Brachy dosimetry Flashcards
What is the TG43 forumlism used for?
To caclulate the dose rate in water from a brachy source
What is the equation for dose using the TG43 formulism?
D = Sk.lambda.G(r,theta)/G(r0,theta0).g(r).F(r,theta)
Why use air KERMA for the formulism?
Only 0.3% of energy is lost from interactions so KERMA = absorbed dose
How is air KEMA strength, Sk, measured?
air KERMA = coulombsx33.97/(charge on an electron x mass of air)
Measured using primary standard by measuring he output for a Brachy unit through a collimator
What is the equation for air KERMA strength?
Sk = K(r) . r^2 where K(r) is the air KERMA rate at r
What are the units for air KERMA strength?
uGym^2/hr = U
What is the equation for the dose rate constant and what are its units?
Dose to water at 1cm/unit air KERMA strength
cGy/hr/U
How is the dose rate constant determined?
MC simulations and measurements
What is the geometry function?
The inverse square law or line integral normalised to 1cm perpendicular to centre of the source long axis
This is only for simple geometries, real sources have complex 3D dose distributions, self attenuation and complexities of scatter in and out of the source at various angles and distances
What are the equations for the geometry function?
Point source: Gp(r,theta) = r^-2
Line source: Gl(r,theta) = beta/L.r.sin(theta) of theta =/= 0
Gl(r,theta) = (r^2 - L^2/4)^-1 if theta = 0
What is the radial dose function?
A curve fit off measured dose fall off along a line perpendicular to the centre of the source long axis due to attenuation and scatter
What fit is often used for the radial dose function and what is its limitation?
Mesberger fit - good over measured data but poor beyond it as the highest power term in the polynomial will dominate at large distances
Is scatter or attenuation dominant for the radial dose function for IR-192?
Initially balanced but attenuation eventually dominates
What is the anisotropy function?
The change in dose with distance and angle not accounted for by G and g
Normalised to 1 everywhere along the line perpendicular to and bisecting the source long axis
Accounts for the effects of absorption and scatter in the medium
How is the anisotropy function determined?
Use the 2D look up table from Daskalov - not TG43 as that is not clinical
Use the full anisotropy function for calculations, not the 1D average for a point source
What is the anisotropy constant?
The anisotropy function averaged over all angles and distances of clinical interest - useful for checks but not planning
What was included in the TG43 update?
Revised definition of AKS
Low energy cut-ff
In vacuo suffixes for dose rate constants
Eliminates apparent activity for specification of source strength as it is a derived quantity
Introduces errors which are difficult to calculate
Anisotropy factor
Extrapolation of data
Consensus on published data sets
More source data
Includes NIST99
Describes methods to obtain parameters
What is NIST99?
An updated method of calculating the dose rate constant using a wide angle free air chamber to measure the source strength rather tha the Ritz low energy free air chamber as this depressed the dose rate constant by 10%
How was air KERMA strength changed between NIST99 and NIST85?
It wasn’t as the dose rate constant actually used in the equation was from MC simulations and was measured by TLDs
What are the future developments for TG43?
It will be replaced by TG186 to take into account water heterogeneity. There will be model based dose calculation methods also published
How does TG43 suggest measuring the strength of a sealed source?
Use a well chamber calibrated at the NPL on a low scatter surface
Use a well chamber from RapidStrand using a special insert
use a PTW checksource using a backscatter block, strand jig and holder
Need to ensure low leakage, can measure 10 seeds in a strand and take an average to improve the SNR
What should the consistency of chambers be? What source should be used to check this?
+/-2%
A long lived source ie Cs-137
What is the Paris system?
It is a set of rules to obtain good geometric coverage and acceptable homogenieity for an interstitial implant, utilising uiform source strength distributions and is based on the reference dose and basel dose for prescription
What are the basel and reference doses?
Basel dose = mean dose in the local minima in the implant
Reference dose = 85% of the mean basel dose
What is needed for the Paris system?
Parallel, straight and equidistant wires, which are perpendicular to the plane through which the midpoints lie
Unifrom linear activity within and between wires
Volumes are equilateral triangles or squares
No obtuse angles
No rectangles
Variation of the BDR about BDR
