First Year Exam: TG-51

Question 1

What is the approximate difference in measured output between TG-21 and TG-51?

Answer 1

TG-51 output is about 1% discrepancy relative to TG-21 output

But for electrons, it can be more at d_max than 1%, because it TG-39 gave some good improvements to further better improve the accuracy for plane-parallel chambers

Question 2

True or False

P_ion can never be > 1?

Answer 2

False

Pion can never be < 1 is the proper answer

Question 3

What does it mean if P_ion is < 1?

Answer 3

The voltage itself is ionizing the air and giving you a false signal

Question 4

What is the limit for P_ion?

Answer 4

P_ion < 1.05

Question 5

Why can you not use TG-51 for <= 4 MeV electron beams?

Answer 5

Because you need to measured at dmax for TG-51, and that’s too close to surface to measure for the lower energy electron beams

Question 6

Why are we allowed to measure dose/MU at 10 cm depth (photons) or d_ref (electrons) when we do TG-51 in our clinic? Why aren’t we doing D_max?

Answer 6

Because we can just divide the measured Dose/MU by PDD to get the Dose/MU at dmax

Question 7

True or False

You can calculate k_Q using either a 100 SSD or 100 SAD setup?

Answer 7

False

The tables in TG-51 only give k_Q for SSD setups. So it doesn’t matter whether you calibrate for SSD or SAD setup, you much find k_Q using an SSD setup regardless

Question 8

Why is TG-51 titled protocol for clinical reference dosimetry of high energy photon and electron beams instead of calibration?

Answer 8

Because TG-51 allows us to tweak the beam

The actually calibration was performed at the ADCL where the chamber was exposed to a standard Co-60 source

Reference dosimetry means we are tweaking a beam relative to the reference, that being a NIST traceable calibration

Question 9

How does NIST figure out an exact dose to chamber for the standard Co-60 beam?

Answer 9

The Co-60 beam is exposed to a standard chamber of known calibration

The known calibration was determined by measuring the standard chamber response using a NIST traceable water calorimetry experiment

Thus, NIST first determined dose to the NIST chamber, then correlates the reading to an equivalent dose to your chamber at the same dose, which is why a calibrationf actor is in units of Gy/C

Question 10

What is the EPOM shift for photons and electrons?

Answer 10

0. 6r_cav for photons
1. 5r_cav for electrons

Question 11

In theory, the effective point of measurement for both electron beams and photon beams is 0.85r_cav upstream of the chamber’s central axis? In reality, however, the shift is not as large? Why is this?

Answer 11

0.85r_cav is assuming that the entrance of the beam to the chamber only dependents on unscattered, direct beam radiation to the surface of the chamber

The reality is that backscatter and lateral scatter shifts the average a bit closer to the center

Question 12

Why is the shift from central axis to EPOM smaller for electrons than for photons?

Answer 12

Because electrons tend to scatter more lateral, thus moving the average a little bit closer to the central axis of the chamber, compared to photons that scatter more forward than electrons do

Question 13

Why for photon beams are we allowed to directly measure a %DD curve, but in electron beams we don’t?

Answer 13

To go from %Ionization curve to %DD curve, you need to multiply by the restrictive stopping power ratio of water to air and depth to dmax

For photons, this ratio stays roughly constant as a function of depth (within 0.1% beyond d_max, so %DD = %I

For electrons, the average electron energies change vs depth, thus the restrictive stopping power also changes vs depth, so %DD = %I * Restrictive stopping power ratio

Question 14

What is standard temperature, pressure and relative humidity?

Answer 14

T = 22 celsius

P = 101.33 kPa or 760 mmHg

Relative humidity is anywhere from 20% to 80%

Question 15

What is the equation for P_TP?

Answer 15

P_TP = (273.2+T) / (273.2 + 22) * (101.33/P)

or

P_TP = (273.2+T) / (273.2 + 22) * (760/P)

Question 16

What does P_TP actually account for?

Answer 16

Variation in the amss of gas in the chamber volume as temperature and pressure variations cause expansion and contraction of air

Question 17

What does P_ion measure? What is it a function of and why? (two things)

Answer 17

Accounts for ion recombination within chamber volume before the ions are collected by the electrodes

Function of dose rate (changes density of the charge cloud) and chamber bias (changes speed of ion collection)

Question 18

What is the equation for P_ion for a pulsed beam (as would be used in Linac)? What about continuous.

If you don’t remember, then atleast state what measurements you need to take in order to measure P_ion?

Answer 18

For Pulsed: P_ion = (1 - V_H/V_L) / (M_H/M_L - V_H/V_L)

For continuous: Just square the ratio of voltages terms

Need to measure voltage and raw readings for low and high voltage

Low and high voltages are usually 150V and 300V respectively

Question 19

How does P_ion change as a function of dose per pulse?

Answer 19

Increases as dose per pulse increases

Question 20

Does P_ion increase or decrease for increasing energies?

Answer 20

P_ion increases as energy increases

Question 21

Is P_ion typically higher or lower for electrons compared to photons?

Answer 21

P_ion is higher for electrons, than for photons

Question 22

What does P_pol account for?

Answer 22

Changes in collection efficiency due to beam quality, cable positioning, and chamber bias

Question 23

What is the tolerance for P_pol?

Answer 23

+- 0.4% from 1.000 and within 0.5% between all energies

Question 24

How do you measure P_pol?

If you don’t remember the equation, atleast say what values need to be measured.

Answer 24

P_pol = (M⁺_raw - M^-_raw) / 2*M^standard_raw

Where M^standard_raw is just whatever you normally use, either pos or negative. We usually use positive.

Need to measure raw reading at positive and negative. Same absolute voltage, different signs.

Question 25

How does P_elec correct reading?

Answer 25

It scales reading of the electrometer to true coulombs

Question 26

How do you measure P_elec?

Answer 26

Typically you don’t. It’s measured at NIST/ADCL

If it’s calibrated with the ion chamber, then whatever correction factor the ion chamber gets will already account for P_elec, so you assume P_elec = 1. If they’re calibrated separately, typically the electrometer measured charge of a known circuit, and the P_elec is a correction based off the ratio of known charge to read charge

Question 27

What does P^Q_gr account for?

Answer 27

Difference in the effective point of measurement versus the point of measurement during output measurements?

Question 28

How do you measure P^Q_gr for photons and electrons for cylindrical chambers?

Answer 28

For photons you don’t have to measure it, because it’s already accounted for in k_Q

For electrons you have to measure it as follows…

P^Q_gr = M_raw(d_ref+0.5r_cav) / M_raw(d_ref)

You need to know d_ref, r_cav and M_raw upstream and at d_ref

Question 29

Why do you have to shift a PDD curve, but yet when you take your reference dosimetry, you’re allowed to center the chamber either at d_ref or 10 cm?

Answer 29

Because the k_Q (for photons) or the P^Q_gr (for electrons) already effectively take into account the shift

Question 30

In general, is P_ion larger for pulsed or continuous radiation sources?

Answer 30

Pulsed

Question 31

If P_pol exceeds tolerance threshold, how do you go about accounting for it?

Answer 31

You divide the N_D,W^Co-60 value by P_pol that was found from time of calibration, and you use that new value in place of N_D,W^Co-60

Question 32

How do you use a chamber that differs from those specified in TG-51?

Answer 32

Simply find the closest matching chamber to the list of chambers that they provide

Question 33

What 4 values determine if your chamber is similar enough to one of the TG-51 chambers?

Answer 33

1. Chamber wall material
2. Radius of cavity
3. Central electrode material
4. Wall thickness

Question 34

What should a waterproofing sleeve be made out of?

Answer 34

PMMA

Question 35

What setup is required for %DD or %I curves?

Answer 35

100 cm SSD, 10x10 cm² field (or 10x10 cm² cone)

Shift using EPOM

Question 36

What are the differences between the following?

%DD(10)

%DD(10)_X

%DD(10)_Pb

Which of these three values do you want as your end result?

Answer 36

%DD(10) - PDD at 10 cm including electrom contamination and all

%DD(10)_X - PDD at 10 cm not including electron contamination

%DD(10)_Pb - PDD at 10 cm with 1 mm of lead in the beam placed 50 cm from the phantom surface (or 30 cm if LINAC doesn’t permit 50 cm)

In the end, you want %DD(10)_X for calculating k_Q

Question 37

Why for photon beams of 10 MV or greater do contaminant electrons need to be removed according to TG-51? Why not for energies < 10 MV?

Answer 37

Contaminant electrons cause an increase in the dose at d_max and a subsequent decrease of %DD(10)

For enegries < 10 MV, the contaminant electrons have a short range so don’t have as much of an effect

Question 38

What is the purpose of the lead foil?

Answer 38

Remove contaminant electrons and introduce a known quantitiy of electrons into the beam

Question 39

How do you find %DD(10)_X?

Answer 39

For E < 10 MV, %DD(10) = %DD(10)_X

For E >= 10 MV, or for FFF beams, you use a lead foil to find %DD(10)_Pb then use analytical equations to convert from that value to %DD(10)_X

Question 40

True or False

For photon output measurements you are allowed to measure in either 100 cm SSD or 100 cm SAD?

Answer 40

True

Just make sure if you have a SSD setup, you use PDD to scale final reading to desired depth, and for SAD setup, you use TMR to scale final reading to desired depth

Question 41

In general, what is greater pas d_max, PDD or TMR? Why?

Answer 41

TMR

Because in TMR, the depth in the phantom is the only thing that changes, distance from source remains the same. In PDD, botht he depth AND the distance from source change as you scan. Meaning, the difference between Dose at max vs dose at a deeper depth is larger for a PDD setup (changes more because of the two factors) than for a TMR setup.

Question 42

How is d_ref determined?

Answer 42

d_ref = 0.6*R₅₀ - 0.1

Question 43

How is R₅₀ calculated?

Answer 43

By using analytical expressions from TG-51 to convert from I₅₀ to R₅₀. These expressions take the form of linear equations

Question 44

Why for electron measurements is a plane parallel chamber always preferred?

Answer 44

In order to limit variations in P_ion, P_pol and P^Q_gr as a function of depth

Question 45

For what energies is a plane parallel chamber preferred for electrons? For what energies is it required.

Answer 45

Preferred for <= 10 MeV

Required for <= 6 MeV

But most people usually use cylindrical chambers for 6 MeV anyway

Question 46

For electrons, what three correction factors replace k_Q?

Answer 46

P^Q_gr

k’_R50

k_ecal

Question 47

What is k_ecal?

Answer 47

A function that takes your chambers calibration factor (which is derived for photons) and applies it to electrons

This factor is chamber specific and constant value

It takes the ADCL calibration factor, and applies it to an arbitary electron beam energy with arbitary beam quality, and is also chamber specific. It’s a singular value because it onlyc ares about the chamber, not the quality of any beam because it just assumes something arbitrary.

Question 48

What is k’_R50?

Answer 48

A factor that accounts for variations in readings due to electron beam quality

It converts from an arbitary electron beam, to your electron beam quality and is a function of R₅₀

Question 49

How do you meausre k’_R50 and k_ecal?

Answer 49

k_ecal is found using a look up table in TG-51

k’_R50 is plottes vs R₅₀. So first find R₅₀, then use the lookup table or an analytically derived fit function

Question 50

Where is your k’_R50 defined?

Answer 50

It’s only defined/valid for measurements taken at d_ref

So it’s derived using R₅₀, but it’s only allowed to be applied to readings at d_ref

Question 51

What are the 5 goals of the TG-51 addendum?

Answer 51

1. Update k_Q tables to include more chambers and models so we can do TG-51 with not just farmer chambers
2. Provide guidance on what makes a “suitable” chamber for reference dosimetry
3. Provide guidance for using TG-51 for new tech (Ex. FFF beams)
4. Discuss more details about the determination of N_D,W
5. Disucss uncertainties present throughout TG-51 and how each part contributes to a final combined uncertainty (establishing an uncertainty budget)

Question 52

What major feature is left out of the TG-51 addendum?

Answer 52

The addendum only talks about photons. It gives no additional guidance for electrons

Question 53

According to the addendum, what 6 features must a chamber have in order to be an “appropriate” reference chamber?

Answer 53

1. Needs to have chamber stabilization capabilities (time required to get equilibrium reading is not too long and that reading from non-equilibrium to equilibrium states are not too drastically off)
2. Should have reasonable leakage current (<0.1% for no correction factor, allowed up to 0.5% if you do have a correction factor to apply. So P_leak < 0.5%, and if < 0.1% then P_leak = 1)
3. Minimal polarity correction (P_pol < 0.4%). and correction shouldn’t change across energies by more than 0.5% (relatively stable vs energy)
4. High degree of chamber stability (Change in calibration coefficient over 2 years should be < 0.3%)
5. Well understood initial and general P_ion
6. Is not small volume (having volume < 0.05 cc) or high-Z

Question 54

What are the following recommendations that the addendum makes for…

Chamber equilibration

P_leak

P_pol

P_ion

Stability

Answer 54

Chamber equilibration: < 0.5% from first irradiation to equilibrium readings

P_leak < 0.1% of chamber readings

P_pol: 1+-0.4% and < 0.5% variation over all photon energies (meaning, if 6X you get 0.997 but 10X you get 1.003, technically they’re both within 1+-0.4%, BUT, the difference between them is 0.6% so it fails

P_ion: Should vary linearly with dose rate, C_init < 0.002, polarity dependence < 0.001

Stability: Calibration cerificates < 0.3% variation year-to-year

Question 55

What does the addendum say about the 0.6r_cav correction?

Answer 55

For now, it’s fine to continue using it, BUT, it has been shown since TG-51 that the correction is not actually true, and that ti dependents on the length of the cavities and central-electrode diameter. So expect in a future addendum that the recommendation for shift becomes chamber specific.

Question 56

What changes to k_Q values did the addendum make?

Answer 56

TG-51 originally used a semi-analytical approach to calculate k_Q values

The addendum used monte-carlo, which is more precise, to calculate k_Q

The addendum values agree within 0.5% of the original values

Experimental k_Q values were collected and showed excellent agreement with MC

Question 57

Approximately how much of an error in k_Q occurs when a change in %dd(10)_X of 1% is measured?

Answer 57

Change in %dd(10)_X of 1% yields a change of ~ 0.15% in k_Q

Question 58

When was TG-51 published?

When was the TG-51 addendum published?

Answer 58

TG-51: 1999

Addendum: 2014

Question 59

True or False

The addendum provides a graph of k_Q for chamber and beam quality using analytical data fit

Answer 59

False

The addendum provides a table. That table has a bunch of difference detectors, and gives 5 k_Q values for each detector depending on the %dd(10)_X (63, 67, 73, 77, 81%). The data is found using monte carlo for the addendum.

Question 60

What is the major flaw in the conversion from %dd(1) to %dd(10)_X equation from TG-51?

Answer 60

The quation is only defined for flat fields, not FFF

Question 61

What uncertainty does using the %dd conversion equation introduce for flattened fields?

Answer 61

0.2% error, so the addendum recommends you can still continue to use the equation for flattened fields, but you should account for the error in your “budget”

Question 62

What recommendation does the addendum make regarding lead foil and FFF beams?

Answer 62

The lead foil should still be used for FFF beams. The equation is not suitable

Question 63

Summarize the recommendations to calculate %dd(10)_X for the following energies…

6X

6FFF

10X

10FFF

15X

Answer 63

6X - you may measure %dd(10). No conversion needed. This is approximate = to %dd(10)_X

6FFF - you should use a lead foil, measure %dd(10)_Pb, then use conversion to get to %dd(10)_X

10X - You can use the equation if you want. It’s a 0.2% error, so just keep that in mind. Or you can use the lead foil and convert to %dd(10)_X.

10FFF - you should use a lead foil, measure %dd(10)_Pb, then use conversion to get to %dd(10)_X

15X - You can use the equation if you want. It’s a 0.2% error, so just keep that in mind. Or you can use the lead foil and convert to %dd(10)_X.

Question 64

Summarize the lead foil recommendations from TG-51 and the addendum

Answer 64

If energy < 10 MV, and beam is not FFF, you don’t have to use foil or any extra conversion

If beam is FFF, you have to use foil and convert to %dd(10)_X using equation

If beam is >= 10 MV and is flat, you can either use a lead foil or not use one. But if you decide not to use one, account for the 0.2% error since %dd(10) is not exactly equal to %dd(10)_X. If you do use the foil, measure %dd(10)_Pb and conver to %dd(10)_X using equation

Question 65

What two correctionf actors does the addendum introduce?

Answer 65

P_rp and P_leak

Question 66

what is P_rp and how do you measure it?

Answer 66

It’s a factor that takes into account radial asymmetry of a beam resulting in differential fluence being delivered to the reference chamber

For flattened beams - you can do a 1D integration along the length of the chamber and ratio the central value quantity to average value

For unflattened beams - you do the same but using a 2D integration (likely with film). Take the average and do central value quantity / average value

Question 67

How do you determine P_leak?

Answer 67

If < 0.1%, you can just ignore it and call P_leak = 1

If < 0.5% and > 0.1%, it’s tolerable, but you need to apply a non-unity P_leak

Question 68

What three requirements are very specific to FFF beams that aren’t as applicable to flattened beams?

Answer 68

1. You need to be extra sure that P_ion < 1.05 (since the higher dose per pulse makes that more challenge to accomplish)
2. You need to be more thorough with calculating your P_rp since it’s now more important
3. You must use lead foil for flattened beams

Question 69

According to the addendum uncertainty budget, what is the approximate combined uncertainty you can expect from the entire TG-51 process?

Answer 69

~ 1%

Question 70

What is the minimum size ion chamber you’re allowed to use for reference dosimetry?

Answer 70

0.05 cc

Question 71

What does the addendum say about ion chambers and Z dependence?

Answer 71

Don’t use ion chambers with high-Z electrodes

Question 72

What does the addendum say about the use of plane parallel chambers for photon-reference dosimetry? What about electron?

Answer 72

Photons: DO NOT USE PLANE PARALLEL CHAMBERS, only cylindrical

Electrons: The addendum doesn’t talk about electrons

Question 73

Does the addendum lean more trowards using or not using lead for flattened photon beams?

Answer 73

It actually leans towards not using lead, and instead just adding the 0.2% uncertainty to the budget

Question 74

Which k_Q values should you use in your clinic? The ones from the addendum, from original TG-51, or it doesn’t really make a big difference?

Answer 74

The addendum

Cause they’re more accurately derived (MC), and align better with experimental results

Question 75

What is the maximum voltage you’re allowed to use for reference dosimetry?

Answer 75

300 V

Question 76

How often should you determine an uncertainty budget?

Answer 76

Everytime your procedure changes or you use different equipment

Question 77

What point do you perform your reference dosimetry output calculation for photons and electrons?

Answer 77

For photons: Always 10 cm depth

For electrons: Always d_ref

Question 78

Why for electrons do we measure reference dosimetry at d_ref and not R₅₀ or d_max?

Answer 78

R₅₀ is in a range of rapid fall-off, so setup uncertainty is much greater

d_max is in a region with significant scatter electron contribution at higher energy fields

d_ref is the perfect balance where there is less scatter electron contribution, and it’s still in the relatively flat region so setup uncertainty doesn’t ruin measurements too much

Question 79

Why in the addendum is it PREFERRED to use the correction formula to go from %dd(10) to %dd(10)_X even for higher energy MV beams, instead of just using lead?

Answer 79

Placement of lead can introduce uncertainty in its own way, and it adds extra complexities. If you use the correction formula, yes you may be less accurate, but the accuracy is actually lower than what was previously expected. The benefit from using the lead foil is small.

TG-51 sites a maximum uncertainty in using the formula for higher energy photon beams of 0.4%

The addendum sites a max uncertainty in using the formula for higher energy photon beams of about 0.2%. It’s not really worth it to use the foil. It will improve uncertainty, but barely.

Question 80

What is the range of photon energies that you may use TG-51 for?

Answer 80

Co-60 - 50 MV

Question 81

What is the full equation for photon D^Q_W using all of the correction factors?

Answer 81

D^Q_W = k_QP_ionP_elecP_TPP_polP_rpP_leakM_raw

Question 82

What is the actual meaning of %dd(10)_X?

Answer 82

It’s the photon component of the percentage depth dose at 10 cm depth for a field size of 10x10cm². This quantity either assumes no electron contamination (as is approximately the case for < 10 MV beams), or it has already factored out the electron component (as is the case of applying the equations)

Question 83

At what energy electron beams does the cone size need to increase from 10x10 to 20x20, and why is this?

Answer 83

For electron energies > 20 MeV

This is because charged particular equilibrium is lost if you continue to use 10x10cm² past 20 MeV

Question 84

What setup is electron clinical reference dosimetry performed with?

Answer 84

Field size >= 10x10 cm²

An SSD value somewhere between 90 and 110 cm (usually 100 cm)

Question 85

How is TG-51 a simplification from TG-21? (Two main benefits)

Answer 85

1. We don’t need all the large stopping-power ratio and mass-energy absorption coefficient tables anymore
2. We don’t need to calculate any theoretical dosimetry factors anymore

Question 86

TG-51 was the 3rd generation of reference dosimetry protocols. TG-21 was the 2nd. What was the major error with the very first generation?

Answer 86

The very first protocol was overly simplified, and was found to lead to errors in beam calibrations of up to 5%

TG-21 fixed these issues, but at the expense of making things overly complex, especially for chamber-dependent factors.

Question 87

What are the TG-51 requirements on phantoms that are appropriate to use?

Answer 87

For annual you MUST use water phantom of dimensions atleast 30x30x30 cm³

For routine and more frequent QA checks, you can use plastic materials (such as solid water) as long as you have a transfer factor (fudge factor) that was established relative to your water measurement

But reference dosimetry measurements in plastics and water-equivalent plastics are not allowed at all. It’s fine for QA, but you shouldn’t be adjusting anything off of it.

Question 88

What are the similarities and differences between %dd(10) and %dd(10)_Pb

Answer 88

Similarities: They are both directly measured and do not require any equation to get to them. Additionally, they both contain an electron component

Differences: One has lead placed, the other is open field. %dd(10)_Pb has a predictable electron component, %dd(10) is not as predictable

Question 89

What is the major difference between %dd(10) and %dd(10)_Pb vs %dd(10)_X?

Answer 89

To find %dd(10)_X you first start at one of the other two, then apply a correction. Also %dd(10)_X does not include the electron component of the photon beam, only the photon component

Question 90

What are the units of D^Q_W? What is the approximate magnitude?

Answer 90

Gy/C

_____ x 10⁷ typically

Question 91

Conceptually speaking, what is k_Q? Why is it actually necessary?

Answer 91

It’s a quality conversion factor that accounts for the change in the absorbed-dose to water calibration factor between the beam quality of interest, Q, and the beam quality for which the calibration factor was first measured (Co-60)

Remember: the correction factor you get for ADCL/NIST only applies for a Co-60 unit. It doesn’t actually apply to your machine. So you need a value to convert the correction factor to your machine. That’s what k_Q does

Keep in mind then, if you’re doing reference dosimetry with TG-51 for a Co-60 unit, k_Q = 1.000

Question 92

What is k_Q for an electron beam broken down into? Why is this necessary?

Answer 92

k_Q = k_R50P^Q_gr

It’s broken into two components, a gradient corrected and non-gradient corrected component. This is necessary because the gradient correction for electron beams is so variable, that it actually changes machine-to-machine. So you need one specifically for your Linac of interest.The k_R50 does not change as much can be found in look up tables in TG-51

Question 93

What are the two components of k_R50? Why are they necessary?

Answer 93

k_R50 = k’_R50k_ecal

k_ecal is used to convert the ADCL calibration factor from photon to some arbitrary electron beam

k’_R50 is needed to convert from an arbitrary electron calibration factor, to one that matches your machine’s beam quality

Question 94

Is N_D,W use dto measure absorbed dose at the chamber’s point of measurement in the absence of or with the chamber in place?

Answer 94

Obviously in the absence of a chamber. That’s the whole point of reference dosimetry

Otherwise why would you need a correction factor for something that you’re measuring directly anyway?

Question 95

In general, if a factor is labeled ‘k’ with anything following it, what is it doing?

Answer 95

Converting the absorbed-dose calibration factor from one quality to another

Question 96

In general, what are any of the ‘P’ correction factors responsible for?

Answer 96

Correcting for equipment or environment discrepencies

Question 97

True or False

There are two independent equations to convert from %dd(10)_Pb to %dd(10)_X. One is for foil at 50 cm +- 5 cm, the other is for foil at 30 cm +- 1 cm.

Answer 97

True

Question 98

How do you convert from a measured I₅₀ value, to your wanted R₅₀ value?

Answer 98

Use the conversion equations in TG-51 (equations 16 and 17)

Question 99

What three options are there to measure R₅₀?

Answer 99

1. Measure using diode detector directly (but then your diode detector needs to be well established)
2. Measure I₅₀ directly, then use conversion equation in TG-51
3. Measure the %I curve, then convert all of it to %dd curve (TG-25 method, but the general way to do this is an equation that tells you stopping power ratios as a function of R₅₀ and depth from a Burns et al paper)

Question 100

What photon beam would in general be hotter? One tweaked to give 1 cGy/MU at reference condition in SSD setup, or one in SAD setup?

Answer 100

Trick question

They both give 1 cGy/MU at d_max

For the SAD setup, the D₁₀ will be higher, but then it just gets divided by TMR which is higher anyway (because the chamber is closer to the source). For a SSD setup, D₁₀ is lower but it just gets divided by PDD which is lower anyway

Question 101

In reality, you can perform clinical reference dosimetry calculations at any depth that you want. Why is this?

Answer 101

Because the only difference would be applying your PDD or TMR to get you to the actual point that you want to make 1 cGy/MU

Question 102

Give the equation for dose to water from an electron beam

(use M_corrected instead of M_raw multiplied by all the correction factors)

(also expand the k_Q factor)

Answer 102

D^Q_w = MP^Q_grk’_R50k_ecalN^60Co_D,w

Question 103

What process is used to avoid obtaining a absorbed dose in water calibration factor from ADCL for plane-parallel chambers? Give a general overview of how it works

Answer 103

Cross Calibration

Your end result that you want to find is (k_ecalN^60Co_D,w)^pp = (D_w)^cyl / (Mk’_R50)^PP

Where the numerator term is just your dose measured using a cylindrical chamber (with all corrections applied), and your denomenator is as it appears.

Question 104

True or False

TG-51 and addendum can be applied to Tomo, GK and CyberKnife?

Answer 104

False

They are not capable of the setups specified in the report (100 SSD or SAD and 10 x 10 cm² fields)

Question 105

True or False

Plane parallel chambers may be used for photon reference dosimetry

Answer 105

True

But just keep in mind, they won’t be anywhere as stable as a farmer chamber or a most cylindrical chambers. Just expect their performance won’t be as good. But yes, if you absolutely wanted to and were willing to account for the added uncerataint, you can use certain models of plane parallel chambers

Question 106

Why is having high-Z electrode such an issue for photon beams?

Answer 106

They’e mainly an issue with FFF beams, where they are not well specified by %dd(10)_X

Question 107

The addendum gives k_Q equation for %dd(10)_X between 63 and 86. What happens if you’re below 63%?

Answer 107

Lineraly interpolate from a k_Q given %dd(10)_X = 63%, to that of a Co-60 unit, k_Q = 1.000 at %dd(10)_X = 58%

Question 108

When must you perform a full TG-51?

Answer 108

Annual and commissioning

Question 109

Why are small voluem chambers not recommended for TG-51?

Answer 109

Anomalous recombination behavior

Large polarity effect

Complex effects from high-Z electrodes

Just overall instability

Question 110

What does a fudge factor do?

Answer 110

Converts from dose in plastic to dose in water

Question 111

Where do the dosimetric uncertainties for each item in the addendum uncertainty budget come from?

Answer 111

TG-142

This assumes your machine is within the tolerances of TG-142

Question 112

Why do we usually only do a temperature and pressure correction, and not a hmidity correction?

Answer 112

Because humidity in most situations in the US and Canada only leads to an error of 0.15% at the absolute maximum. It’s small enough to ignore

Labs usually give a range of relative humidity for which the calibration coefficient is valid, and this range is very large (20-80% but ideally 40-60%)

Question 113

What uncertainty k value do ADCL calibrations report? What about the recommended uncertainty budget?

Answer 113

Uncertainty budget recommends k = 1

ADCL reports k = 2

You would need to convert from k = 2 to k = 1 to get the uncertianty of the calibration coefficient

Question 114

What does the stability (or time needed to get to equilibrium) (or chamber settling) of an ion chamber depend on?

Answer 114

Irradiation History

How long its been since it was last used

Whether polarity is being changed back and forth or not

Question 115

If k_Q values are calculated using montecarlo and taking into account the exact build of a chamber and the flatness and symmetry of the field, and would therefore already account for volume averaging, then why do we need a P_rp factor at all?

Isn’t this redundant?

What uncertainty occurs from using both the implicit volume averaging correction in k_Q and the P_rp?

Answer 115

That’s because it doesn’t actuallya ccount for the correct flatness and symmetry. It assumes a relatively much more uniform profile than what is typically found in most LINACs

You have an error of at most 0.05% if you use both the k_Q and P_rp. Very insignificant. That’s why you always also measure the P_rp

Question 116

What is the chamber settling tolerance recommendation given by the addendum for a suitable ion chamber?

Answer 116

Less than a 0.5% change in chamber reading per monitor unit from a beam-on for a warmed up machine, to stabilization of the ionization chamber

So unstable to stable shoudl have a change in reading less than 0.5%

Question 117

What is the restricted mass stopping power?

Answer 117

Quantity used to calculate dose to a medium with the restriction that only interactions below a certain cutoff energy are used in calculating dose

Reasoning: electrons of higher energy will travel away from site and contribute dose non-locally

Question 118

What affect does not accounting for electron contamination have on your %dd at 10 cm?

Answer 118

Electron contamination will increase dose at d_max. As such your %dd at 10 cm will decrease if you don’t account out the electron contamination.

This effect is very small for < 10 MV because the electron contamination doesn’t penetrate that deep, but it reaches up to a 0.2% error in k_Q for >= 10 MV beams

Question 119

Ideally where should the lead be placed? What about if it can’t be placed in the ideal situation?

Answer 119

Ideally: 50 cm from phantom surface

Also acceptable: 30 cm from phantom surface

Question 120

How thick is the lead foil?

Answer 120

1 mm

Question 121

For reference dosimetry, what TMR/%DD value should you use to scale dose at 10 to dmax? What source should you get that data from, the commissioning data or the data you take on the day of measurement?

Answer 121

You must use the COMMISSIONING DATA! That’s because it’s what the TPS uses as well. If your TPS thinks you’re delivery 1 cGy/MU at dmax SAD or SSD setup, then you need to be consistent.

Do not use whatever %dd(10)_X you measured on the day of. Even if it’s annual. You have to use whatever is in TPS

Question 122

For electron and photon beam dosimetry, where are you trying to get 1 cGy/MU?

Answer 122

At d_max for both

Question 123

What is the single item in the uncertainty budget that contributes the largest uncertainty?

Answer 123

The calibration factor you get from ADCL

That’s about a 0.75% uncertainty typically

Question 124

What 5 values compose k_Q?

Answer 124

k_Q = [P_wallP_celP_flP_gr(L/p)^w_air]_Q / [P_wallP_celP_flP_gr(L/p)^w_air]_60Co​

So notice, k_Q accounts for properties of the physical chamber makeup that vary with respect to beam quality. All of them are pretty obvious in their naming, the only one that might not be is P_cel which is the central electrode correction factor

Question 125

Which of the following is the most important characteristic that must match when choosing an appropriate chamber to resemble your chamber?

Central electrode material

Wall thickness

Wall material

Radius of cavity

Answer 125

Central electrode material

Wall thickness

Wall material

Radius of cavity

The only just the wall material is the same, you will be within 0.5%

Question 126

Which correction factors are likely to be larger in FFF beams than in flattned beams?

Answer 126

P_rp and P_ion

Question 127

How is the P_ion value further decomposed in the TG-51 addendum?

Answer 127

Break it down into a C_gen and a C_init value

C_gen is related to the dose per pulse in pulsed linac beams

C_init should be equal to that measured by ADCL in Co-60, and should be < 0.002

P_ion = 1 + C_init + C_genD_pp

The whole point of this breakdown is mainly for chamber commissioning purposes, but also suggests that P_ion increases linearly with dose per pulse

Question 128

Simply put, what is the end result of Bragg-Gray cavity theorem?

Answer 128

To relate dose measured in air to dose measured in water

Question 129

What is the effective point of measurement for a single electron defined as?

Answer 129

The location at which the electron enters the chamber or detector

Question 130

What is the addendum rule of thumb for uncertainty of output per 1 cm error in field size?

Answer 130

1%/cm uncertainty due to field size uncertainty