeters.

Electron absorption in human tissue is greatly influenced by the presence of air
cavities and bone. The dose is increased when the electron beam passes through
an air space and is reduced when the beam passes through bone.

Common uses The most common clinical uses of electron beams include the
treatment of skin lesions, such as basal cell carcinomas, and boosting of areas
that have previously received photon irradiation, such as the postoperative
lumpectomy or mastectomy scar in breast cancer patients, as well as select
nodal areas in the head and neck.

MEASURING RADIATION ABSORPTION
The dose of radiation absorbed correlates directly with the energy of the beam.
An accurate measurement of absorbed dose is critical in radiation treatment.
The deposition of energy in tissues results in damage to DNA and diminishes
or eradicates the cell's ability to replicate indefinitely.

Gray The basic unit of radiation absorbed dose is the amount of energy (joules)
absorbed per unit mass (kg). This unit, known as the gray (Gy), has replaced
the unit of rad used in the past (100 rads = 1 Gy; 1 rad = 1 cGy).

Exposure To measure dose in a patient, one must first measure the ionization
produced in air by a beam of radiation. This quantity is known as exposure.
One can then correct for the presence of soft tissue in the air and calculate the
absorbed dose in Gy.

Percentage depth dose The dose absorbed by tissues due to these interactions
can be measured and plotted to form a percentage depth dose curve.
As energy increases, the penetrative ability of the beam increases and the skin
dose decreases.

How radiation is deliveredTREATMENT MACHINESLinear accelerators
High-energy radiation is delivered to tumors by means of a linear accelerator.
A beam of electrons is generated and accelerated through a waveguide that
increases their energy to the keV to MeV range. These electrons strike a tungsten
target and produce x-rays.

X-rays generated in the 10-30-keV range are known as grenz rays, whereas the
energy range for superficial units is about 30-125 keV. Orthovoltage units generate
x-rays from 125-500 keV.

Orthovoltage units continue to be used today to treat superficial lesions; in
fact, they were practically the only machines treating skin lesions before the
recent emergence of electron therapy. The maximum dose from any of these
low-energy units is found on the surface of patients; thus, skin becomes the
dose-limiting structure when treating patients at these energies. The depth at
which the dose is 50% of the maximum is about 7 cm. Table 1 lists the physical
characteristics of several relevant x-ray energies.

Megavoltage units The megavoltage linear accelerator has been the standard
radiotherapy equipment for the past 20-30 years. Its production of x-rays is
identical to that of lower-energy machines. However, the energy range of
megavoltage units is quite broad-from 4 to 20 MeV. The depth of the maximum
dose in this energy range is 1.5-3.5 cm. The dose to the skin is about 30%-
40% of the maximum dose.

Most megavoltage units today also have electron-beam capabilities, usually in
the energy range of about 5-20 MeV. To produce an electron beam, the tungsten
target is moved away from the path of the beam. The original electron
beam that was aimed at the tungsten target is now the electron beam used for
treatment. Unlike that of photons, the electron skin dose is quite high, about
80%-95% of the maximum dose. A rule of thumb regarding the depth of penetration
of electrons is that 80% of the dose is delivered at a depth (in cm)
corresponding to one-third of the electron energy (in MeV). Thus, a 12-MeV
beam will deliver 80% of the dose at a depth of 4 cm.

Altering beam intensity and field size When measurements are made at the
point just past the target, the beam is more intense in the center than at the
edges. Optimal treatment planning is obtained with a relatively constant intensity
across the width of the beam. This process is accomplished by placing a
flattening filter below the target.

For the radiation beam to conform to a certain size, high atomic number collimators
are installed in the machine. They can vary the field size from 4 4 cm
to 40 40 cm at a distance of 100 cm from the target, which is the distance at
which most treatments are performed.

If it is decided that a beam should be more intense on one side than the other,
high atomic number filters, known as wedges, are placed in the beam. These
filters can shift the dose distribution surrounding the tumor by 15o-60o. Wedges
can also be used to optimize the dose distribution if the treatment surface is
curved or irregular.

Shielding normal tissue Once the collimators have been opened to the desired
field size that encompasses the tumor, the physician may decide to block
out some normal tissue that remains in the treatment field. This is accomplished
by placing blocks (or alloy), constructed of a combination of bismuth, tin, cadmium,
and lead, in the path of the beam. In this way, normal tissues are shielded,
and the dose can be delivered to the tumor at a higher level than if the normal
structures were in the field. These individually constructed blocks are used in
both x-ray and electron treatments. A more modern technique involves multileaf
collimators mounted inside the gantry. They provide computerized, customized
blocking instead of having to construct a new block for each field. (See
section on "Intensity-modulated radiation therapy.")

PRETREATMENT PROCEDURES
Certain imaging procedures must be performed before radiation therapy is
begun.

Pretreatment CT Before any treatment planning can begin, a pretreatment
CT scan is often performed. This scan allows the radiation oncologist to identify
both tumor and surrounding normal structures.

Simulation The patient is then sent for a simulation. The patient is placed on a
diagnostic x-ray unit that geometrically simulates an actual treatment machine.
With use of the CT information, the patient's treatment position is simulated
by means of fluoroscopy. A series of orthogonal films are taken, and block
templates that will shield any normal structures are drawn on the films. These
films are sent to the mold room, where technicians construct the blocks to be
used for treatment. CT simulation is a modern alternative to "conventional" simulation and is described later in this chapter.

Guides for treatment field placement Small skin marks, or tattoos, are
placed on the patient following proper positioning in simulation. These
tattoos will guide the placement of treatment fields and give the physician
a permanent record of past fields should the patient need additional treatment
in the future.

It is imperative that the patient be treated in a reproducible manner each day.
To facilitate this, Styrofoam casts that conform to the patient's contour and
place the patient in the same position for each treatment are constructed. Lasers
also help line up the patient during treatment.

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