TG43 Flashcards
Where are low-energy photon emitting brachy sources used?
mostly for prostate cancer
-some in eye plaques or permanent lung implants
retro-pubic prostate brachy
would actually do open surgery- put needles or seeds into prostate
2 types of assumed source distributions
point source
line source
define seed
cylindrical brachytherapy source with active length, L, or effective length,Leff
know and draw out the parameters for the 2D dose rate equation
r is distance from center of active source to point of interest
ro is reference distance (1 cm)
theta is the angle specifying the pt of interest relative to souce longitudinal axis
reference angle is 90 degrees (source transverse plane)
D(r, theta) = Sk * A * (Gl(r, theta)/Gl(r0, theta0)) * gl(r) * F(r, theta)
-applies to sources that are cylindrically symmetric about source longitudinal axis
concensus data sets also assume that sources are symmetric about the transverse plane
where should the source coordinate system origin be?
geometric center of the radionuclide distribution~as determined using positioning information obtained fromthe markers!, not the geometric center of the exterior surfaceof the capsule or marker.
units of air kerma strength
1 U = uG m^ 2/ h = cGy cm^2/h
formula for air kerma strengh
air kerma rate in vacuo due to photons at disance d, multiplied by d^2 , photons have energy greater than delta
-delta cutoff is intended to excludelow-energy or contaminant photons~e.g., characteristicx-rays originating in the outer layers of steel or titaniumsource cladding!that increaseK ̇d(d) without contributingsignificantly to dose at distances greater than 0.1 cm in tis-sue. The value ofdis typically 5 keV for low-energy photon-emitting brachytherapy sources, and is dependent on the ap-plication
what does in vacuo entail?
mea-surements should be corrected for photon attenuation andscattering in air and any other medium interposed betweenthe source and detector, as well as photon scattering fromany nearby objects including walls, floors, and ceilings.
what is A dose rate constant
ratio of dose rate at reference position and Sk. ie D (ro,theta0)/ Sk.
unit of A
cGy/(U*h) = 1/(cm^2)
what affects dose rate constant A
radionuclide and source model
influenced by source internal design and exerimental methodology used by primary standard to calc Sk
what does geometry function G do?
provides IS correction based on approximate model of spatial distribution of radioactivity within the source
neglects scattering and attenuation
used to interpolate between tabulated dose rate values to replicate original dosimetry results
pt source: Gp (r, theta) = 1/r^2
line source: GL (r, theta) = beta/(Lrsin theta) if theta not 0 degrees, (r^2-(L^2/4))^-1 if theta = 0 degrees
beta is angle (radians) from one end of source to point and other end of source to point (see diagram)
what is effective length?
-active length of the cylinder
-for brachy sources that contain uniformly spaced multiple radioactive components, Leff = delta S X N
N is discrete number of pellets and delta S is the pellet center-to-center distance
If Leff is greater than the physical length of the source capsule~usually;4.5 mm) , the maximum separation~dis-tance between proximal and distal aspects of the activity dis-tribution should be used as the active length,L
radial dose function
g(r)
accounts for dose fall-off on transverse plane due to photon scatter and attenuation and excluding IS as included by geometry function
g(r0= 1 cm) = 1
g(r) = (D(r, theta0)/D(r0,theta0))*(G(r0,theta0)/G(r,theta0))
2D anisotropy function
accounts for variation in dose as a function of polar angle relative to the transverse plane
F(r,theta) = (D(r, theta)/(D (r, theta0))*(G(r,theta0)/G(r, theta))
how does F(r, theta) change with different parameters?
F(r, theta) = 1 on transverse plane
F decreases as r decreases, as theta approaches 0 or 180 degrees, as encapsulation thickness increases, and as photon energy decreases
when can F(r, theta) exceed unity?
at absolue value (theta - 90 degrees) > +/- arcsin(L/2r)for right-cylinder sources coated with low-energy photonemitters due to screening of photons by the active element at angles towards the transverse plane
1D anisotropy function, phi_an(r)
ratio of solid angle weighted dose rate, averaged over enture 4 pi stereidian space, to the dose rate at the same distance r on the transverse plane
phi_an(r) = integral from o to pi (D(r, theta)sin(theta)dtetha / 2D(r, theta0)
what is advantage of using 1 D approximation?
don’t have to determine orientation of source longitudinal axis from imaging studies
where does data in tables come from?
either experimental or monte carlo
assumptions of TG-43
No source-to-source shielding effects
all tissues in and around implant are water equivalent
scattering volume within patient is equivalent to that used in the concensus data sets (at least 5 cm of water-equivalent material surrounds the point of calculation)
criteria used to evaluate dosimetry parameters for each source model
1.
Internal source geometry and a description of the source,
2.
review of the pertinent literature for the source,
3.
correction to I125 values due to the 1999 anomaly in NIST air-kerma strength measurements (if applicable),
4.
solid water-to-liquid water corrections,
5.
experimental method used: TLD or diode,
6.
active length assumed for the geometry function line-source approximation,
7.
name and version of the Monte Carlo transport code,
8.
cross-section library used by the Monte Carlo simulation,
9.
Monte Carlo estimator used to score kerma or dose, and
10.
agreement between Monte Carlo calculations and experimental measurement.
recommended mass density for moist and dry air
0.00119 g/cm^3
recommended relative humidity of 40%
interpolation for F(r, theta)
linear
-based on 2 data points immediately adjacent to point of interest
for extrapolation, use zeroth order approach (i.e. use F(rmin) for r< rmin and F(rmax) for r> rmax)
interpolation for g(r)
log-linear
what does F(r,theta) approach with increasing radial distance?
unity
for all angles except theta 0
how does 1D anisotropy function vary with r?
nearly constant or linear for r>1 cm
values significantly increase with decreasing r for r < 1 cm
This behavior is caused by volume averaging of larger dose rates near the source long-axis due to the increasing ellipsoidal shape of isodose distributions in comparison to the dose rate at the same r value along the transverse plane
i.e. volume averaging of higher dose rates for smaller polar angles.
how to interpolate 1D anisotropy function?
log-linear
-base it on 2 immediately adjacent points
extrapolating 1D anisotropy function
- recommended equation for r< rmin
- linear for r> rmax
what characteristics define the radial dose function?
- attenuation and scatter in a 15 cm radius liquid water medium
- broad beam attenuation is based on u/p and absorbed dose is based on uen/p
how to interpolate radial function for r < 1 cm
log-linear
using points immediately adjacent to the radius of interest
how to extrapolate radial function
zeroth orer for r< rmin
log linear for r> rmax
interpolating/extrapolating point radial dose function vs line radial dose dunction
apply interpolation or extrapolation to linear function, because point function changes more rapidly for r<1 cm
apply ratio of point and line source gemoetry function to then get radial dose function for a point function
inhomogeneities are more significant for dose calcs of what type of source?
low energy source
anisotropy in “along away format”
anisotropy decreases as you go “away” and also decreases as you go “along” the source
what is mort important, gamma rays or x-rays?
gamma for high enery source
x-ray for low energy source
most commonly employed dosimeter for experimental dosimetry of low-energy brachy sources
TLDs
what were calculations done on before TG-43?
- based on apparent activity, equivalent mass of radium, exposure rate constants, and tissue-attenuation coefficients
- did not account for source-to-source differences in encapsulation or internal construction
-Except for radium, the exposure-rate constants and other input parameters to these algorithms depended only on the radionuclide. In contrast, TG-43 employed dose-rate constants and other dosimetric parameters that depended on the specific source design, and are directly measured or cal-culated for each source design.
what does AAPM recommend for informatuon about a source before it is used clinically?
at least one experimental and one Monte Carlo determination of the TG-43 dosimetry parameters be published in the peer-reviewed literature be-fore using new low-energy photon-emitting sources thosewith average photon energies less than 50 keV in routine clinical practice
definition of source
any encapsulated radioactive material used for brachy
point source approximation
dose distribution is assumed to be radially symmetric at a given radial distance r
line source approximation
ra-dioactivity is assumed to be uniformly distributed along a 1 D line-segment with active length L
definition of seed
cylindrical source with Leff < 0.5 cm
assumptions for the line source
cylindrically symmetric with longitudinal axis
also symmetric with transverse plane, but latter ca be accounted for if needed
what does TG43 dose calc reproduce?
exactly the measured orMonte Carlo-derived dose rates from whichg(r) andF(r,u)tables were derived
as long as geometry function used consistently
where are concensus data for the sources taken from?
averaged experimental data are averaged with MC data
what was TG-43 specifically written for?
interstitial brachy
why not use source strength as an activity?
activity might be inferred by the manufacturer using one
value of the constant and the dose might be calculated by the
user from a different value
i.e. exposure rate constant
what did TG-43 change from previous calculation methods?
gamma ray constant, exposure
rate constant, tissue attenuation factors, apparent activity, and
exposure-to-dose conversion factors are not needed in the
new formalism. Instead, only quantities directly derived from
dose rates in water medium near the actual source are used.
Some of these quantities are dose rate constant, radial dose
function, anisotropy function, anisotropy factor, and geometry
factor
traditional equation for dose rate
D(r)= AappfmedTAU(1/r^2)T(r)*Phian
Aapp is apparent activity fmed is exposure to dose conversion factor TAU is exposure rate constant T(r) is tissue attenuation factor Phian is anisotropy factor
In new TG43 system, each of the quantities is measured or calculated for the specific type of source in questionand therfore depends on source construction and geometry
-for older calcs, inout data is fundamental property of radionuclide
fundamental problem with pre TG43 protocols
based upon photon fluence around the source
in free space, whereas clinical applications require dose distributions
in a scattering medium such as a patient. Determination
of two-dimensional dose distributions in a scattering
medium from a knowledge of the two-dimensional distribution
of photon fluence in free space is easily accomplished
only for a point isotropic source. An actual brachytherapy
source exhibits considerable anisotropy
-could not handle non-point sources
TG43 solves this issue by measuring dose distribution in water equivalent phantoms
what was apparent activity replaced by?
air kerma strength
what was exposure rate constant replaced by?
dose rate constant
what was IS replaced by?
geometry factor
what was tissue attenuation T(r) replaced by
radial dose function
difference between anisotropy function and radial dose function
radial= depth dependence along transverse axis
anisotropy: anisotropy of dose relative to dose on transverse axis
why decouple F(r, theta) and g(r)
when better measurements are obtained they are easy to update
only absolute quantity in TG43
dose rate constant A
dose rate to water
at a distance of 1 cm on the tranverse axis of a unit air kerma
strength source in a water phantom
not simplified as point dose
what effects are included in the dose rate constant?
source geometry
spatial distribution of radioactivity in the source
encapsulation and self filtration
scatter in water surrounding the source
standardization measurements
to which the air kerma strength calibration of the source is
traceable
why do we suppress IS law effects from g(r) and F(r, theta)?
Due to the large dose rate gradients encountered near interstitial
sources, it is difficult to measure dose rates accurately
at distances less than 5 mm from the source. In addition,
the large dose rate variation arising from inverse square
law makes accurate interpolation of intermediate dose rate
values difficult without an excessively large table of measured
data. By suppressing inverse square law effects, extrapolation
to small distances from dose rate profiles measured
at distances of 5 and 10 mm as well as interpolation
b e t w e e n s p a r s e l y d i s t r i b u t e d m e a s u r e d v a l u e s i s u s u a l l y
more accurate.
when can you use point source approximation?
degree of dose anisotropy around
single sources is limited
what is 1-D anisotropy factor?
the ratio of the dose rate at distance r ,
averaged with respect to solid angle, to dose rate on the
transverse axis at the same distance.
per Mike R, essentially the average 2D variant for each radius
use when you don’tr know or care about orientation, or you expect it to average out and want an easier calculation
constant is averaged over radius too- not recommended to use in update (inverse square law weighted average). IS because phi does not take IS into account like F does.
For instances phi(r) data are not available over con-stant increments of r, linear interpolation of phi(r) may be used for derivation of phi constant
how was g(r) determined?
radial dose functions from solid water were fitted to a polynomial series expansion
how were F(r, theta) values determined
measured in solid water medium 2-D dose distributions
how were dose rate constants in water determined?
measurements or MC
or measurements converted to measuement in water using MC
uncertainty in dose rate constant A
5 %
3% in dose determination and 3 % in measurement of air kerma strength used in determination of A
uncertainty in F(r,ther)
5 %
3% in each of the dose rates in the ratio
similar for radial function
uncertainty in geometry factor
unlike g(r) and F(r,theta), it is a mathematical function with minimal uncertainty
overall uncertainty in dose rate at a point around a source
10%
add uncertainty of A, g(r), F(r,theta), air kerma strenght (5%) in quadrature
how can physicist check validity of correct implementation of TG43?
calculate dose rates at various points using point source approximation and compare results with those in tables
how does dose rate constant need to be updated if the standards underlying
vendor calibration of these sources change
Anew= A old * Skold/Sknew
what was used to measure absolute dose for TG43 measurements
TLD
As TLD detectors are secondary dosimeters,
their sensitivity to dose is determined by measuring their
response to a known dose delivered by a calibrated reference
beam. Since reference beam calibrations are traceable to
N I S T6 0Co and x-ray beam air kerma standards, the absolute
dose rate measurements endorsed by this report are subject to
change should the underlying air kerma standards be revised.
This qualification applies only to the 103Pd dose rate constant
value as the Monte Carlo dose rate constant values used for
1 2 5I a n d 1 9 2I r d o n o t d e p e n d o n p r i m a r y d o s i m e t r i c s t a n -
dards. Any future revisions of the air kerma standards are
expected to be small in relation to. the overall uncertainty of
the reported measurements.
historically what did TG43 accomplish?
replace the often bewildering
array of historical quantities with a single explicit output
quantity defined in terms of SI-compatible quantities and
units
can convert from old measurements to new using conversion factors that yield source strength per equivalent mass of radium, apparent activity, or reference exposure rate, and then apply TG
what is air kerma strength over reference exposure rate?
W/e (33.97 J/C)
Sk/equivalent mass of radium
exposure rate constant for Ra 226 filtered by 0.5 mm Pt * W/e
Sk/ apparent activity
exposure rate for unfiltered point source * W/e
why are I125 dose rate constants reduced by 7-10 % from historical?
previous studies were contaminated by titanium characteristic x-rays, which have significant penetration in air but not in water
sievert model
Since every commercial treatment planning system does
not permit tabular entry of two-dimensional brachytherapy
data, incorporation of the two-dimensional dose distribution
data contained in this report may be difficult or impractical
for many users. Nearly all currently available treatment planning
systems make use of the Sievert model to generate twodimensional
dose rate arrays for filtered line sources.
Because the effect of filtration is
assumed to be independent of distance (μ´ is a constant), the
Sievert model as described probably cannot accurately model
sources for which the scatter-to-total-dose ratio varies significantly
with distance
TG 43 appendix with regards to old TPS
explains how to try to adapt TG43 parameters into old calculation models
for what r does TG43 formalism using a polar coordinate system break down?
r < L/2
points are inside the source capsule
issue with eye placque brachy
the sources are placed in a nearly sphericalcup and within the target volume most of the seeds contrib-ute dose along their transverse directions. In this setup, thetarget volume receives very little dose in the longitudinaldirections of the sources
make seeds point doses and use dose rates on transverse plane alone
extrapolating phi(r)
TG43 yields dose rate functions for r< rmin
then says this is not meanginful - as none of the 1D models will yield meanginful in this case
why does phi(r) rapidly increase for r< 2L?
doesn’t exclude IS fall-off as F does
what is apparent activity?
the ac-tivity of an unfiltered point source of a given radionuclidethat has the same air-kerma strength as that of the givenencapsulated source
why did TG43 get rid of use of apparent activity?
vendors get Aapp by dividing Sk by exposure rate constant. Then user multiplied by this constant- introduces source of mistakes
-have to use same constant as manufctuer- source of errors as constants are updated
AAPM wants everything in terms of Sk
point source sievert integral
D= K * uen/p (water to air) * IS correction * phi(r)
phi(r) was for scatter and attenuation at distance r from source when compared to in vacuo
issues with sievert integral
u were mathematical best fits, not physical quantities
-did not take into account real scatter behaviour
-reasonable for 137Cs and 192Ir but 25 % error for 125I
TG 43 concept
calculate and measure dose distribution around a source
parametrize TG-43 oarameters to fit to the measurements (consencus datasets)
how is data entered into TPS?
lookup tables
fit to a math model
where to find data?
low energy sources- TG 43 and ESTRO
high energy sources- ESTRO
literature
can you use 1D anistropy function for line source?
yes, if oritentation of line source not known but still want to model source as line source
issue with lack of heterogneity corrections in TG43
high energy source: nearly same behaviour as water
low energy source: importance of PE effect increases as energy decreases
I125- difference of 9-20 % in lung vs water
predominant effect on dose
distance
issue with assumption of full scatter condition
- not true if close to skin (ex breast)
- breast dose 14-20% less than calculated
- shielding creates lack of scatter- 2-15% less dose when using shilded vaginal implants
uncertainty regarding transit dose
source entry, interdwell movements, exits
-depends on geometry, interdwell velocity, source intensity, prescribed dose
intersource effect
depends on number of sources, composition, gemoetry
prostate with many seeds- peripheral dose reduction of 6 %
tip of tandem of 137 Cs- 20% reduction
what does applicator do that isn’t accounted for in TG43?
absorb dose
could be accounted for during calibration if its always the same applicator
1-6% reduction
why is shielding used in vaginal applicators?
protect rectum, bladder, urethra
-reduction of bladder-rectum dose of 6-50%
TG 43 is itended for what line sources?
short, few mm in length
need extension for longer sources
formula for 1D anisotropy function
phi(r)= integral from 0 to pi of (D(r,theta)*sin(theta)dtheta all divded by 2D(r,theta knot)
FINAL extrapolation/interpolation guidelines per TG43 S1
F- linear-linear interpolation using 2 nearest adjacent points
for r< rmin, use F(rmin). For r> rmax, use F(rmax)
Phi(r) - log linear using adjacent data points. This differs from TG43U. Gives equation for r< rmin. For r> rmax, use Phi(rmax)
g(r)- log linear usding adjacent neighbours
- use nearest g(rmin) for r< rmin
- for r> rmax, there is equation with exponential function
- for gp, interpolate gl instead (as gp changes drastically with r), then multiply by ratio of gp/gl
where does TG 43 not work?
- inhomogeneous
- not enough scatter material
- at small r where line source approximation breaks down
- inter-source shielding effects and shielding from applicators
- orientation of seeds unknown (argue can use pt dose approximation or 1D anisotropic function)
- transit time not accounted for
Per TG43 update, where did concensus data sets come from?
For dose rate constant:
- experimental were averaged
- MC were averaged
- experimental and MC averages were equally weighted averaged together
- F and g: for most sources, taken from transformed MC data set
- The radial dose function was corrected for non-liquid water measurement medium if necessary. Assuming that the different datasets agreed within experimental uncertainties, the consensus data were defined as the ideal candidate dataset having the highest resolution,covering the largest distance range, and having the highest degree of smoothness.
what sized water phantom is TG43 based one?
spherical homogeneous water phantoms with radii of 15 cm for low energy and 40 cm for high energy
tolerance for hand calculation checks
2%
if different using TPS then examine why
Ir-192 seed design
0.6 mm diameter core of 30% Ir-70% Pt surrounded by 0.2 mm thick cladding of stainless steel - welded to source cable
- 3.5 mm long (active part) and 0.9 mm in diameter
- including inactive parts, 4.5 mm long
(0.7 mm between cable and active part)
I-125 seed design
- 5 mm long
- 8 mm in diameter
- 05 mm thick titanium wall sealed by end welds
Internally, one
model (6711) contains a silver wire of about 3 mm length
with the active material as silver iodide on the surface. This
model is available in air kerma strengths up to 6.3 U (equivalent
to an apparent activity of 5 mCi). The other model
(6702) contains the radioisotope absorbed in 3-5 tiny resin
spheres, and is available with strengths up to 51 U (equivalent
to an apparent activity of 40 mCi).
Pd-103 seed design
4.5 mm long
0.8 mm diameter
103 Pd is encapsulated in 0.05 mm thick titanium tube that is laser welded on the ends
The active material is coated
onto two graphite pellets 0.9 mm long and 0.6 mm in diameter.
Between the active pellets is a 1 mm long lead marker
that provides good source visibility on radiographs.
Ir-192 dose rate constant
1.12 chGy/hU
I-125 dose rate constant
- 93 cGy/hU (model 6702)
0. 88 cGy/hU (model 6711)
Pd-103 dose rate constant
0.74 cGy/hU
air kerma strengths of Ir-192 sources
for HDR
29000-41000 U
I-125 air kerma strength
6.3 U (6711) 51 U (6702)
Pd-103 air kerma strength
2.6 U
what type of TLD would you have to use for brachy
small because of high dose gradient
calibrated to appropriate energy
purpose of G
quantifies contribution of geometry to fall-off
Co-60 air kerma strength
15,000-18,000 U
Cs-137 air kerma strength
56 U
