EXAM 2: X-ray Beam Intensity and Transmission

TAR method, power law method, isodose shift method, equivalent tissue-air ratio method

4 types of methods for tissue inhomogeneities

manual treatment planning; individual point dose calculations

isodose shift method is used more for ________________________ while effective SSD method and Tissue-air ratio method are used to make _______________

ETAR method

method best suited for energies 6 MV or <

TAR method

method for tissue inhomogeneities that applies dose at point of interest, doesn't take into account the position of the inhomogeneity relative to that point of interest

isodose shift method

method for tissue inhomogeneities that is convenient for correcting isodose charts for the presence of inhomogeneities

equivalent tissue-air ratio (ETAR)

method for tissue inhomogeneities that is used in computer models which takes into account the primary and scatter dose component of different volumes based on their geometric arrangements with respect to the point of interest

Batho method (power law)

method that is best suited for energies 10 MV or >

effective TAR method

method that is used to calculate tissue inhomogeneities that helps determine what depth to use

TAR method

method that overestimates dose for all energies

electronic equilibrium; cavity surface

most important effect of air cavities for megavoltage beams is the loss of _________________________________ at the ____________________________

surface beyond the cavity

most significant decrease occurs at the __________________________ for large cavities and small field size

5%

none of the methods can claim an accuracy of +/-______ for all irradiation conditions

skin sparing (beam skims surface, reducing skin sparing)

one effect of non-perpendicular beam incidence (such as tangents, i.e breast tangents) is on _____________________

clarkson calculation

performed by calculating doses at different points of interest within an irregularly shaped field

the inhomogeneity; the build-up region; Batho

power law method does NOT apply to points inside __________________________ or in __________________________ and was first proposed by _________

an energy dependent shift factor less than 1.0; height of the tissue excess or deficit

the amount that the line on an isodose chart moves is by an amount K x H where K is _________________________ and H is _______________________

1

the closer to ________ a homogeneity coefficient is, the more homogeneous the half value layer is

beam energy, field size, depth, and SSD

the factor K in the isodose chart is dependent on ________________, ________________, _________________, and _________________

original strength of the beam

the intensity of a photon beam at a target depends primarily on the __________________________________

small; higher

the loss of lateral electronic equilibrium is more prominent with ___________ fields and with ______________ energies

transmission

the ratio of the beam intensity I to Io

scatter air ratio

the ratio of the scattered dose at a given point in the phantom to the dose in free space at the same point

effective SSD method, Tissue-Air (Tissue-Max) Ratio Method, Isodose shift method

the three methods recommended for angles of incidence when correcting for contour irregularities

45 degrees; 30 degrees

there are three methods are recommended for angles of incidence of up to ________ for megavoltage beams and of up to ______ from the surface normal for Orthovoltage x-rays

high

there is a loss of loss of lateral electronic equilibrium when using __________ energy beams because of the increased number of electrons that scatter outside of geometrical limits of beam (less sharp dose profile)

half-value layer

thickness of a given material necessary to reduce the intensity of a photon beam to 50% of its initial value

attenuation; scatter

tissue inhomogeneities alter the dose distribution from the standard curves due to ______________________ and __________________

volumes within the patient that have non-uniform tissue densities

tissue inhomogeneities are:

irradiated scatter volume

weighted densities are assigned by the volume of the ________________________

attenuation

when a photon beam passes through matter energy is removed from the beam

under

when treating a lung tumor with small fields and higher energy there is a possibility of ______________dosage in the periphery of the fields

uncorrected for tissue inhomogeneity; corrected for the inhomogeneity

with TAR method, multiply the dose at the point of interest _________________________ by the correction factor to give you the dose at the point of interest ______________________________

scatter; dose

with clarkson calculation, we determine how much _____________ at different point of interest in irregular fields to determine _____________

water equivalent path length

with effective TAR method, radiation is sent through three materials to three points that help determine the _____________________________ which is the thickness of water that will provide the identical attenuation of the actual beam path

increased

with soft tissue IN bone, dose is _________________ because of increased contamination (photoelectric, compton, or pair production)

electron backscattering

with soft tissue surrounding bone, on the entrance side of bone, there is an increased in soft tissues near bone primarily due to ______________________________________ which is nearly the same for all energies

decrease

with the isodose shift method, the shift factor will ____________________ with higher energy beams because there is a smaller change in dose per cm of missing tissue

oblique incidence

calculations to account for inhomogeneities and heterogeneity corrections inside the patient are handled in a similar fashion to _______________________

mass attenuation coefficient

characterizes attenuation in terms of probability per unit mass, as radiation beams enter the body, they are attenuated by the body tissues which are composed of various densities characterized in terms of probability per unit mass

bolus or compensator

contour corrections may be avoided by using ___________________ or __________________

scattering

difficult to equate for small inhomogeneities because of enhanced _______________ effects

density

dose in lung is governed by its _______________

PDD (%) x (TAR @ depth from closest surface/TAR @ depth from other edge of the gap)

equation for tissue air ratio method to correct for air gap

approximation

equivalent path length and isodose shift are ____________________ methods, so they do not account for scatter of nearby structures but give answers that are within a few percent of other methods

pair-production

for energies greater than 10 MV there is an increase in dose due to _____________________ electron interaction

toward; negative; away from; positive

for isodose shift method, you shift ______________ the skin for bone so the number is ____________ and shift __________ the skin for lung or air cavities so the number is __________________

CET's - Coefficient of Equivalent Thickness

for large volumes of inhomogeneities, we use _____________

beyond

for points that lie _______________ the inhomogeneity, the primary effect is the attenuation of the primary beam

within and at

for points that lie _______________ the inhomogeneity, the primary effect is the changes in the secondary electron fluence

beam energy

for soft tissue surrounding bone, on the transmission side the forward scatter of electrons depends on _________________

reduced

for superficial and orthovoltage beams, dose beyond bone is ______________ due to the increased electron density of bone compared to muscle tissue

less; builds up

for up to 10 MV with soft tissue surrounding bone, the dose is initially __________ and then __________________ to a dose slightly higher than in a homogeneous tissue

small

for x-ray beams in megavoltage range corrections for bone are usually _______________

linear attenuation coefficient

gives the probability that a given photon will be attenuated in a unit thickness of a particular attenuator

1st HVL/2nd HVL

homogeneity coefficient equation

μ (linear attenuation coefficient)

if the thickness of x is given in a particular unit of length, then the units of __________________ must be the inverse of that length

1.5

in the effective TAR method, when calculating for the point going through bone, the measured length of the amount of bone is multiplied by _______

low; high

instead of the isodose curves shifts 1/2 or 2/3 in the direction of the tissue deficit, the curves shift away from the patient surface with _________-density inhomogeneities or toward the patient surface with _________-density inhomogeneities

tissue excess or deficit along that line

isodose chart follows the central axis and moves up and down depending on ______________________________

thickness

isodose curves beyond the inhomogeneity are moved by an amount equal to "n" times the _________________ of the inhomogeneities

changes in the absorption of primary beam and associated pattern of scattered photons; changes in the secondary electron fluence

***2 general categories of effects caused by tissue inhomogeneities

pair production

***above 10 MV the shielding effects of bone increase slightly due to ________________________ in which atomic number increases with shielding effect

photoelectric absorption

***absorbed dose within or near bone may be several times higher than in soft tissue due to increased _____________________________ in the mineral content of bone

compton interactions

***between 4 and 10 MV, _______________ are the predominant mode of interaction in which attenuation of the beam in any medium is governed by electron density

monoenergetic or homogenous

***for ________________________________________ beams, the μ of the material remains unchanged and when plotted on a semi-log graph paper an exponential function produces a straight line

polyenergetic or heterogeneous

***for _________________________________________ beams the μ varies in value and when plotted on a semi-log graph it does NOT produce a straight line

power law method (Batho's method)

***method for tissue inhomogeneities that does depend on location of inhomogeneity relative to point of interest but NOT relative to surface

beam divergence; attenuation

***the beam's intensity at the target or simulator film is reduced by two effects:

tissue-air or tissue max ratio

***the one method for correcting for contour irregularities that gives the most accurate results is the:

SSD; depth; field size at that depth

***the tissue air ratio method depends on the principle that it is NOT dependent on ____________ and is a function only of ______________ and __________________

PDD (%) x *_____________________________*

*equation to correct for contour irregularities with SSD method*

1.65 cm

CET for hard bone =

1.1 cm

CET for spongy bone =

scatter

SAR's are primarily used in calculating ___________ in a field of any shape, so they are tabulated as functions of depth and radius of a circular field

Io; I

________ is the initial intensity at a detector before filtration and _____________ is the final beam intensity after filtration

lower

____________ density lung gives rise to higher dose within and beyond the lung

beam divergence

____________________ is explained by the inverse square law, intensity gets smaller as the distance increases

energy, field size, location and extent of inhomogeneity and location of point of calculation

accuracy of different methods depends on conditions such as:

inverse square law

area over which a radiation beam spreads is proportional to the square of the distance from the source

decreases

as energy increases, and depth and field size decreases, the SAR __________________

increases

as the filter thickness _____________________ the intensity of the attenuated beam drops