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Residency > 2nd Year: TG-43 > Flashcards

2nd Year: TG-43 Flashcards

1
Q

What is the purpose of TG-43?

A

Propose protocol/formalism that allows you or a researcher/manufacturer to measure dose in a medium using quantities measured only in the medium

To tabulate data, since measuring the data yourself is very difficult

To verify the tabulated measured data in water with MC calculation

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2
Q

When was TG-43, TG-43U1 and supplement to TG-43U1 published?

A

TG-43: 1995
TG-43 U1: 2004
Supplement to TG-43 U1: 2007

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3
Q

What type of data is TG-43? Model based or retrospective?

A

Retrospective

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4
Q

What are the main differences between TG-43 and TG-43U1? (5 listed)

A

TG-43U1 revised the definition of air kerma strength to include cutoff energy and specify in vacuo measurement

TG-43U1 eliminated apparent activity for source strength specification

TG-43U1 replaces the anisotropy constant with the distance-dependent 1D anisotropy function

TG-43U1 provides guidance on extrapolating TG-43 parameter data

TG-43U1 provides updated dataset that pools more literature

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5
Q

What is the main difference between the factors used in TG-43 vs other formalisms?

A

The TG-43 factors are all directly derived from dose rates in water medium near the actual source used

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6
Q

How are Ir-192, I-125 and Pd-103 produced?

A

In reactors with neutrons bombarding stable isotopes

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7
Q

What is the name of the first-order solution of the Sievert integral?

A

Modular dose rate equation

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8
Q

What does the Sievert integral aim to solve?

A

Dose calculation formalism with a non-finite shaped source distribution which comprises of infinite distribution of point sources

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9
Q

What are all of the terms in the 2D modular dose rate equation?

A

Air kerma strength (Sk)
Dose rate constant (Λ)
Geometry factor (G(r,θ))
Radial Dose function (g(r))
Anisotropy function (F(r,θ))

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10
Q

What are the units of the air kerma strength?

A

U (or cGy cm2/hr)

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11
Q

What is the equation for air kerma strength?

A

Air kerma rate multiplied by the distance squared

Air kerma rate is measured in free space, meaning it must not include effects of attenuation or scatter in a medium and must be measured at a distance much larger than source length

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12
Q

What is the TG-43 decided reference point determined?

A

1 cm away from source on transverse axis

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13
Q

How is air kerma strength measured?

A

At NIST using a wide angle free air chamber at 1 meter distance from the source

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14
Q

What are the units of the dose rate constant?

A

cGy / hr U

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15
Q

What is the equation for dose rate constant?

A

Dose rate at reference point divided by the air kerma strength of a unit air kerma strength source. NOTE: IT IS NOT DIVIDED BY THE AIR KERMA STRENGTH OF YOUR SOURCE, IT IS DIVIDED BY THE AIR KERMA STRENGTH OF AN IMAGINARY SOURCE OF EXACTLY 1 U

That is, dose rate to water at a distance of 1 cm on the transverse axis of a unit air kerma strength source in a water

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16
Q

What do you get when you multiply the dose rate constant by the air kerma strength?

A

Dose rate **in water **for your source at the reference point

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17
Q

What does the dose rate constant account for?

A

Source geometry, spatial distirbution of radioactivity in source, encapsulation, self-filtration, and scattering in water

Remember, it’s a measurement in water using the source model you care about. It accounts for everything. BUT, it is only measured at the reference point

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18
Q

What are the units of the geometry factor G(r,θ)

A

cm-2

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19
Q

What does the geometry factor account for?

A

Dependence of photon fluence around a source in free space

Variation of relative dose due only to spatial distribution of activity within the source

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20
Q

How is the geometry factor measured?

A

Trick question, it isn’t

It is very difficult to get an exact known composition of the source, so instead a line source approximation is used instead

So, the geometry factor is a mathematically derived value and does not need interpolation or datasets

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21
Q

What does the geometry factor fail to take into account?

A

Photon absorption and scattering in the source structure

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22
Q

What does the radial dose function (g(r)) account for?

A

Attenuation and scatter in the medium surrounding source in the transverse midplane of the source

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23
Q

True or False

The radial dose function only applies to the transverse axis?

A

True

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24
Q

What are the units of the radial dose function?

A

Unitless

It’s a relative quantity

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25
Q

What does the anisotropy function (F(r,θ)) account for?

A

Attenuation and scatter in medium surrounding the source outside of the transverse midplane of the source

So this is the anisotropy of dose relative to dose on the transverse axis

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26
Q

Why is the anisotropy function needed? In other words, why is the source dose distribution anisotropic?

A

Due to self-filtration, oblique filtration, or primary photon attenuation through encapsulating material, and scattering of photons in medium

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27
Q

What are the units of the anisotropy function?

A

Unitless

It’s a relative quantity

28
Q

Why is it difficult to measure dose rates in the 5 mm closest to the source? What is done to extrapolate for dat in this region instead?

A

Due to ISL, the dose rate gradient is very large near the source, so physically measuring is extremely difficult

By using the anisotropy function to suppress influence of ISL, an extrapolation to small distances can be made by extrapolating dose profiles of 10 mm to 5 mm from source

29
Q

In practice, what is the one limiting factor that can destroy the accuracy of the anisotropy function and its counterparts?

A

Angular orientation of the seed in the actual body vs TPS

Even a slight tilt will make all of the values completely off

30
Q

What assumption is made to go from the 2D dose formalism to the 1D dose formalism?

A

Dose depends only on radial distance from the source and not on the self-filtration or source obliquity

31
Q

Mathematically, how does the 2D formulation (shown below) transform to the 1D?

A
32
Q

What is the 1D countepart to the anisotropy function?

A

The anisotropy factor ϕ_an (r)

33
Q

Mathematically, what is the anisotropy factor?

A

The ratio of dose rate at distance r, averaged with respect to solid angle, to dose rate on the transverse axis at the same distance

Recall, the 1D function assumes everything is on the transverse axis, but you still need some way to account for anisotropy, so the factor will average the surrounding anisotropy and apply it to transverse axis

34
Q

What are typical values for the anisotropy factor?

A

0.91 to 0.97

35
Q

The anisotropy constant is an alternative to the anisotropy factor. Although it is not recommended for usage in the later TG-43 updates, what was the original idea of the constant in TG-43?

A

It would be a distance independent constant which averages the anisotropy factors over all distances to give a singular value

36
Q

What values are provided with datasets by the TG-43 and updated reports?

A

Dose rate constants
Radial dose function (g(r))
Anisotropy function (F(r,θ))
Anisotropy factor (ϕ_an (r) )
Anisotropy constant (not given in the later ones)

37
Q

True or false,

The anisotropy factor is directly measurable

A

False

It’s derived from the anisotropy function data tables

Remember, all it is is an average of the anisotropy function tables

38
Q

What value must the user measure themselves for their source when entering the source into the TPS?

A

Air kerma strength

That is the only thing that will change source-to-source

39
Q

True or False

For low energy brachytherapy sources, the dose rate constant is dependent on the source model?

A

True

40
Q

What isotopes are TG-43 data tables given for?

A

I-125, Pd-103 and Ir-192

41
Q

Where did the datasets for I-125 and Ir-192 come from in TG-43?

A

Monte carlo simulation varified with agreement with measured data in solid water

42
Q

Where did the datasets for Pd-103 come from in TG-43?

A

Measurement data in solid water, with conversion factor calculated from MC

But the MC did not show close agreement with the solid water, so only a conversion factor was used

43
Q

What are the approximate uncertainties of all TG-43 parameters?

A

5% for all parameters except for the geometry factor which has a 0% error (since it’s mathematically derived)

44
Q

What is the combined TG-43 uncertainty considering uncertainties of all associated parameters?

A

10%

45
Q

Draw out the shape of the dose distribution of a single brachytherapy source

A
  • Close to source is elliptical
  • Further from source is circular because source behaves as point source
  • Dip in curve close to source axis is due to oblique filtration
46
Q

Per TG-43U1, what is the minimum barrier to entry for datasets to be used as consensus data?

A

Atleast one experimental and one MC comparison in peer-reviewed literature showing agreement. Most datasets had multiple of each in agreement

Un-noisy data

Only low-energy photon-emitting sources (avg energy less than 50 keV) are considered in this update

47
Q

How were updated dose-rate constants determined in TG-43U1?

A

A weighted average of the averaged experimental values and averaged MC derived values

48
Q

In TG-43U1, which isotopes is conensus data given for?

A

I-125 and Pd-103

49
Q

True or False

In TG-43U1, it is highly recommended that users do NOT measure their own data

A

True

It is very difficult to accurately measure your own data in clinic. Thus it is much safer to rely on the conensus data instead

BUT, if for some reason you did want to, especially to publish data, the report does give recommendations on how to do it

50
Q

How does TG-43U1 recommend acceptance testing and commissioning the TPS for brachytherapy?

A

Comparison of TPS calculated dose rate-distributions with each source thta will be used clinically vs conensus data published in the reports

Distances from source in which calculations and conensus data disagree by more than 2% should be noticed and documented

51
Q

Why does the TG-43U1 report discourage use of the anisotropy constant?

A

Because the value does not agreewith either MC or measured data for r < 1 cm

52
Q

In 1999 it was discovered by NIST a 9% error in determining the air kerma strength for Pd-103. What is the recommendation of the TG-43U1 report following this discovery?

A

To keep in mind that any study/publication published prior to the discovery will cite the incorrect Pd-103 air kerma strength

53
Q

What are the two purposes of the TG-43U1 supplement?

A
  1. To provide even more consensus data for new low-energy photon-emitting sources that became available since the publication of TG-43U1
  2. To clarify information regarding interpolation and extrapolation methods used to determine datasets
54
Q

In one sentence, what is the purpose of TG-43 and the two following reports?

A

Give conensus datasets for commonly used sources and to define 1D and 2D formalisms for Ir-192, Pd-103 and I-125 (for TG-43) and Pd-103 and I-125 (for the other two)

55
Q

True or False

For our TPS, we use the TG-43 dataset for Ir-192 sources given in the original report

A

False

We use a dataset from a paper that follows the same formalism, but just has different values

56
Q

What is the change in the dose rate formalism between TG-43 and TG-43U1?

A

For the 2D formalism, there is no change

For the 1D formalism, the only change is the addition of subscripts L or P to represent if formalism is for line or point source

57
Q

Between these two 1D dose formalisms from TG-43, which is the recommended and which is the most commonly used?

A

Equation 10 is most commonly used

Equation 11 is recommended. But either is fine according to the report

58
Q

What is the purpose of each colored section, conceptually speaking?

A

Red: gives dose rate at reference point by multiplying calibrated air kerma strength by a factor that converts it to dose rate

Green: Accounts for variation of dose rate due only to spatial distribution of activity within source

Blue: Accounts for variation of dose rate due to self-filtration, obliquity filtration, encapsulating material attenuation, scattering of photons in medium on both transverse axis (g(r)), and anisotropically (F(r,theta))

59
Q

Where do we get our seed model TG 43 data?

A

From a paper that followed the TG 43 formalism to measure the factors for our seed model

60
Q

What dosimeters were used to experimentally measure TG43 data?

A

TLD is the method of choice/recommended (mainly because they have been validated with MC comparison)

Diodes (have also been validated with MC comparison and are useful for relative dosimetry)

diamond detectors, miniature ion chambers, plastic scintillators, film, polymer gels, and chemical dosimeters can also, and have been used. But their validity in measuring TG-43 parameters has not been completely validated.

61
Q

What are some known sources of uncertainty to using TLDs for TG43 dosimetry? (5 listed)

What is the biggest source of uncertainty?

A

A) TLD dose calibrations
B) Repetitive measurement inconsistency
C) LiF energy correction (the biggest source of uncertainty)
D) Measurement medium correction factor
E) Seed/TLD positioning

62
Q

What are some known uncertainties to using MC for TG43 dosimetry? (5 listed)

A

A) Statistical limits
B) Photoionizations
C) Cross sectional data
D) Seed geometry
E) Source energy spectrum

63
Q

Since neither TLDs nor MC are perfect for measuring TG43 data, what is the LIBD recommendation?

A

Perform an equally weighted average of the average measured using TLDs, and the average calculated using MC

Each method has complementary strengths and weaknesses

64
Q

For the paper we base our Ir-192 TG43 tables off of…

What is the point of the paper?

Is the data MC or measured data?

A

What is the point of the paper?
Give consensus data for high energy brachytherapy seed source models

Is the data MC or measured data?
One or the other, or both. This is because the source is deemed as “conventional encapsulated” similar in design to an existing or previously existing one such that a comparison to that study can be made. They don’t specify which of the three options is used for our source specifically though.

65
Q

Why is a very small ion chamber (Ex. A16), not used for TG43 data measurement?

A

Because you need a high sensitivity detector due to the low dose rates far from source

For this reason, TLDs are preferred

66
Q

True or False

On the transverse plane, the anisotropy function = 1

A

True

The function is relative to the dose on the transverse plane for that depth.

Radial dose function accounts for distance from active length
Anisotropy function accounts for angle (so should therefore be unity whenever it’s on the transverse axis)

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