Download Dosimetry/ Radiation Therapy Terms

Dosimetry/ Radiation Therapy Terms
1) Digital reconstructed radiograph (DRR) - based on acquired CT information, these are
images that render a beam’s eye view display of the treatment field anatomy and areas of
treatment interest.
2) Attenuation- the change in a beam of radiation as it passes through matter. The
intensity of the electromagnetic radiation decreases as its depth of penetration increases.
3) Beam Hardening- the process of increasing the average energy level of an x-ray beam
by filtering out the low-energy photons.
4) CT Window and leveling- arbitrary numbers used for image display based on various
shades of gray, window width controls the overall gray level and affects image contrast.
Window level- the digital processing that produces changes in density/brightness.
5) Dmax- when a radiation beam interacts with tissue, a certain depth of tissue is required
before we see the maximum amount of radiation (maximum dose or dmax). This region
is called the build-up region and is responsible for the skin sparing effect. The dose at
dmax is defined as 100% and then the radiation dose decreases as the depth increases, the
energy being absorbed within the tissue.
6) Tomotherapy- a type of radiation therapy in which the radiation is delivered slice-byslice. This method of delivery differs from other forms of external beam radiation therapy
in which the entire tumor volume is irradiated at one time.
7) Effective Field Size- the equivalent rectangular field dimension of the open or treated
area within the collimator field dimensions when blocks are used to customize the shape
of the treatment area.
8) Equivalent Square- rectangular field size that demonstrate the same measurable
scattering and attenuation characteristics of a square field size.
9) Given dose-the applied dose, entrance dose, peak absorbed dose, or DMAX dose.
Point where PDD is 100%. Given dose = TD/ PDD x 100 TD: tumor dose, dose at depth.
10) Gray (Gy) - the SI unit of absorbed dose. One gray is equal to an absorbed dose of 1
Joule/kilogram (100 rads).
11) Hinge Angle- measure of the angle between central rays of two intersecting treatment
beams. HA=180-(WA x 2)12) Heterogeneity corrections- corrections that account for the
presence of irradiated media other than water.
13) CT Simulation-CT simulation is a process used by the radiation therapy team to
determine the exact location and size of the area to be treated. This is done in a room that
contains a narrow, movable table and special X-ray equipment. During simulation, you
will lie on the table. The radiation therapist will position you and then watch you through
a window while the CT scanner takes pictures or “slices” of the treatment site. Your
radiation oncologist looks at the cancerous area with the radiation therapist. The radiation
therapist may use special pillows, pads, or other devices to help hold your body in the
proper position. Sometimes contrast material (liquid with dye in it) may be swallowed or
injected into your body so internal organs are easier to see on the CT scan. After your
radiation oncologist locates the area to be treated, the radiation therapist will outline the
treatment field (area to be treated) on your skin. These tiny, permanent dots will be used
as a guide for correct positioning during your treatments.
14) Hot spots- hot spots are volumes of tissues that receive doses greater than prescribed
dose.
15) Half Value Layer- thickness of absorbing material necessary to reduce the x-ray
intensity to half of its original value.
16) Bolus- a tissue equivalent material used to reduce the depth of the maximum dose (D
max). Also called a “build-up bolus”. A bolus can be used in place of a compensator for
kilovoltage radiation to even out the skin surface contours. In megavoltage radiation
bolus is primarily used to bring up the buildup zone near the skin in treating superficial
lesions.
17) Isodose curve- plotted percentage depth dose at various points in the beam along the
central axis and elsewhere.
18) Magnification factor- magnification is directly proportional to the distance of the
object from the target or source and is dependent on the distance of the object from the
film. Magnification factor= image size/object size.
19) Minification factor- objects on film are smaller than actual size. To find the min
factor you can use this method, minification factor= object size/image size.
20) Maximum target dose- the maximum dose a specific organ/target should receive. For
organs that can only tolerate a certain dose, a maximum dose is than set so that organ
doesn’t receive more dose than it can tolerate.
21) Minimum target dose- the minimum dose that a specific organ/target should receive.
For example, you want the tumor to get the prescribed dose, so the minimum dose to the
tumor would be the prescribed dose.
22) Output Factor- the ratio of the dose rate of a given field size to the dose rate of the
reference field size.
23) Orthogonal films- two images taken 90 degrees apart. They are required for treatment
planning purposes to define the location and relationship of various anatomic structures
relative to the field’s isocenter.
24) Percent Depth Dose (PDD) - the ratio expressed as a percentage, of the absorbed dose
at a given depth to the absorbed dose at a fixed reference depth usually Dmax. PDD=dose
@ d/ dose @Dmax.
25) Penumbra-the area at the edge of a beam where the dose decreases rapidly.
26) Prescribed dose- the total dose that is prescribed by the doctor, which depends on the
goal of treatment (cure, palliation, or prophylaxis) and the area that is being treated. The
prescribed dose may be split into daily fractions or just one fraction.
27) Primary radiation- radiation that comes out of the machine before it hits the patient.
33) HDR/LDR Brachytherapy- an internal type of cancer treatment that allows for the
radiation source to be inserted into a tumor or directly on top of a tumor, depending on
the need. There are two types of brachytherapy.
Low-dose-rate (LDR) brachytherapy is meant to be a permanent treatment and the
radiation sources are left in place for extended time frames. High-dose-rate (HDR)
brachytherapy is a temporary radiation treatment and the amount of time that the
radiation source is left in place is determined by the physician.
34) Coplanar- geometrical principle describing two radiation fields configured in such a
way that the beam edges lie on the same plane. Non-coplanar-fields that are not in the
same plane.
28) RAD - the special unit of absorbed dose. One rad is equal to an absorbed dose of 100
ergs/gram or 0.01 joule/kilogram (0.01 gray).
29) Scatter radiation- radiation that has been deflected from its path by impact during its
passage through matter. Any interaction of the primary radiation may result in scatter
radiation.
30) Skin sparing - property of megavoltage irradiation where the maximum dose occurs
at some depth beneath the skin surface.
31) SAD technique- where the field size is defined at a depth calculated within the patient
(Isocenter)
32) SSD technique- where the field size is defined on the surface/ skin.
35) 3-D planning- three-dimensional image visualization and treatment planning tools are
used to conform isodose distribution to only target volumes while excluding normal
tissues as much as possible.
4-D planning- uses three-dimensional treatment planning plus time.
36) Dose (absorbed dose)- measured at a specific point in a medium and refers to the
energy deposited at that point.
37) Activity- rate at which a radioactive isotope undergoes nuclear decay; units are the
Curie or Becquerel.
38) Linear Attenuation Coefficient- describes the fraction of a beam of x-rays or gamma
rays that is absorbed or scattered per unit thickness of the absorber. This value basically
accounts for the number of atoms in a cubic cm volume of material and the probability of
a photon being scattered or absorbed from the nucleus or an electron of one of these
atoms. U= .693/HVL
39) Build- up region- region between the skin surface and the depth of dmax. Build-up
region is a characteristic of megavoltage irradiation. In this region, the dose increases
with depth until it reaches a maximum at the depth of dmax.
40) Calibration- to check, adjust, or determine by comparison with a standard for linear
accelerators.
41) Decay constant- the decay constant can be expressed as the total number of atoms
that decay per unit time.
42) Dynamic wedge- use of a moving collimator jaw to produce a wedged isodose
distribution.
43) Entrance dose- an "entrance dose" of x-rays is the dose absorbed at the surface of the
skin where the x-ray beam enters. An x-ray beam enters the body from the direction of
the x-ray tube.
44) Gap calculation- used when you have abutting fields, two fields next to each other.
This is one method to avoid over lap. (L1/2+d/SSD1) + (L2/2+d/SSD2)
45) GM meter- an example of an ionization chamber. It is used for finding lost seeds,
monitoring deliveries of radioactive materials and searching for holes in walls of the linac
room. It is good for measuring low intensity radiation.
46) Normalization- the dose at some point has been forced to 100% and the dose
everywhere else is changed by the same ratio.
47) Heterogeneity- all photons in a beam have different energies.
48) Independent jaw- also called a half beam technique. This technique is used for
abutting fields, two fields next to each other. For this technique you match fields at the
CA because there is no divergence there.
49) ICRU- the International Commission on Radiation Units and Measurements is a
standardization body set up in 1925 by the International Congress of Radiology. Its
objective is to develop internationally acceptable recommendations for quantities and
units of radiation and radioactivity, as well as associated measurement procedures and
physical data.
50) Intensity- radiation intensity is the amount of energy passing through a given area
that is perpendicular to the direction of radiation travel in a given unit of time.
51) Inverse square law- a law stating that the amount of radiation reaching a surface is
inversely proportional to the square of the distance between the source and the surface.
For example, a person standing 1 m from a patient being treated with radium is exposed
to four times more radiation than a person standing 2 m from the patient.
52) Irregular field- certain treatments for certain tumors require irregular fields. Irregular
fields are considered any field that is not a rectangle, square, or circular field. The reason
why irregular fields are used is to shield critical structures from the primary beam.
Examples of irregular fields include mantle and inverted Y fields.
53) Dose Volume Histogram- plot of target or normal structure volume as a function of
dose.
54) Mass attenuation coefficient- used in treatment planning for different types of tissue
densities and how they attenuate the beam. Mass attenuation coefficient = u/density.
55) Monitor Units (MU) - a measure of output for linear accelerators.
56) MLC- multileaf collimators are a bank of large number of collimating blocks or
leaves can be moved automatically independent of each other to generate a field of any
shape. 40 pairs of leaves or more having a width of 1 cm on less (projected at the
isocenter).
57) Monte Carlo method- any method which solves a problem by generating suitable
random numbers and observing that fraction of the numbers obeying some property or
properties. The method is useful for obtaining numerical solutions to problems which are
too complicated to solve analytically.
58) Non conformal radiation- also known as traditional radiation. Traditional radiation is
limited to between two and four ports of entry. The shape of the fields and the depth of
penetration can be manipulated, but that is the limit of the ability to conform the
radiation. It does not allow for any specific conformation to avoid normal structures in
the area of treatment. The treatment is therefore limited by the normal structure’s
radiation tolerance. The treatment area can be moderated over time, but the entire plan is
still delivered based on the acceptable dose limits of the normal tissues involved.
59) Natural Background Radiation- ionizing radiation from natural sources including
cosmic rays from outer space and the sun, terrestrial radiation from radioactive materials
in the earth, and internal radiation from radioactive materials normally present in the
body.
60) Inverse planning- treatment planning in which the clinical objectives are specified
mathematically and computer software is used to determine the best beam parameters that
will lead to the desired dose distribution (IMRT).
Forward planning- process of entering dose altering parameters and beam modifiers into
the treatment plan by the planner (3D).
61) Peak Scatter factor- peak scatter factor is a backscatter factor sometimes normalized
to a referenced field size, usually 10X10cm, for energies of 4MV and above.
62) Patterson-Parker method- it was developed to deliver uniform dose (+/- 10%) to a
plane or volume. The system specified rules of source distribution to achieve the dose
uniformity and provided dosage tables for these idealized implants.
64) Quimby method- it is characterize by a uniform distribution of sources of equal linear
activity. Planar implants: the dose stated is the dose at the center of the plane (maximum
dose).Volume implants: the dose stated is the minimum dose within the volume.
Corrections are the same as the Patterson-Parker system.
65) Quality factor- the value of the quality factor for each type of radiation depends on
the distribution of the absorbed energy in a mass of tissue. X-ray and gamma rays quality
factor is 1. Neutrons and protons quality factor is 10 and alpha particles quality factor is
20.
66) Scatter Air Ratio (SAR) - the difference between the TAR for a field of definite area
and that for a zero area.
67) Collimator scatter (Sc) - the larger the field size, the more scatter created. The Sc is
1.0 for a 10x10 field size, less than 1.0 for fields under 10x10 and more than 1.0 for fields
larger than 10x10.
68) Phantom scatter (Sp) - amount produced in tissue.
69) Tissue- Maximum Ratio (TMR) - ratio of the absorbed dose at a given depth in
phantom to the absorbed dose at the same point at the level of dmax in phantom.
70) Thermoluminescent dosimeter (TLD) - device for measuring dose. It uses the
phenomenon that some solid materials, when irradiated, will subsequently give off light
when heated. Amount of light emitted is proportional to the dose delivered to the crystal.
71) Tissue tolerances- is the ability of the skin and it’s supporting structures to endure the
effects of pressure without adverse effects.
72) Wedge Angle- angle between the slanted isodose line and a line perpendicular to the
central axis of the beam.
73) Intensity Modulated Radiation Therapy (IMRT) - therapy that delivers non-uniform
exposure across the beam’s eye view using a variety of techniques and equipment. The
main advantage of using a Dynamic MLC is that the continuous leaf motion enables the
delivered intensity to closely match with the optimal fluence calculated by the inverse
treatment planning algorithm, accurately preserving both the spatial and intensity
resolutions. On the other hand, the static MLC approach resembles a conventional multisegmented treatment and requires approximating the intensity profile into discrete
intensity levels (briefly described in the methods and materials section), resulting in a
lower resolution. The Static MLC IMRT may be convenient to verify and is technically
less demanding than a Dynamic MLC treatment. A Dynamic MLC-based delivery
requires more monitor units (MU) than the Static MLC method, as the beam is kept on
throughout the delivery of radiation.
74) Equivalent dose- is a computed average measure of the radiation absorbed by a fixed
mass of biological tissue that attempts to account for the different biological damage
potential of different types of ionizing radiation.
75) Exit dose- the dose absorbed by a point that is located at the depth of DMAX at the
exit of the beam.
76) Bremsstrahlung- a German term for “braking” radiation.
77) Fluence pattern- refers to an intensity pattern of the IMRT beam. This may be
described at the sequence and the progression of dose delivered per beam, as a product of
several segments.
78) Past pointing-moving isocenter farther away from the source in order to evenly cover
the intended target.
79) Isocenter- the intersection of the axis of rotation of the gantry and the axis of rotation
of the collimator for the treatment unit.
80) Electron therapeutic range- common isodose depths are 80% and 90%. 90% is the
most common=therapeutic range.
81) Off axis ratio (OAR) - the OAR is the ratio of the dose at a point (Q) relative a point
on the central axis at the same depth (P). OAR=DQ/DP. The OAR is used for calculation
of dose at points away from the central axis.
82) Backscatter- radiation that is deflected back toward the patient. The term backscatter
factor (BSF) is simply the tissue-air ratio at the depth of maximum dose on central axis of
the beam. BSF= TAR Backscatter Factor (BSF): the ratio of the dose rate with a
scattering medium to the dose rate at the same point without a scattering medium at the
level of maximum equilibrium.
83) GTV- the tumor - this is called the GTV or Gross Tumor Volume.
CTV- the tumor plus a margin for microscopic spread - this is called the CTV or Clinical
Target Volume.
PTV- the tumor plus a margin to account for microscopic tumor spread and plus another
smaller margin to account for day to day set up variability. This is called the PTV or
Planning Target Volume.
84) Cone beam CT (CBCT) - based image guided systems have been integrated with
medical linear accelerators to great success. With improvements in flat-panel technology,
CBCT has been able to provide volumetric imaging, and allows for radiographic or
fluoroscopic monitoring throughout the treatment process. Cone beam CT acquires many
projections over the entire volume of interest in each projection.
85) Stereotactic radiation therapy- SRT is fractionated and can be done on linac, the brain
lab, Cyberknife, or Gammaknife. Stereotatic radiosurgery (SRS) is a single dose
treatment and involves the use of a halo brace for the head. Both SRS and SRT involve a
1-3cm diameter cones (no blocks used) and the gantry moves in arcs as the dose is
delivered in thin strips.
86) IGRT- image-guided radiation therapy is a process of using various imaging
technologies to locate a tumor target prior to a radiation therapy treatment. This process
is aimed to improve the treatment accuracy so that the need for large target margins
which have traditionally been used to compensate for errors in localization. As a result,
the amount of healthy tissue exposed to radiation can be reduced, minimizing the
incidence of side effects.
87) EPID- electronic portal imaging is the process of using digital imaging, such as a
CCD video camera, liquid ion chamber and amorphous silicon flat panel detectors to
create a digital image with improved quality and contrast over traditional portal imaging.
The benefit of the system is the ability to capture images, for review and guidance,
digitally. These systems are in use throughout clinical practice.
88) CT- computerized tomography scans. Pictures of structures within the body created
by a computer that takes the data from multiple X-ray images and turns them into
pictures on a screen.
PET- positron emission tomography. A highly specialized imaging technique that uses
short- lived radioactive substances to produce three-dimensional colored images of those
substances functioning within the body.
MRI- a magnetic resonance imaging scan is a radiology technique that uses magnetism,
radio waves, and a computer to produce images of body structures. The MRI scanner is a
tube surrounded by a giant circular magnet. The patient is placed on a moveable bed that
is inserted into the magnet. The magnet creates a strong magnetic field that aligns the
protons of hydrogen atoms, which are then exposed to a beam of radio waves. This spins
the various protons of the body, and they produce a faint signal that is detected by the
receiver portion of the MRI scanner. The receiver information is processed by a
computer, and an image is produced.
Bat- B-mode Acquisition and Targeting is an ultra sound-based targeting system used
with IMRT and external beam radiation, to precisely localize targets. To accomplish great
precision it uses technologies of 3-D computer imaging, a sophisticated linear accelerator
alignment and delivery system, and advanced ultrasound technology. BAT is useful for
prostate cancer, breast cancer, liver cancer, and bladder cancer.
89) Mayneord- F factor- a special application of the inverse square law. Does not account
for changes in scatter because of a change in beam divergence. New PDD = PDD x
(SSD1 + d / SSD1 + DMAX)2 x (SSD2 + DMAX / SSD1 + d)2