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Residency > First Year Exam: Instrumentation (no TG-51) > Flashcards

First Year Exam: Instrumentation (no TG-51) Flashcards

1
Q

How do you calibrate NanoDots?

A
  1. Make sure you know an exact dose to some depth for your LINAC
  2. Place Nanodots ontop of atleast 5 cm of backscatter solid water
  3. Arrange 4 Nanodots in a square, so that the average reading corresponds to dose at central axis
  4. Place bolus on top of the nanodots
  5. Place more solid water on top of bolus, in order to get to a depth that you know the exact dose for
  6. Irradiate Nanodots and plot your curve
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2
Q

What types of dosimeters are typically used as surface dosimeters? (4 common types)

A

OSLDs

TLDs

Diodes

MOSFETS

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3
Q

Why would you prefer not to use OSLDs for out of field measurements for high energy beams?

A

They do not measure neutron component

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4
Q

What are the common applications of a farmer chamber?

A

Absolute dosimetry in water, solid water phantoms, and air

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5
Q

What do farmer chambers measure?

A

Charge (which is then corrected to absorbed dose in water)

Exposure

Air kerma

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6
Q

What is the minimum field size that a farmer chamber can measure dosimetry for?

A

5 x 5 cm2

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7
Q

What is a typical range of operating voltage for a farmer chamber?

A

100 to 400 V

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8
Q

What is the sensitive volume of a PTW farmer chamber?

A

0.6 cc

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9
Q

What is the typical radius of a farmer chamber?

A

3.05 mm

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10
Q

What is the nominal response of a farmer chamber (that is, typical nC/Gy)?

A

20 nC/Gy

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11
Q

What is the approximate maximum instability of a farmer chamber?

A

<= 0.5% change in sensitivity per year

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12
Q

Approximately what magnitude is leakage charge in a ion chamber + electrometer system? What causes the majority of this leakage?

A

pico Coulombs

(most of it is due to the wire)

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13
Q

What is a farmer chamber’s collection electrode made out of?

A

Aluminum

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14
Q

In general, what are the walls of most cylindrical ion chambers made out of?

A

Graphite and PMMA

(graphited acrylic wall)

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15
Q

What is the markus (parallel plate) chamber sensitive volume?

A

0.055 cc

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16
Q

What is the nominal response (typical nC/Gy) of a parallel plate chamber?

A

0.67 nC/Gy

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17
Q

What is the sensitive volume of the PTW semiflex 31013 chamber that we use?

A

0.3 cc

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18
Q

What is the radius of the sensitive volume of the semiflex 0.3 cc chamber?

A

2.75 mm

NOTE: IT’S ACTUALLY NOT EXACTLY THE SAME AS THE FARMER CHAMBER. BUT THE DIFFERENCE IS SO SMALL, THAT FOR THE PURPOSE OF A TG-51 EPOM CORRECTION, THERE IS BARELY ANY DIFFERENCE

(also for absolute dosimetry (with solid water for example), the kQ factor for it already takes into account any shifts specific to that chamber)

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19
Q

What is the nominal response of a semiflex 0.3 cc chamber?

A

10 nC/Gy

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20
Q

What is the PTW TN 31013 semiflex chamber used for?

A

Absolute dosimetry (alternative for the farmer chamber)

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21
Q

In general, what does any ion chamber measure?

A

Charge (to be converted to dose in water)

Exposure, air kerma

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22
Q

What is the unit of radiation exposure?

A

Roentgen

Or Charge/mass (C/kg)

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23
Q

What is the value of 1 R?

A

2.58 x 10-4 C/kg

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24
Q

What is the sensitive volume of a PTW TN31014 pinpoint ion chamber?

A

0.015 cc

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25
Q

What is a PTW TN31014 pinpoint ion chamber used for?

A

Dosimetry of high energy photons with high spatial resolution

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26
Q

What is the smallest field size that a pinpoitn detector can measure dosimetry for?

A

2 x 2 cm2

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27
Q

What is the A16 chamber used for?

A

SRS and some IMRT dosimetry application

(due to its excellent spatial resolution and exact pinpoint beam profile characterization)

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28
Q

What is the collection volume of a A16 chamber?

A

0.007 cc

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29
Q

What is the smallest field size that a A16 detector can be used for?

What is the biggest field size it could be used for?

A

3.4 x 3.4 cm2

5 x 5 cm2

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30
Q

What are some main advantages of ion chambers?

A

Good uniform response

Accurate dose reading

Capable of measuring high dose rates

Designed with different sizes, shapes, and volumes (variety)

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31
Q

What are some disadvantages to ion chambers?

A

Easily susceptible to moisture, temperature and pressure

Since air is not very dense, the typical nominal response is low. So you need larger volumes to get enough reading. Size limited

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32
Q

Fill in the diagram

A
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33
Q

What is the main role of a guard ring?

A

Reducing leakage of extraneous charge to the collecting electrode

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34
Q

Fill in the diagram

A
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35
Q

Fill in the diagram

A
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36
Q

Briefly describe the region 1

A

Response is voltage dependent and energy dependent

Large number of ions recombine prior to collection (not enough voltage to separate)

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37
Q

Briefly describe the region 6

A

Clinically useless

Too many secondary ionizations just burn through the gas in the chamber

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38
Q

Briefly describe the region 5

A

Townsend avalanche creates a very large number of secondary avalanches

No more information exists from the original radiation dose

Horrible for dosimetry. Great for detection

Secondary electrons begin to neutralize the local electric field, preventing further recombinations (hence why it levels)

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39
Q

Briefly describe the region 4

A

This voltage region is not used clinically

Response to collected energy diminishes due to dilution of counts due to secondary ionizations

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40
Q

Briefly describe the region 3

A

Response is proportional to energy collected and applied voltage

Some secondary ionization begins to occur

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41
Q

Briefly describe the region 2

A

Sufficient voltage to prevent recombination

Insufficient to produce secondary ionizations

Measured signal is directly proportional to number of ionizations produced by incident radiation

Very little increase in response vs voltage

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42
Q

How do diodes work?

A
  1. ionizing radiation produces electron-hole pair
  2. Electrons are attracted towards N side due to positive bias
  3. Holes attracted to P side due to negative bias
  4. This diffusion is quantified with an electrometer
  5. No external bias needed since the electric field between the P and N junction is already enough for charge pair separation
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43
Q

What is measured with a diode by an electrometer?

A

Accumulated charge or induced current

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44
Q

What is a depletion zone of a diode?

A

Sensitive volume between the P and N junction

The large magnetic field separates all ions to either the P or N zones, further strengthening the field

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45
Q

What are the advantages to diodes? (6)

A

Immediate readout

No bias voltage required

High sensitivity

Very small

Good stability

Small energy dependence for mass stopping power ratios between silicon and water (can measure %DD curve directly)

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46
Q

What are diodes typically used for?

A

Small field dosimetry

Array devices

Electron PDD

In-vivo dosimetry

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47
Q

What are some disadvantages to diodes? (6)

A

Tempearture dependence (0.5% / C)

Dose per pulse dependence

Energy dependence (over response to low energy photons)

Angular dependence

Changes in sensitivity over time due to radiation damage

Need to be hooked up via wires/electronics during time of irradiation for you to record a measurement

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48
Q

How do OSLDs work?

A
  1. Small crystals mounted on discs tightly encased in a very thin case
  2. Ionizing radiation enter through the medium, excite and release electrons from valence band
  3. Electrons from valence band enter conduction band
  4. Due to crystal impurities, some of the electrons, as they drop from conduction to valence, are trapped in the in-between energy range
  5. Holes are also trapped
  6. These trapped electrons are freed during reading via light energy or heat
  7. The freed electrons and holes escape their trap, allowing them to recombine at the luminescent center and release light
  8. Emitted light is measured using either a PMT or a camera (CCD or CMOS)
  9. Light is correlated to absorbed dose
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49
Q

How long after OSLD exposure can you get a readout?

A

10 mins after irradiation

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50
Q

What light is used to excite the trapped electrons in OSLDs?

A

Green light

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51
Q

What is one major benefit of OSLDs vs TLDs?

A

OSLD readout is non-destruction. Meaning you get enough of a readout to get a dose, but you can continue to keep measuring over and over again

TLDs are destructive. You can really only get the readout once

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52
Q

What is the dose range that OSLDs can measure?

A

1 uGy to 15 Gy

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53
Q

What are some advantages to OSLDs?

A

Small

Reproducible

Passive/active detector

No angular dependence

Can be re-read

No dose rate dependence below 100 keV

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54
Q

What are some disadvantages to OSLDs?

A

Delayed readout (10 mins)

Not water equivalent

Cerenkov stem effect

Sensitivity to light

Small temperature dependence

Sensitivity change vs accumulated dose (due to traps getting filled)

Energy dependent

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55
Q

What are OSLDs used for?

A

in-vivo dosimetry (nanodots)

Personnel dosimeters

End-to-end testing

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56
Q

What detectors are used in the chest badge? What about finger badge?

A

Chest badge - OSLD

Finger badge - TLD

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57
Q

What is a band gap?

A

Difference in energy between valuence and conduction bands

(minimum energy required to an excited valence electron to enter conduction band)

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58
Q

In terms of bandgaps, what are conductors, semiconductors and insulators?

A

Conductors: allow for electron flow/current because they have no band gap (valence electrons can flow freely)

Semiconductors: have some intermedia band gap

Insulators: have a very large band gap. Make it very difficult for electrons to flow

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59
Q

What is a “trap” in luminescent dosimeters?

A

Energy levels between the valence band and conduction band caused by imperfections in crystal than can trap some electrons

The trap centers allow detectors to hold some of the absorbed dose energy until readout

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60
Q

What is a competitive center?

A

Trap charge carriers that don’t contribute to luminescence other than removing charge from being able to recombine

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61
Q

What causes the supralinear response of TLDs vs accumulated dose?

A

Competitive centers start to fill up and aren’t emptied

So electrons moving in the band gap are less likely to get trapped in a competitive center. So recombinations become more probable

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62
Q

What does a glow curve measure?

A

Thermoluminescence vs temperature

Note: the cumulative probability of emission is proportional to temperature AND time

This is why having a consistent heating protocol is so vital. Because your glow curve is directly dependent on the protocol used

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63
Q

What is the process used to reuse TLDs? How does it work?

A

Annealing

Trap centers are emptied and redistributed through a consistent heating protocol (ex. 400 celsius for 1 hour to reset lattice structures, then reduce heat to 80 celsius for 24 hours to rearrange traps that result in a certain peak)

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64
Q

What is the process used to reuse OSLDs called? How does it work?

A

Bleaching

Treat OSLD with light from halogen or fluorescent lamp, or green LED

Empties most trap centers

Deep trap centers will NOT be emptied during bleaching, thus changing OSLD sensitivity over time

You can actually avoid this, however, by annealing OSLD at 900 celsius to empty even the deep traps

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65
Q

Aluminum is the most common central electrode material for ion chambers. But what is the 2nd most common?

A

Graphite

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66
Q

What is the design difference between a farmer vs thimble chamber?

A

Farmer chambers have pointed ends

Thimble chambers have rounded ends

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67
Q

What are extrapolation chambers used for?

A

Measuring dose buildup region

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68
Q

How do extrapolation chambers work?

A

Very tiny collection volume

You can find tune the collection volume

You can extrapolate to depth zero (which is impossible to directly measure, because you would need 0 cc of collection volume. Thus dose would always read 0 which isn’t always true)

Chamber is at water surface

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69
Q

What are the advantages to TLDs? (5)

A

No angular dependence

Wireless

Dose rate independent

Reusable

Energy independent above 100 keV

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70
Q

What are the disadvantages to TLDs? (6)

A

Can only be read out once

Requires significant time for readout

Supralinear response with reuse

Require special prep and calibration

Temperature dependent

Light sensitive

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71
Q

True or False

For diodes %DD scans, the EPOM is actually BELOW the point of measurement?

A

True

This is due to shielding in the diode

So EPOM shifts in %DD measurements give + something, instead of - as they would be in ion chambers

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72
Q

What type of detector is an edge detector?

A

Diode

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73
Q

When is the edge detector used?

A

Small field relative dosimetry and electrons

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74
Q

What are the uses of TLDs?

A

Secondary dose check

In-vivo dosimetry (ring badge)

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75
Q

What is the accuracy of a TLD?

What about OSLD?

A

3% for both

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76
Q

Why does solid water need a fudge factor vs actual water?

A

Because solid water has a charge retaining effect and liquid water doesn’t. Literally, solid water an hold charge for a small amount of time.

Also the electron density of solid water is not exactly equal to that of actual water

And additional uncertainties in the construction of the solid water slabs and any inhomogeneities

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77
Q

Why are triaxial cables needed for ion chambers?

A

Because ion chambers have 3 electrodes…

  1. Collector
  2. Guard
  3. HV bias

So you need 3 channels

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78
Q

What is the approximate leakage of a triaxial cable?

A

10-13 - 10-14 Amperes

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79
Q

What is the general construction of an IC Profiler?

A

251 parallel plate ion chambers

5 mm spacing on x and y axes

7.07 mm spacing on diagonal arrays

32x32 cm2 measurement range on x an y axes

45 x 45 cm2 measurement on diagonal axes

Trigger diodes within 5 mm of ion chambers to tell detectors when to start measuring

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80
Q

How do you measure a 40 x 40 cm2 field using ICP?

A

Just move the detector up

Flatness, symmetry and beam shift are not affected by SSD, so you can do this just find

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81
Q

equation for symmetry used in ICP?

A

Da - Db / DCAX * 100%

Where a and b are two points oriented symmetric of one another across the central axis

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82
Q

True or False

ICP has temperature and pressure corrections?

A

True - recall the collecting chambers are unsealed ion chambers

But it only uses them for absolute dosimetry (which doesn’t apply for us at all). For flatness and symmetry, it’s not necessary at all

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83
Q

What can the ICP be used for?

A

Measures of flatness, symmetry, field size, beam center, penumbra width, light radiation field

Can also be used to measure beam constancy, steering, collimator and rotational sag QA

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84
Q

How many diode detectors are in an ArcCheck? How does this compare to MapCheck?

A

ArcCheck: 1386

MapCheck: 1527

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85
Q

What is daisy chaining? (you don’t need to know how to do it yet)

A

A method for measuring small field output factor using two different dosimeters

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86
Q

What are some main points of the Georgia State Regulations (111-8-90) for calibration and spot check QA?

(Don’t spend too long on this card)

A

Keep records for everything as long as you can

Time between calibration should not exceed 12 months

Need to calibrate after change to machine performances

Need a “qualified expert” to do calibrations

Spot check calibrations which are our weekly/periodic QA’s should be regularly performed

A qualified expert is a ABR certified medical physicist

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87
Q

What is the most common scintillation detector we may see in the clinic?

A

W1

88
Q

Why do scintillation detectors need to be coupled with PMTs?

A

Because their light output is too low so it needs to be amplified

89
Q

What symmetry equation does DailyQA3 use?

A

(Arealeft - Arearight) / (Arealeft + Arearight) * 100%

90
Q

How does DailyQA3 measure energy?

A

Electrons: has different materials of different densities and measures output at chambers under these materials for electron beams

Photons: measures flatness on the premise that changes in energy of beam will produce changes in beam flatness

91
Q

What are some benefits to scintillator detectors? (9)

A

Water equivalent

Fast response

Temperature independent

Energy independent

Dose rate independent

Linear response to dose

Great spatial resolution

No angular dependence

Can come as organic, inorganic or plastic

Small volume

92
Q

What are some disadvantages to scintillator detectors? (2)

A

Cerenkov noise

Change in sensitivity due to plastic yellowing

93
Q

What are scintillation detectors used for?

A

Small field dosimetry

Electron measurements

Nuclear medicine

94
Q

How do scintillation detectors account for Cerenkov signal? (3 ways)

A
  1. Temporal avoidance (Cerenkov signal usually lasts 500 ns less than the actual scintillation signal. So you want to only measure the final 500 ns of a measurement)
  2. Optical filtration (wavelength selection)
  3. Dual light pipe design
95
Q

What are three things that degrade SNR in scintillators?

A
  1. Cherenkov
  2. Dark current (electrical current that flows through photodetectors even when no photons enter) (can usually just be subtracted out)
  3. Direct interactions between radiation and detector
96
Q

How does a dual light pipe work for scintillation detectors?

A

One light pipe connects to the photodetector

The other is near scintillator but shielded from it. So it collects only background (Cherenkov) reading

97
Q

What is the pro and con to GM counters?

A

Pro: Can easily detector contamination

Con: Cannot give a trustworthy exposure rate unless measuring the source that it was calibrated with

98
Q

What is the general design of EPIDs? Give both the historical and current designs.

A

Historical: camera (radiation hits photophosphur, producing light, which bounces off a 45 deg mirror and hits a camera) or ion chamber type (array of liquid ion chambers)

Current: Amorphus silicon photodiode arrays. Metal plate on top, scintillator (phosphur) beneath, amorphus silicon later underneath those

99
Q

How does an amorphus silicon EPID work?

A

Metal plate convers photons to compton electrons (4% interact with plate, 1% actually convert properly) (MASSIVE loss in signal)

Some photons that are missed by the copper plate may be caught by the phosphor

Phosphor scintillator converts photons and electrons into visible light

Photodiodes implanted on a amorphus silicon panel detect visible light and send signal to read-out electronics

100
Q

What are EPIDs used for?

A

Patient setup verification

Assessment of target and organ motion

In-vivo dosimetry distributions

Patient specific QA

Machine QA (PF, HBB, WL, etc)

101
Q

What is the resolution of amorphus silicon EPID?

A

< 0.5 mm

102
Q

what units do survey meters measure in?

A

Either exposure over time (mR/hr) or counts per minute (cpm)

103
Q

Which survey meter is best for detection of beta radiation?

A

Scintillation probes (which measure in cpm)

104
Q

Which survey meter is best for photon contamination surveying?

A

Geiger counters

105
Q

Which survey meter is best for survey and measurement of exposure in air from neutron contamination?

A

REM balls

106
Q

What does a REM ball measure?

A

It means both neutron AND photon exposure

107
Q

How does a REM ball work?

A

Has a sphere filled with polythylene, which slows down neutrons, creating gamma rays in the process which are detected by a survey meter

108
Q

If you want to measure only exposure from neutrons, how would you do it?

A

You have to have a REM ball and a separate survey meter that doesn’t have neutron capabilities

REM ball will measure photons and neutrons, so you want something to allow you to subtract out the photon component

109
Q

What is the unit of measurement of most neutron detectors?

A

mrem/hr

(it gives dose equivalent using the known quality factor for the neutron energies it’s designed to detect)

110
Q

How does bonner sphere spectroscopy work?

A

Multiple spheres with thermal neutron detectors embedded between the moderating spheres

The moderating spheres will slow down the neutrons to thermal neutron range, which will be measured by the thermal neutron detector

Having multiple moderating thicknesses allows for spectroscopy (measuring of multiple energies)

111
Q

Since a rem ball is just a single bonner sphere, it only measures a certain range of neutrons. How then does it account for all neutron dose?

A

By correcting sensitivity with an approximation for radiation weighting factor across a range of neutron energies

112
Q

Explain the general design of a rem ball

A

A rem ball is an example of an activation foil neutron detector

Most outer layer is a cadmium coating (to filter out thermal neutrons)

In-between layer is a moderator (polythylene or BF3)

Inner most layer is a thermal activation foil to absorb the neutrons and produce photons

ontop of the REM ball is a survey meter that measures photon exposure

113
Q

True or False

Unlike REM detectors, Bonner spheres are able to filter out photon exposure by themself

A

True

114
Q

What are bubble detectors used for?

A

In-vault neutron dose monitoring

Personnel neutron monitoring

115
Q

What is the useful range of TLDs and OSLDs?

A

mR to 100 Gy

116
Q

What are MOSFETs useful for measuring?

A

In-vivo dosimetry

Penumbra

Very small field sizes

117
Q

What is the quantity measured in a MOSFET detector?

A

Threshold voltage

118
Q

Why does EPID dosimetry require many corrections?

A

Due to the high-Z materials found in the EPID

119
Q

What is the lifetime of a MOSFET detector?

A

100 Gy

120
Q

What are some cons to MOSFET detectors?

A

Not water-equivalent

Limited life-time

Temperature dependent

Response degrades with accumulated exposure

Energy dependent

Expensive

121
Q

What are some pros to MOSFET detectors?

A

Small

Available in arrays

No angular dependence

Real-time in vivo capabilities

Dual bias eliminates temperature dependence

Permanent dose storage

122
Q

For cross calibration of a PP chamber, what is the most appropriate energy to use (according to TG-21, TG-39 and TG-51)? Why?

A

Use the highst available electron energy

Therefore Prepl is no smaller than 0.98 (infact for energies > 15 MeV it approaches unity)

123
Q

How do you cross calibrate a PP chamber?

A

Remember: the only thing you don’t have with cross calibration is the kecal, so that’s included in your end result

124
Q

What is the sensitivity reduction rate of diodes vs dose?

A

< 1-2% reduction per 250 Gy

125
Q

If you have two diodes, one for 6 MV and one for 18 MV, and you have a treatment that uses both 6 and 18 MV, but you can only use one diode, which should you use?

A

Use the 18 MV diode

That way the buildup is sufficient enough and reduces electron contamination still

126
Q

What causes directional dependence of a diode?

A

Partly by detector construct (transmission through varying thicknesses or cable)

Partly by back scattering from patient or phantom

127
Q

What phantom is used for QA of respiratory motion of CT and in-room EBRT tracking? How does it work?

A

Quasar phantom

It simulates a patient breathing by moving cylindrical inserts on the superior/inferior direction

The inserts can also have high density spheres that simulate tumors

128
Q

How does a Las Vegas phantom work?

A

Varies contrast by increasing or decreasing hole depths, and varies resolution by increasing hole diameter

Allows you to measure EPID contrast and spatial resolution

129
Q

What is the other version of a Las Vegas phantom, but circular orientation and uses line spacings in the center for the resolution test instead of varying hole thicknesses?

A

Leed Phantom

130
Q

What are the planar imaging phantoms that we use for kV and MV OBI image quality?

A

the SNC kV-QA and MV-QA phantoms

131
Q

What is the phantom that Charlotte uses to measure light vs radiation field coincidence and jaw positioning?

A

The SNC FS-QA phantom

132
Q

What does a WL cube allow you to QA?

A

Imaging, Radiation and Mechanical isocenter coincidence

And the size of your isocenter

133
Q

Approximately how much error exists between solid water and real water, on average?

A

0.5% error

134
Q

How often do electrometers need to be calibrated?

A

Once every 2 years at ADCL

135
Q

How is a electrometer calibrated at the ADCL?

A
  • Set up a simple single capacitor circuit
  • Capacitor is calibrated with known capacitance
  • A known voltage is applied to the circuit
  • Q = CV
  • Vary capacitance from a range of pico to nano Coloumbs to measure linearity
  • Charge reading should increase linearly with capacitance
  • Unit of calibration: C/reading
  • Estimated uncertainty to k=2, 95% confidence
136
Q

What is typical electrometer leakage on low mode? What about high mode?

A

1 fA or 1 fC on low mode

1 pA or 1 pC for high mode

137
Q

Which mode do we use on our electrometer? Low or High?

A

High mode

138
Q

Briefly, how does radiochromic film work?

A

Film has an active layer of a radiation-sensitive polymers that changes chemical structure to a blue shade when exposed

Polymers sit on an inert polyster substrate

You measure optical density and plot that with dose

139
Q

What are the advantages to radiochromic film? (7)

A

Large measuring area

Very good spatial resolution

Tissue equivalent

No chemical processing needed

Accurate within 2-3%

Energy independent

Not sensitive to visible light (only UV)

140
Q

What are the disadvantages to radiochromic film? (5)

A

Need a calibration curve

Need a scanner

Need to wait 24 hours before reading due to continuing polymerization

High cost per film

Scanner readout is directional dependent

141
Q

What is the useful range of radiochromic film?

(this is across all film, not just EBT3 or EBTXD)

A

0.1 Gy - 100 Gy

EBTXD is capped at 50 Gy

But some fi;m, particularly those used for Trigeminal neuralgia, will need to go up as high as possible

142
Q

What are the advantages to radiographic film? (4)

A

Un-matched spatial resolution (micrometer)

Large measuring area

Cheaper than gafchromic film

Dose rate independent

143
Q

What are the disadvantages to radiographic film? (7)

A

Require development

Accurate to only within 3-5%

Sensitive to visible light

Strong energy dependence below 400 keV (due to the silver)

Not tissue equivalent

Bulky readout system

Emulsion differences among films and batches

144
Q

In general, what are some things that film good for measuring? (that we use in our QA)

A

Some IMRT QA

Light vs radiation field

Star shots

Profiles (although we don’t really do it for anything other than Prp)

145
Q

What type of radiation (and energy ranges), does the Fluke 451P detect?

A

Beta above 1 MeV

Gamma above 25 keV

146
Q

What is the maximum reading you can get with a 451P?

A

5 R/hr (50 mSv/hr)

147
Q

What is typical background for a 451P?

A

< 10 uR/hr

148
Q

What is the battery lifetime of a 451P?

A

200 hrs when using two batteries

You can also use only one baterry, and it operates for 100 hrs instead

149
Q

What’s approximate warmup time for a survey meter that has been off for 12 hours or more?

A

4 mins

150
Q

In general, measuring from what area of the 451P is the most sensitive? Side, face or front?

A

Face

151
Q

True or False

The 451P energy response is near unity for the entire detection range

A

False

It’s low for lower energies, highest for 80-100 keV (1.2), and then levels out near unity for higher energies

152
Q

What is the calibration energy range of the 451P?

A

20 keV to 2 MeV gamma and xrays

153
Q

When a 451P “LOW BATTERY” indicator first appears. How many hours of operation are remaining?

A

6 hours

154
Q

What batteries doe sa 451P take?

A

9V

155
Q

Which is the positive side of a PN junction?

A

The N is the positive side

Remember: N-type is the electron donors. Meaning when the P and N junctions were initially placed together, the electrons of the N flowed to the P, giving the N a overall positive side

This electron flow happened for enough time until an electric field occured, retarding additional flow

156
Q

When we say that a diode is losing sensitivity due to radiation damage, what is actually being damaged?

A

The crystal structure. Defects occur over time in a silicon diode due to radiation

The defects produce RG centers and carrier traps (defects which capture carriers but have a very small probability of recombination)

157
Q

What are some correction factors that need to be applied when using diodes for in-vivo dosimetry? (8)

A

Entrance SSD

Exit SSD

Entrance field size

Exit field size

Accessories

Temperature

Angular

Patient thickness

158
Q

What are the axes of a H&D curve?

A

Y: Optical density

X: log exposure or log dose

159
Q

What types of film have better contrast, high speed or low speed?

A

High speed

160
Q

What types of film have a larger linear region (or measurable region). High speed or low speed?

A

Low speed

161
Q

In the following curve, what does “base” and what does “fog” mean?

A

Base is OD caused by the natural attenuation of the film

Fog is the OD caused by darkening of the film due to background radiation or, (for radiographic film only), light exposure

162
Q

What is the dosimetric accuracy of radiographic film?

What about radiochromic?

A

Radiographic: 3-5%

Radiochromic: 2-3%

163
Q

What is the approximate spatial resolution of radiochromic film?

A

Sub mm

164
Q

What is the difference between un-symmetric and symmetric radiochromic film? What kind is EBT3?

A

Symetric film, as the name would suggest, is constructed symmetrically when an equal amount of upper and lower material sandwiching the active layer. Non-symmetric has an unequal amount sandwiching the active layer

EBT3 is symmetric

165
Q

What is one consideration you need to have when working with non-symmetric film, that you don’t need to have when working with symmetrical film?

A

For non-symmetric film, orientation during measurement and readout matter

(in reality, it’s shown that this impact is very minimal, but worth accounting for)

166
Q

Which detector did we use to measure cone output factors?

A

Edge detector

167
Q

What are the 4 modules for the catphan 504 used for?

A
  1. Uniformity
  2. High resolution (21 aluminum line pair high resolution. Most systems can only distinguish 7-9)
  3. Low contrast resolution
  4. Slice geometry, contrast linearity, HU constancy, geometric distortion
168
Q

What is the name of our parallel plate chamber?

A

The Markus Chamber

169
Q

In terms of cost, what is cheaper, EPID or film for portal verification?

A

EPID is more expensive at first (initial installation and purchase), but over time the cost per scan for EPID is much lower than for film. So over time film cost outweighs EPID

170
Q

What is the collection diameter of the A16 chamber?

A

0.33 mm

171
Q

What kind of QA would you do on an electrometer? What would you check for? (5 things in mind)

A

Linearity checks

V=QC to get an exact expected charge and make sure your electrometer confirms it

Cross checking two electrometers

Expecting a nominal response of roughly 20 nC/Gy for a farmer chamber

Inherent leakage and dark current checks

172
Q

Why would you not use a PP chamber for photons?

A

Small volume, not as sensitive, too much noise

You need to be extra careful that the chamber is flat and perpendicular to beam

Not isotropic

Less researched

It’s one good use for photons, buildup dose, is usually given by varian

173
Q

Where do you want to use a REM ball to measure neutron contamination from LINACS?

A

Outside vault door

At the shadow shields (extra shielding added to to some kind of hole or compromised shielding for architectural reasons)

174
Q

What are the upper and lower limits of a GM counter?

A

mR/hr - 2 R/hr

175
Q

What is the design of a hankins rem ball?

A

Outer layer of very thin cadmium foil

Inner moderator of 6 cm Polysthylene sphere

Ludlum 12-4 or Eberline NRD detectors in center (both of which are BF3 or He-3 detectors), and both of which measure gamma

176
Q

What 4 detectors that are commonly known would you consider using for a small field measurement?

A

A16, W1, W2, Edge detector

177
Q

For DailyQA3 dose calibration, are you getting a energy specific factor, or a machine specific factor?

A

It’s machine specific

You only collect one dose calibration factor per machine, as opposed to ArcCheck and MapCheck where it’s energy specific.

Note: ArcCheck and MapCheck should actually also be machine specific, but our beams are matched so we just do energy specific.

178
Q

What is a typical value for a diode’s sensitivity variation with temperature?

A

0.5% / celsius

179
Q

How soon after irradiation are TLDs read? Why?

A

24 hours after irradiation

The lowest energy traps willr elease their electrons in the first 10 hours. After this, the accuracy of the TLD becomes maintained and stable to within 12 weeks after exposure. So 24 hours is a safe strategy to avoid the first 10 hour releases

180
Q

When reading a OSLD, what percentage of trapped electrons are released per reading?

A

0.2% released per reading

181
Q

Between TLDs and OSLDs, which is smaller? Which has a smaller angular dependence?

A

TLDs are both smaller AND have a lower angular dependence

182
Q

What are the advantages and disadvantages of diodes vs ion chambers?

A

Diodes are much more sensitive, and can therefore be built much smaller than ion chambers (2 eV/ip on average, vs Ion Chambers which need 34 eV/ip)

BUT

Diodes have energy dependence, especially in the kV range, and lose sensitivity with accumulated dose

183
Q

To what depth are diodes typically calibrated?

A

Either skin for skin in-vivo (0.5 mm deep) or to dmax of the beam energy

184
Q

List all the detectors that can be used for absolute calibration of a linear accelerator

A

Ion Chamber…

Nothing else

185
Q

True or False

Radiochromic film is not sensisitive to visible light

A

False

It’s very slightly sensitive. Can be close to negligible

186
Q

True or False

Radiographic film is not sensisitive to visible light

A

False

187
Q

What device is used to measure OD of radiographic film?

What device is used to measure OD of radiochromic film?

A

Radiographic - densitometer

Radiochromic - Laser scanner, microdensitometer, spectrophotometer

188
Q

What does ADCL stand for?

A

Accredited Dosimetry Calibration Laboratory

189
Q

What dependencies do MOSFETs have?

A

Angular dependence and energy dependence (not water equivalent)

190
Q

What are three dosimeters that measure absolute dose without needing calibration?

A

Calorimeter, Fricke Dosimeter and Free Air Chamber

Below is an image of a fricke dosimeter

191
Q

What is the most famous anthropomorphic phantom named?

A

RANDO phantom

192
Q

True or False

All TLDs are able to measure neutron component

A

False

Some TLDs can measure neutron component, some cannot, and some even over-respond to neutrons

193
Q

What causes the “SSD Dependence” in a diode, leading to the need to calibrate diodes at specific SSDs?

A

Diodes are dependent on instantaneous dose rates, and depending on the SSD, the instantaneous dose rate will differ

Thus, the SSD factor for diodes is actually a measure of dependence of instantaneous dose rate dependence

194
Q

Why are diodes not sensitive to the time between pulses (dose rate set at console)?

A

Because the collection time of a diode is much shorter than the time between pulses. So no deadtime affect

195
Q

What is the sensitivity variation of a diode from 80 - 120 cm SSD? What causes this sensitivity change?

A

1%

Caused by change in instantaneous dose rate per pulse

196
Q

Up to what percent are diodes field size dependent?

A

Change in reading up to 5% with change in field size

197
Q

Does diode sensitivity increase or decrease with increasing temperature?

A

Increases with increasing temperature

0.5% / celsius

198
Q

For our truebeams, how many diodes, at minimum, should we have?

A

3 diodes at minimum

One for low energy photons (6 - 10 MV)

One for high energy photons (15 -18 MV)

One for all electrons

199
Q

Why can’t we use one diode for all energies?

A

A diode needs to have enough buildup to shield contaminate electrons, but not enough that the dose shadowing (shielding) below the diode becomes significant (this is especially pronounced when a photon diode is used in electron therapy)

200
Q

Does diode sensitivity increase or decrease with increasing field size?

A

Increases

201
Q

For diode in-vivo dosimetry for TBI, which of the following correction factors are of particular concern?

Temperature

Field Size

Leakage

Angular

SSD

A

For diode in-vivo dosimetry for TBI, which of the following correction factors are of particular concern?

Temperature

Field Size

Leakage

Angular

SSD

202
Q

According to TG-235, approximately what dpi should you use to scan radiochromic film?

A

70 dpi

203
Q

Why can you not use a GM counter for detection of LINAC fields?

A

Because the pulsed beam will produce excessive dead time accumulation

204
Q

Give the general design of a triaxial cable

A

It has an inner cable/electrode and is surrounded by two outer layers, an insulator and a conducting sheath.

This protects from noise and leakage signal

205
Q
A
206
Q

What is the minimum 3D tank dimensions as required by TG-106 for LINAC commissioning?

A

Each dimension must be atleast 5 cm larger than the maximum field size at max depth, and the max depth itself.

So max depth is 40 cm, so depth of tank must be atleast 45 cm. Max field size is 40x40, but at 40 cm depth it’s larger, so do the math for how large the tank needs to be. Conservative estimate is atleast 75x75 cm2

207
Q

Why is the use of distilled water recommended for commissioning per TG-106?

A

Because the length of time of data collection is so long, algae may grow and mess up the setup/equipment.

208
Q

Below what field size does TG-106 recommend using a micro-ion chamber or diode?

A

4 x 4 cm2

209
Q

For diodes used for electron scans, what region would they fail in on the electron depth dose curve?

A

They fail in the Bremmstrahlung region

This is because diodes are designed for either photons or electrons. So when you use an electron diode, and you measure in the photon region of the electron curve, there’s gonna be issues

210
Q

Why can you not use a farmer chamber for beam profile commissioning?

A

Because of volume averaging in the penumbra

211
Q

What device would you use to measure wedge profiles during beam commissioning?

A

Detector Arrays

212
Q

In a cylindrical ion chamber, where is the electric field occuring?

A

Between the central collection electrode, and the outer electrode which is the inner layer of the chamber wall, and sometimes even in the wall itself is a electrode.

213
Q

What is the most common material diodes are made out of?

A

Silicon

214
Q

What materials are OSLDs?

A

Aluminum oxide doped with carbon

215
Q

What materials are TLDs?

A

Lithium Fluoride doped with Magnesium and Titanium

216
Q

Why can you not calibrate a surface dosimeter by having it on surface and delivering a corresponding known dose?

A

You can, if all you want to do is have the dosimeter as a form of secondary check on machine output. That’s allowed

But if you want something for In-Vivo dosimetry, it makes no sense since in real life most of your dose is not coming from en-face beams

Residency (23 decks)

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