Abscopal Effect A measurable response of tumor tissue
definitely separate from the area treated; found in chronic leukemias or in
lymphomas where irradiation of an enlarged spleen or lymph nodes causes a
generalized disease remission in the former and relief of obstructive symptoms
in the latter.
Absorbed Dose A measure of the amount of energy absorbed
from a radiation beam, by a medium in the path of the beam, described in units
of rads. One rad is equivalent to 100 ergs of energy deposited per gram of
Absorption of Radiation Any material placed in the path of
radiation beam will absorb some of the radiation, that is, some of the energy of
that beam. The amount of absorption will depend upon the type of radiation, the
density of the absorbing material, and the atomic number of the absorbing
material. This may result in ionization, heating, production of scattered
radiation, and rearrangement of atomic bonds.
Accelerator (Particle) A device that imparts kinetic energy
to charged particles such as electrons, protons, deuterons, and helium ions.
These particles may be used in medical irradiation either directly or indirectly
(via the production of x-rays or neutrons). See Linear Accelerator, Van de
Graaff Generator, Betatron, and Cyclotron.
Afterloading Techniques In these techniques, hollow
applicators (designed to carry a radioactive material) are implanted or inserted
into the volume to be treated. These are checked by various methods (x-ray films
or image intensifier) before the radioactive material is inserted, a step that
is usually done when the patient has been transferred back to his room. Its main
advantage is that it allows the operation to be carried out with little exposure
of personnel to radiation. It also limits the radiation exposure to specified
personnel who are closely monitored. The most commonly used afterloading
technique is that used in the treatment of carcinoma of the cervix. See
Brachytherapy and Internal Sources of Radiation.
Arc Therapy External beam teletherapy in which the source
of radiation is moved about the patient on an arc during treatment. Multiple
areas may be used. In some cases, the beam is stationary and the patient rotated
in a vertical plane.
Backpointer An attachment to a therapy treatment machine
used in conjunction with a frontpointer or front light beam to align the
direction of the radiation beam. It consists of a curved arm, arcing over the
patient with a pointer lying along the central axis, pointing to the exit point
of the radiation beam.
Backscatter Some of the radiation after entering the tissue
will scatter back toward the surface. This portion of the radiation is called
backscatter. Therefore, the surface or skin dose may be defined as the air dose
plus the backscatter dose.
Beam Quality The spectral-energy distribution of the
radiation beam. Beam quality determines both the penetration of the beam through
tissue and the relative absorption of the energy in different types of tissue.
Beam Shaping The use of special blocks, wedges,
compensators, and other devices to create a treatment beam of the geometric
proportions required for a treatment plan that is beyond the capabilities of the
Beta Plaque A flat plate or curved surface that is coated
with a radioactive material and emits electrons. It is placed on the surface of
the area to be treated. The depth of penetration is very small, in the order of
millimeters. A common isotope used is strontium-90. An example of a beta plaque
is an ophthalmological applicator, curved to conform to the surface of the eye.
Betatron A megavoltage treatment machine capable of
delivering high energy x-rays and, in some instances, an electron beam. The
electrons are accelerated in a circular orbit.
Bolus A material of density nearly equivalent to tissue
placed within the treatment beam to compensate for unevenness of body contour or
to enhance the buildup of electrons on the surface of the skin.
Brachytherapy The administration of radiation therapy by
applying a radioactive material inside or in close approximation to the patient.
This material may be contained in various types of apparatus, may be on the
surface of plaques, or may be enclosed in tubes, needles, wire, seeds, or other
small containers. Common materials for the administration of brachytherapy are
radium, cobalt-60, cesium-137, iodine-125, and iridium-192. Brachytherapy is
sometimes called plesiotherapy.
Brachytherapy-Intracavitary Application The radioactive
material, usually in the form of sealed sources, is placed into various types of
applicators, which are then (or have been previously) inserted directly into a
cavity in the patient through natural or surgically produced apertures. See
Internal Sources of Radiation.
Brachytherapy-Surface Application The radioactive material
is usually contained on the surface of a plaque or mold, which is applied
directly or close to the surface of the patient.
Bragg Peak The region near the end of a charged particle
track in matter, in which the rate of energy loss is maximum.
Breast Bridge A jig used to set up tangential fields, in
particular, for treatment of the breast or a remaining skin flap.
Bremsstrahlung X-rays produced when energetic electrons are
deflected in the field of an atomic nucleus; they may have any energy up to the
energy of the electrons producing them.
Cast A shell, usually of plastic, individually formed to
fit the outer shape of the patient. All relevant external radiation field
markings are placed on the cast. During treatment, the cast holds the patient in
one position and ensures that the tumor volume is treated exactly.
Central Axis Depth-Dose Depth-dose (defined below) along
the central axis of a radiation beam.
Cesium-137 Tubes Used instead of radium for intracavitary
insertions, especially in the treatment of carcinoma of the cervix. They are
cylinders two centimeters in length and approximately four millimeters in
diameter, usually calibrated as milligram equivalent of radium.
Cobalt-60 Teletherapy The administration of external beam
radiation therapy by means of Cobalt-60 gamma-rays: energies 1:17 and 1.33 MeV.
Compensator A specially measured slab of material placed in
the treatment beam to allow selective transmission of areas of the treatment
beam to compensate for unevenness of machine output or body contour.
Compensators vary greatly in complexity depending upon use requirements.
Contour Preparation The utilization of some means of
transferring a sectional shape of the patient's body onto a suitable surface for
the purpose of dosimetry calculations. Normally followed by location of
consequential internal structures and the suspected tumor volume.
Contact Therapy Treatment by an x-ray machine designed for
use at very small TSD (target-to-skin distance) of one to five centimeters at
relatively low energy (approximately 60 kv.). Useful for small skin lesions.
Conventional Therapy Treatment by x-ray beams, not in the
Curie Special unit of (radio) activity, equal to 3.70 x
1010 disintegrations per second. Historically, the curie (Ci) was based on the
disintegration rate of one gram of radium.
Cyclotron A circular accelerator used to produce high
energy protons, deuterons, and other relatively heavy charged particles.
Energies over 100 million volts may be achieved. Such particles may be used for
basic physics research and to produce radionuclides for medical applications.
They are sometimes used directly for experimental therapy or to produce neutron
beams for therapy.
Depth-Dose Radiation dose at some specified depth or depths
in tissue relative to the dose at a fixed reference point on the beam axis.
Depth-dose is usually expressed as a percentage.
Diaphragm (or Collimator) The part of a radiation beam
therapy machine that limits the field to the desired size. The thickness of the
metal required increases with the energy of the beam.
Dosimetrist An individual who has training and knowledge in
treatment planning and who, under the supervision of a qualified radiological
physicist or radiation therapist, is capable of performing dose calculations and
of assisting in calibration and verification of dose distribution within the
patient. A dosimetrist shall have a Bachelor's degree in physical science with
at least one year of additional experience in radiation physics, or be a
registered radiation therapy technologist with at least two years of additional
experience in radiation physics and treatment planning.
Dosimetry Strictly, the measurement of dose. In practice,
calculations, measurements, and other activities required for determining the
radiation dose delivered.
Electromagnetic Radiation Electromagnetic radiation
consists of a transport of energy through space as a combination of an electric
and magnetic field both of which change in magnitude as a function of time and
space. (Examples: visible light, infrared and ultraviolet, x- and gamma-rays).
Electron Beam Therapy Treatment by electrons accelerated to
high energies by a machine such as the betatron. Used mainly for lesions
situated at or near the surface. The advantage is that, unlike x-rays, electrons
deliver the maximum dose to the first few centimeters of tissue, with rapidly
decreasing dose as depth increases. The depth of the high-dose region can be
varied by varying the electron energy.
Entrance Port The area on the surface of a patient or
phantom on which a radiation beam is incident.
Exit Dose The dose at the point where the axis of the beam
emerges from the patient.
External Irradiation A method of irradiation in which the
source of radiation is outside the body. The radiation beam must always traverse
the skin and some normal tissue except with a superficial lesion.
Field Block A quantity of attenuating material utilized to
shape a treatment beam, especially to produce a beam of complex shape.
Field Size The measure of an area irradiated by a given
beam. There are two most useful conventions. The first is the geometric field
size; the geometric projection on a plane perpendicular to the central ray of
the distal end of the collimator as seen from the center of the front surface of
the source. The second is the physical field size, defined as the area included
within the 50 percent maximum dose isodose curve at the depth of maximum dose.
Filter An insert, composed of various layers of different
metals (aluminum, copper, lead) put in the x-ray beam to filter out the lower
energy rays of the beam.
Fractionation A technique of administering radiation
therapy in multiple doses over a number of days or weeks to achieve a maximum
Gamma-Ray A photon emitted from the nucleus of a
radioactive atom, different from an x-ray photon only with respect to origin. A
photon may be thought of as a moving electromagnetic disturbance which behaves
sometimes as a wave and sometimes as a particle.
Gold-198 A radioactive isotope used in interstitial therapy
in the form of seeds or wire, or in intracavitary therapy i n the form of a
colloidal solution. The isotope has a half-life of 2.7 days and emits photons at
0.41 MeV, as well as electrons.
Half-Life The time during which half the atoms initially
present in a sample of radioactive material will have decayed.
Half-Value Layer (Thickness) (HVL) A measure of the quality
of the radiation. Indicates the thickness of the specified material required to
reduce the flux density of the radiation beam to one half of its initial value.
For example, the HVL of 60Co is 11 mm. of lead.
High LET Radiation Energetic charged particles, or other
radiation capable of liberating secondary charged particles, for which the
spatial rate of energy loss (linear energy transfer or LET) is greater than
about 10 kilo electron volts per micron of particle path, i.e. a significantly
higher rate than that for electrons. Examples are protons, pions, alpha
particles, and neutrons. Neutrons are uncharged but liberate protons and other
charged particles in tissue.
Immobilization Device A mechanical device to maintain the
patient in a fixed position during treatment. This may take the form of a
plaster cast, a vacuum pillow, or some other holding device.
Internal Sources of Radiation (See Brachytherapy)
Removable Implant The radioactive material is enclosed in
needles, seeds, or tubes, which can be removed after the desired dose is given.
Examples are radium-226 or cesium-137 needles and iridium-192 seeds or wire. The
sources may be inserted directly into tissue or inside an applicator of some
Permanent Interstitial Implant Seeds containing the
radionuclide are inserted directly into tissue, either individually or in
ribbons of absorbable suture material, and left indefinitely. The technique is
used to treat intrathoracic and intra-abdominal tumors (lung, pancreas, bladder,
prostate) and in the palliative treatment of accessible tumors. Radionuclides
used include iodine-125 (60 day half-life) and radon-222 (3.8 day half-life).
The seeds are inert after total decay and do not cause any foreign body
Distributed Internal Source Intracavitary application of
radiocolloids, such as Au-198, or P-32 (chromic phosphate), allows uniform
distribution of radioactivity over serosal surfaces.
Metabolically Located Sources Administered radionuclide is
transported and concentrated by normal metabolic activity, e.g. I-131 in
Inverse-Square Law A physical law describing the intensity
of radiation at various distances from a point source. The flux density or
radiation is inversely proportional to the square of the distance from a point
Ion Chamber A special radiation measuring device in which
the collected electrical charge from ionization in a gas-filled cavity is taken
to be the proportion to some parameter (e.g. dose or exposure) of radiation
Isodose Curve A curve on which all points receive an equal
radiation dose. A series of them will map out the relative intensities of a
radiation field in a phantom or patient.
Isodose Distribution In a selected plan intersecting the
treatment region, a representation of dose distribution by a set of curved
lines, each line tracing the locations of points at which a specified dose is
delivered. Its calculation is very time consuming unless performed with the aid
of a computer. The dose at each point is the sum of doses from all intersecting
beams (in teletherapy) or from all implanted sources (in brachytherapy).
Isotopes Atoms of identical chemical properties (same
configuration of orbital electrons) but with a different atomic weight
(different number of neutrons contained in the nucleus of the atom).
LD 50/30 A term that represents a single total-body
irradiation lethal in 30 days to 50 percent of a group of animals. For man it is
about 350 to 450 rads.
Linear Accelerator Essentially a pipe in which charged
particles may be accelerated by applying a high frequency potential difference
during the particle transit along the pipe. In electron accelerators for
radiotherapy, the pipe becomes a "wave-guide", which may be a corrugated tube
with continually increasing spacing between corrugations. Microwave frequency
electromagnetic fields generated at one end travel down the wave-guide in a wave
of increasing velocity. A "bunch" of electrons injected at precisely the right
time is accelerated by "riding the crest" of this wave. The electrons produce
x-rays by striking a target at the far end of the tube.
Linear Energy Transfer (LET) The spatial rate of energy
loss by a charged particle along its path, often expressed in units of kilo
electron volts per micron.
Loading (Field Weighting) Among several beams in a
treatment plan, the relative distribution of radiation exposures to the
individual beams may be expressed in percentage of radiations applied, one field
to another or total dose, centering on the same tumor volume. May also be
expressed as a ratio (one to one, one to three, etc.).
Localization Films X-ray films taken with various
radiopaque markers in order to localize the position of the tumor relative to
the external markings.
Low LET Radiation Radiation that delivers most of its dose
by way of charged particles having spatial rates of energy loss (linear energy
transfer or LET) less than about 10 kilo electron volts per micron of particle
path. Low LET values are associated mainly with electrons and with x- or
gamma-rays (which liberate electrons in tissue).
Nuclide A species of atom having specified numbers of
neutrons and protons in its nucleus.
Orthovoltage X-Ray Therapy X-ray therapy applied with a
machine producing x-rays having a peak energy of 140 to 680 kVp.
Oxygen Enhancement Ratio (OER) The ratio between the dose
required to produce a given biological effect under hypoxic conditions and the
dose required to produce the same effect under well-oxygenated conditions. The
OER for x-rays typically has a value of 2.6 as compared to a value of 1.6 for
Penumbra The region at the edge of an irradiated volume
that receives some radiation, but not the full dose of the beam. It exists
because of the finite source size and because of scattered radiation.
Phosphorus-32 A radioactive isotope that emits beta rays
and has a half-life of 14.3 days. It is administered internally, in solution,
and tends to concentrate in the bone marrow, spleen, liver, and lymph nodes.
Useful in treatment of polycythemia vera. Phosphorus-32 is also used in
colloidal form by injection into the serous cavities (pleural and peritoneal) in
order to control the malignant accumulation of fluid.
Pin and Arc An attachment for a therapy unit used to
position the beam at a specific angle and aimed toward a specific point in the
Point A An imaginary point described by Todd and Meredith
as being two centimeters lateral to the cervical canal and two centimeters above
the cervical os. The point is supposed to lie in the paracervical tissues.
Point B A reference point that lies three centimeters
lateral to Point A and is used as a means of evaluating pelvic wall dosage.
Port Film A radiograph taken with the patient interposed
between the treatment machine portal and an x-ray film. The purpose of this film
is to demonstrate radiographically that the treatment field, as externally set
on the patient, adequately encompasses the desired treatment volume and at the
same time avoids adjacent critical structures.
Primary Beam The direct radiation beam emanating from the
head of the irradiating unit. Scattered radiations result when this beam
collides with any object (patient, treatment table, walls)
Quality The penetrating power of a photon beam, described
in terms of half-value layer.
Rad Unit of the radiation quantity "absorbed dose". One rad
is equal to an energy absorption of 0.01 joule per kilogram of any material.
Radioactivity The property of certain nuclides of
spontaneously emitting particles or gamma radiation or of emitting x-radiation
following orbital electron capture or of undergoing spontaneous fission.
Radiation The propagation of energy through space or
matter. In radiology, it can be divided into two main groups: charged particles
(e.g. electrons, protons, alpha particles) and electromagnetic (x-rays,
Radiation Beam The flow of therapeutically useful radiation
energy through a defined area: includes x-rays, gamma-rays, electrons, and other
radiation from a treatment machine.
Radioactive Half-Life The time during which half the atoms
initially present in a sample of radioactive material will have decayed.
Radium-226 A radioactive isotope commonly used for
radiotherapy. It has historical importance in that it was the first isotope to
be used medically and is used as a radiation standard. The half-life is about
1,620 years and photons of many discrete energies are emitted up to a maximum of
2.2 MeV. It is used in the form of needles and tubes for interstitial and
Radon It is available in the form of sealed gold seeds and
is widely used for permanent implants.
Relative Biological Effect (RBE) The ratio between the dose
required to produce a given biological effect using a reference radiation (of
specified type and energy) and the dose required to produce the same effect
using the radiation in question.
REM (Roentgen-Equivalent-Man) Special unit of the radiation
protection quantity "dose equivalent". Dose equivalent is obtained by
multiplying absorbed dose by a "quality factor", which has higher values for
higher LET radiations. When dose is expressed in rads, dose equivalent is in
Roentgen (R) Special unit of the radiation quantity
"exposure", equal to an electrical charge (produced by ionization) of 2.58 x
10-4 coulomb per kilogram of air.
Rotation Therapy External beam teletherapy in which the
source of radiation moves circumferentially around the patient while being
centered in the volume of interest. Some devices allow the patient to rotate in
the vertical plane within a stationary beam.
Scatter When a material is in the path of a radiation beam,
the material not only absorbs some of the radiation, it also scatters some in
all directions, usually reducing the quality of the beam at the same time.
Therefore, the radiation received at a point has two components: scattered and
primary. It follows that the exposure rate at a point in air will be increased
if a patient or phantom is placed behind it. This is caused by "backscattered"
Simulation The use of a simulator to determine the various
treatment field outlines and orientations to be used in the course of radiation
Simulation Films X-ray films taken with the same field
size, source-to-skin distance, and orientation as a therapy beam in order to
mimic the beam and for visualization of the treated volume on an x-ray film.
Simulator A radiation generator operating in the diagnostic
x-ray range with the mechanical capability to orient a radiation beam toward a
patient with parameters imitating that proposed for therapy, and affording
direct x-ray fluoroscopic visualization and roentgenographic images of the area.
Machine not capable of delivering radiation therapy.
Skin Sparing Because of the buildup of the absorbed dose in
supervoltage radiation deep to the skin, the skin surface does not receive the
maximum dose delivered. The skin reaction is therefore much less than would be
expected from conventional radiation.
Source Axis Distance The distance from the source of
radiations to the center of rotation in the case of rotational or arc therapy.
Source-to-Skin (SSD) The distance from the radiating
isotopic source to the skin of the patient.
Split-Course A course of radiotherapy delivered in two or
more parts separated by planned rest periods.
Strontium-90 Applicator A beta-ray source used for contact
therapy, usually for eye lesions. Most of the dose is delivered by beta-rays
(maximum energy of 2.3 MeV) from the equilibrium activity of the yttrium-90
Superficial Therapy Treatment with an x-ray machine of
relatively low energy, approximately 100 kv. Penetration is not large.
Supervoltage Therapy 680 kVp to two MeV and above including
cobalt-60 and cesium-137.
Teletherapy The delivery of radiation treatments to the
patient from a source located far (usually more than 50 cm.) from the region to
Thermoluminescent Dosimetry The use of radiation-sensitive
materials to determine dose at selected locations by measuring light emitted
when materials are heated, i.e. thermoluminescence. Frequently placed in the
path of the treatment beam to verify radiation exposures comprising a treatment
plan. The action of radiation on certain crystalline materials, e.g. lithium
flouride, is to "trap" some electrons at higher energy levels, in the crystal
lattice. These electrons may be released from their traps by heating of the
material, and the light that they emit in returning to their former energy
levels is directly proportional to the absorbed dose.<
Tolerance Dose The maximum radiation dose which may be
delivered to a given biological tissue at a specified dose rate and throughout a
specified volume without producing an unacceptable change in the tissue.
Treatment Beam Parameters The data required for the
complete specification of an individual treatment beam. Includes: radiation
energy, field size, use of wedges, blocks, bolus, etc., orientation with respect
to patient and prescribed exposure time, dose, and distance, etc.
Treatment Field A plane section of a beam, perpendicular to
the beam axis, as defined by the collimator of the treatment machine. Term often
used synonymously with treatment port.
Treatment Plan An ensemble of radiation beams or sources
designed to produce a prescribed dose pattern in and for the patient; includes
spatial and temporal distributions.
Treatment Planning A complex process carried out prior to
the administration of radiation therapy. Usually includes such items as tumor
localization, treatment volume determination, contour preparation, and treatment
dose determination to prescribe the dosage pattern required; also, composition
of radiation exposure spatially and their time distribution to effect the
prescribed dose pattern.
Treatment Port The machine opening through which the
radiation beam is delivered. See Treatment Field.
Tumor Localization The various studies and procedures to
determine the volume of tumor involvement, e.g. simulation, tomography,
computerized transaxial scanning, ultrasonic scanning, and radionuclide
Tumor Volume That volume encompassing all known or presumed
Van de Graaff Generator An electrostatic machine that
utilizes a moving belt to carry electrons to a high-voltage collector or
terminal and then accelerates these charged particles to high energies.
Verification Field The radiograph taken with the patient
interposed between the treatment machine portal and an x-ray film. The film is
left in place for the entire course of treatment and its purpose is to verify
the area treated by the original portal film.
Wedge Filter A tapered block of attenuating material,
designed to produce a differential distribution of radiation exposures over the
area of a radiation beam.
X-Rays or X-Radiation Electromagnetic radiation of energy
greater than 100 electron volts, emitted when electrons (or other charged
particles) experience a sudden loss of energy, either falling into a vacant
orbital energy level of an atom (in which case "characteristic" x-rays are
produced) or being sharply deflected in the field of an atomic nucleus (in which
case "bremsstrahlung" x-rays are produced).