Download Dosimerty/Radiation Therapy Terms

Dosimerty/Radiation Therapy Terms
DRR (digital reconstructed radiograph)- 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. These images resemble conventional radiographs
Attenuation- removal of photons and electrons from a radiation therapy by scatter or
absorption as it travels through a medium, typically tissue or tissue equivalent materials
Beam hardening- increasing the average energy level of the beam by filtering out lowenergy photons
CT window and leveling- the CT number setting in Hounsfield unites of the midpoint of
the window width, which is the gray scaled of the image.
Dmax- the depth of maximum buildup, in which 100% of the dose is deposited beneath
the skin. Depth at which electronic equilibrium occurs for photon beams. This is also the
depth of maximum absorbed dose and ionization, for photons, from a single treatment
field. Depth of maximum ionization and maximum absorbed dose are usually not the
same depth for electrons
Tomotherapy- IMRT treatments using a continuously rotating gantry. Radiation is
helically delivered slice by slice
Effective field- blocked field size. Effective field size is the equivalent rectangular field
dimensions of the open or treated area within the collimator field dimensions. Effective
field is the actual area treated
Equivalent square- square field that has the same percentage depth dose and output of a
rectangular field. This method takes different rectangle field sizes and compares them to
square fields that demonstrate the same measurable scattering and attenuation
characteristics
Given dose- the dose delivered at the depth of maximum equilibrium (Dmax) through a
single treatment field. Aka applied dose or Dmax dose
Gray- SI derived unit of absorbed dose
Hinge angle- measure of the angle between central rays of two intersection treatment
beams
Heterogeneity correction- corrections that account for the presence of irradiated media
other than water
CT Simulation- computed tomography scanner equipped with software that can provide
information needed to design the patient’s treatment parameters
Hot Spots- volume outside the PTV which receives dose larger than 100 % of the
specified PTV dose
HVL- thickness of absorbing material necessary to reduce the x-ray intensity to half its
original value
Bolus- tissue equivalent material that is usually placed on the patient to increase the skin
dose and/or even out irregular contours in the patient
Isodose curve- plotted percentage depth dose at various points in the beam along the
central axis and elsewhere
Magnification factor- the degree of image enlargement how magnification is expressed
and measured by formula: Image size/object size
Minification factor- objects on a film are smaller than actual size
Maximum target dose- (105%) largest dose consistent with safety
Minimum target dose- (97%) smallest does that will produce an appreciable effect
Output factor- ratio of the dose rate of a given field size to the dose rate of the reference
field size
Orthogonal films- two images taken 90 degrees apart. They are required for treatment
planning purpose to define the location and relationship of various anatomic structures
relative to the field’s isocenter
Percentage Depth Dose- ratio, expressed as a percentage of the absorbed dose at a given
depth to the absorbed dose at a fixed reference depth usually (Dmax)
Penumbra- area of region at the beam’s edge where the radiation intensity falls to 0
Prescribed dose- planned treatment dose
Primary radiation- radiation arriving directly from its source without interaction with
matter
Rad- unit of absorbed dose
Scatter radiation- type of secondary radiation that occurs when the useful beam
intercepts any object
Skin Sparing- property of megavoltage irradiation where the maximum dose occurs at
some depth beneath the skin surface
SAD technique- isocentric technique, three dimensional treatment using multiple
imaging modalities, including fluoroscopy,CT, MRI,PET and ultrasound planning where
the isocenter is placed in or near the target volume
SSD technique- non isocentric technique, based on the distance from the source of
radiation to the patients skin (PDD)
High Dose Rate brachytherapy (HDR)- delivery of brachytherapy on an outpatient
basis using HDR brachytherapy equipment. Actual treatment delivery lasts about 5 to 10
minutes in contrast to a hospital stay that might take several days for low-dose rate
brachytherapy
Low Dose Rate brachytherapy (LDR)- Brachytherapy that is delivered in a
conventional low dose rate regimen that lasts several days and requires a hospital stay
Coplanar- geometrical principle describing two radiation fields configured in such a way
that the beam edges lie in the same plane
Non coplanar- beam edges don’t lie in the same plane
3-D planning- three-dimensional image visualization and treatment-planning tools are
used to conform isodose distributions to only target volumes while excluding normal
tissues as much as possible
4-D planning- uses three dimensional treatment planning +time =4D
Absorbed dose- energy absorbed per unit mass of any material; units are the cGy or rad
Activity- rate at which a radioactive isotope undergoes nuclear decay; units are the Curie
or Becquerel (Bq=1 disintegration per second) 1 Ci=3.7x10^10Bq
Linear attenuation coefficient- constant that describes the rate of energy loss by a
photon beam. Describes the probability that each photon in the beam will interact with
the medium and lose its energy per centimeter of material that the photons pass through
and has units of cm^-1
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
Calibration- determation of the accuracy of an instrument, usually by measurement of its
variation from a standard, to ascertain necessary correction factors
Decay Constant- total number of atoms that decay per unit time
Dynamic wedge- use of a moving collimator jaw to produce a wedged isodose
distribution
Entrance dose- measure of radiation dose absorbed by the skin
Gap Calculation- treatment fields overlapping at a prescribed depth in a patient. Exact
length of gap must be calculated by knowing the length of each treatment field and depth
at which the intersection of the fields is to be positioned
[(1/2L1/SSD1)+(1/2L2/SSD2)]x match depth
GM detector- highly sensitive, best for measuring contamination and other low levels of
radiation
Normalization- re-expressing of isodose distribution in terms of percents. All dose
distribution points are re-adjusted to be specified 100% point and then the distribution of
isodose lines is able to be selected in terms of percentages
Heterogeneity- consisting of different frequencies, various energies or a variety of
particles
ICRU( international commission on Radiation Units and Measurements)international standard that determines the radiation tumor volumes
Inverse square law- mathematical relationship that describes the change in beam
intensity as the distance from the source changes, where intensity is inversely
proportional to the distance squared
DVH (dose volume histogram)- plot of target or normal structure volume as a function
of dose
Mass attenuation coefficient- describes the fraction of a beam of xrays or gamma rays
that is absorbed or scattered per unit thickness of the absorber
Monitor Unit (MU)- unit of output measure used for linear accelerators. Accelerators are
calibrated so that 1 MU delivers 1 cGy for a standard, reference field size at a standard
reference depth at a standard source to calibration point
Multi Leaf Collimator (MLC)- distinct part of the linear accelerator that allows
treatment field shaping and blocking through the use of motorized leaves in the head of
the machine
Monte Carlo Method- any method which solves a problem by 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.
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’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.
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.
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).
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.
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.
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.
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.
Scatter Air Ratio (SAR) - the difference between the TAR for a field of definite area
and
that for a zero area
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.
Phantom scatter (Sp) - amount produced in tissue.
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.
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.
Tissue tolerances- is the ability of the skin and it’s supporting structures to endure the
effects of pressure without adverse effects.
Wedge Angle- angle between the slanted isodose line and a line perpendicular to the
central axis of the beam.
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 multi-segmented
treatment
and requires approximating the intensity profile into discrete intensity levels (briefly
described in the methods and materials section), resulting inlower 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.
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.
Exit dose- the dose absorbed by a point that is located at the depth of DMAX at the exit
of
the beam.
Bremsstrahlung- a German term for “braking” radiation.
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.
Past pointing-moving isocenter farther away from the source in order to evenly cover
the intended target.
Isocenter- the intersection of the axis of rotation of the gantry and the axis of rotation of
the collimator for the treatment unit.
Electron therapeutic range- common isodose depths are 80% and 90%. 90% is the most
common=therapeutic range.
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.
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.
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
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.
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.
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.
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
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.
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
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
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.
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