TG43 Flashcards

Question

interpolation for F(r, theta)

Answer 1

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)

Answer 2

log-linear

Answer 3

unity | for all angles except theta 0

Answer 4

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.

Answer 5

log-linear | -base it on 2 immediately adjacent points

Answer 6

- recommended equation for r< rmin | - linear for r> rmax

Answer 7

- 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

Answer 8

log-linear | using points immediately adjacent to the radius of interest

Answer 9

zeroth orer for r< rmin log linear for r> rmax

Answer 10

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

Answer 11

low energy source

Answer 12

anisotropy decreases as you go "away" and also decreases as you go "along" the source

Answer 13

gamma for high enery source | x-ray for low energy source

Answer 14

- 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.

Answer 15

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

Answer 16

any encapsulated radioactive material used for brachy

Answer 17

dose distribution is assumed to be radially symmetric at a given radial distance r

Answer 18

ra-dioactivity is assumed to be uniformly distributed along a 1 D line-segment with active length L

Answer 19

cylindrical source with Leff < 0.5 cm

Answer 20

cylindrically symmetric with longitudinal axis | also symmetric with transverse plane, but latter ca be accounted for if needed

Answer 21

exactly the measured orMonte Carlo-derived dose rates from whichg(r) andF(r,u)tables were derived as long as geometry function used consistently

Answer 22

averaged experimental data are averaged with MC data

Answer 23

interstitial brachy

Answer 24

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

Answer 25

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

Answer 26

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

Answer 27

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

Answer 28

air kerma strength

Answer 29

dose rate constant

Answer 30

geometry factor

Answer 31

radial dose function

Answer 32

radial= depth dependence along transverse axis | anisotropy: anisotropy of dose relative to dose on transverse axis

Answer 33

when better measurements are obtained they are easy to update

Answer 34

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

Answer 35

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

Answer 36

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.

Answer 37

degree of dose anisotropy around | single sources is limited

Answer 38

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

Answer 39

radial dose functions from solid water were fitted to a polynomial series expansion

Answer 40

measured in solid water medium 2-D dose distributions

Answer 41

measurements or MC | or measurements converted to measuement in water using MC

Answer 42

5 % | 3% in dose determination and 3 % in measurement of air kerma strength used in determination of A

Answer 43

5 % 3% in each of the dose rates in the ratio similar for radial function

Answer 44

unlike g(r) and F(r,theta), it is a mathematical function with minimal uncertainty

Answer 45

10% | add uncertainty of A, g(r), F(r,theta), air kerma strenght (5%) in quadrature

Answer 46

calculate dose rates at various points using point source approximation and compare results with those in tables

Answer 47

Anew= A old * Skold/Sknew

Answer 48

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.

Answer 49

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

Answer 50

W/e (33.97 J/C)

Answer 51

exposure rate constant for Ra 226 filtered by 0.5 mm Pt * W/e

Answer 52

exposure rate for unfiltered point source * W/e

Answer 53

previous studies were contaminated by titanium characteristic x-rays, which have significant penetration in air but not in water

Answer 54

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

Answer 55

explains how to try to adapt TG43 parameters into old calculation models

Answer 56

r < L/2 | points are inside the source capsule

Answer 57

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

Answer 58

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

Answer 59

doesn't exclude IS fall-off as F does

Answer 60

the ac-tivity of an unfiltered point source of a given radionuclidethat has the same air-kerma strength as that of the givenencapsulated source

Answer 61

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

Answer 62

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

Answer 63

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

Answer 64

calculate and measure dose distribution around a source | parametrize TG-43 oarameters to fit to the measurements (consencus datasets)

Answer 65

lookup tables | fit to a math model

Answer 66

low energy sources- TG 43 and ESTRO high energy sources- ESTRO literature

Answer 67

yes, if oritentation of line source not known but still want to model source as line source

Answer 68

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

Answer 69

- 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

Answer 70

source entry, interdwell movements, exits | -depends on geometry, interdwell velocity, source intensity, prescribed dose

Answer 71

depends on number of sources, composition, gemoetry prostate with many seeds- peripheral dose reduction of 6 % tip of tandem of 137 Cs- 20% reduction

Answer 72

absorb dose could be accounted for during calibration if its always the same applicator 1-6% reduction

Answer 73

protect rectum, bladder, urethra | -reduction of bladder-rectum dose of 6-50%

Answer 74

short, few mm in length | need extension for longer sources

Answer 75

phi(r)= integral from 0 to pi of (D(r,theta)*sin(theta)dtheta all divded by 2D(r,theta knot)

Answer 76

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

Answer 77

- 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

Answer 78

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.

Answer 79

spherical homogeneous water phantoms with radii of 15 cm for low energy and 40 cm for high energy

Answer 80

2% | if different using TPS then examine why

Answer 81

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)

Answer 82

4. 5 mm long 0. 8 mm in diameter 0. 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).

Answer 83

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.

Answer 84

1.12 chGy/hU

Answer 85

0. 93 cGy/hU (model 6702) | 0. 88 cGy/hU (model 6711)

Answer 86

0.74 cGy/hU

Answer 87

for HDR | 29000-41000 U

Answer 88

``` 6.3 U (6711) 51 U (6702) ```

Answer 89

small because of high dose gradient | calibrated to appropriate energy

Answer 90

quantifies contribution of geometry to fall-off

Answer 91

15,000-18,000 U