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Radiotherapy Year 2 > Radiation Detectors > Flashcards

Radiation Detectors Flashcards

1
Q

Examples of pulsed and continuous radiation

A

Pulsed: LINAC, CT
Continuous: Isotopes, 60Co, 192Ir

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2
Q

What does dosimeter measure?

A

The amount of ionisation as a result of radiation
The amount of energy deposited in air or tissue as a result of ionisation
Any other related quantities

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3
Q

Properties of an ideal dosimeter

A

Stable, accurate, precise, tissue equivalent
Linear over dose range
Linear with dose rate
Flat energy response (no change in sensitivity with energy)
No angular dependence
Large dynamic range (measure over wide energy range)
High spatial resolution
Independent of radiation type
Perform consistently with environmental changes (T,P)
Require minimal correction
Have direct read out

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4
Q

What is precision and accuracy and relevance to dosimeters

A

Precision: degree of reproducibility of a measurement
Accuracy: how close the mean of measurement is to true value

Precise dosimeter can be calibrated to give accurate reading

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5
Q

Why do we want tissue equivalent dosimeters?

A

Want to replicate the scenario in the patient to get an accurate estimate of the dose being delivered

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6
Q

How can tissue equivalence be described?

A

In terms of atomic composition or density

Or energy absorption coefficient and stopping power, which is more accurate

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7
Q

What do radiochromic films consist of and how do they differ from radiographic films?

A

A single or double layer of radiosensitive microcrystal monomers on a thin polyester base with transparent coating

Differ as self developing, not processed

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8
Q

Size and dynamic range of radiochromic films

A

Depends on type:
Gafchromic EBT2: 20 x 25cm x 0.29mm and 0.01 - 5Gy

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9
Q

Applications of radiochromic film

A

QA in imaging and dosimetry
Verification of patient plans
Dose monitoring of staff - film badges

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10
Q

How does radiochromic film work?

A

When irradiated, energy absorbed by receptive part of photomonomer molecules in active layer
Polmerization results in formation of blue coloured polymer molecules with absoption maxima in red and blue regions of light spectrum
Intensity of maxima proportional to dose
Read out

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11
Q

Why do some gafchromic films have marker dyes?

A

To help minimise response variations which can be caused by coating anomalies (for example)
EBT3 appears green

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12
Q

How is radiochromic film read out?

A

Spectrophotometer (measures light intensity as function of light source wavelength)
Laser scanning densitometer
Flatbed scanners

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13
Q

Advantages of radiochromic films

A

Self developing - no processing or QA
Effectively grainless - very high resolution
Independent of dose rate
Less energy dependence than radiographic film
Large dynamic range: up to 1000Gy

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14
Q

Disadvantages of radiochromic film

A

Less sensitive than radiographic
Not reusable

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15
Q

Dose linearity of radiographic film

A

Degree of linearity depends on scanning medium: spectromphotometer more accurate than scanner

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16
Q

Energy dependence of radiochromic film

A

Weakly dependent on energy (significant dependence at low kV)
Degree of dependence can vary between films, depending on constituents in active layer mostly

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17
Q

Temperature dependence of radiochromic films

A

Insensitive to range of room temperatures

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18
Q

What are n-type semiconductors doped with?

A

Group V elements such as phosphorus
Gives extra electrons
Also called electron donors

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19
Q

What are p-type semiconductors doped with?

A

Group III elements such as boron
Gives rise to excess holes
Also called electron acceptors

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20
Q

Applications of semiconductor diodes

A

Used in in-vivo dosimetry
Relative dosimetry measurements

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21
Q

How can depletion zone also be described?

A

Potential hill
Sizes of which can be controlled by bias voltage

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22
Q

Forward and reverse bias

A

Forward: potential hill lowered, more electrons can now cross junction and current flows
Reverse: potential hill increased, less electrons have energy to climb hill. Some in p side may still be swept across, won’t be replaced.

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23
Q

How is pn junction created in reality?

A

Begin with base type semiconductor, such as p type Si doped with B
Something diffused onto this, eg Ph, some parts of silicon protected by SiO2
Region of n type material created, n+ because very highly doped compared to p type
Because bulk is p type this is called p type

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24
Q

What is measured to estimate dose in semiconductor?

A

Current produced which is proportional to dose

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25
How does radiation detection with semiconductor work?
RT radiation provides boost through creation of electrons/holes created when radiation interacts with diode E/Hs created everywhere Any electrons in p type that go through depletion region contribute to current, like holes with n type Diffusion length is function of carrier concentration: diffusion length on very doped side smaller than less doped, mostly ionisations in base that contribute to current
26
Response sensitivity of semiconductors
Sensitivity changes with accumulated dose due to radiation damage, becomes non-linear with dose rate. N-type more sensitive Due to displacement of Si atoms from lattice sites which leads to recombination centres that siphon off charge carriers
27
Temperature dependence of semiconductors
Response sensitive to change in temperature for pulsed and continuous, but linear so can be characterised
28
Dose rate dependence of semiconductors
DR dependence for p-type less than for n-type at lower energies so p-type used more Due to saturation of radiation induced traps
29
Energy dependence of semiconductors
Response changes with energy for both p and n type
30
Pre-irradiation of semiconductors
Semi-conductors are pre-irradiated due to their sensitivity varying over time.
31
Advantages of semiconductors
Small size, high resolution No external bias needed Good mechanical stability High sensitivity Instant readout Simple instrumentation
32
Disadvantages of semiconductors
Energy dependent DR dependent Temperature dependent Requires cables/ connection during irradiation for readout Sensitivity changes over time
33
What is a diamond detector?
Detector using diamond to measure dose, works because a diamond with impurities can be used like a semiconductor Under biasing potential a current is produced that is proportional to dose
34
Application of diamond detector
Small field dosimetry (although very expensive so few places use them)
35
Response stability of diamond detector
Response stable after pre-irradiation of 5-15Gy Drop in sensitivity due to polarizing effect caused by captured electrons in forbidden zone which creates E field opposed to that applied
36
Bias voltage dependence of diamond detector
Response is sensitive Current increase proportional to increase in bias voltage, but response less stable at high voltage Optimum is 125 p/m 25V
37
Dose response of diamond detector
Linear
38
DR dependence of diamond detector
Significant dependence on dose rate Mainly due to impurities in sensitive volume creating polarizing effect when bias voltage applied across detector
39
Response sensitivity with accumulated dose of diamond detector
Detector can withstand a significant amount of accumulated dose without showing degradation of response sensitivity, i.e. not susceptible to radiation hardening
40
Energy dependence of diamond detector
Only weakly energy dependent
41
Advantages of diamond detectors
Small size, high resolution Waterproof Almost tissue equivalent Sensitive response
42
Disadvantages of diamond detectors
Weakly energy dependent - must correct Dependent on dose rate Requires pre-irradiation Sensitive to bias voltage Expensive
43
What is luminescence?
Process where a material upon stimulation releases trapped energy in form of UV, visible, IR
44
What does delay between stimulation and emission determine?
<10^-8s, fluorescence >10^-8s, phosphoresence
45
Thermoluminescence vs optical stimulated luminescence
If stimulation is done by heat, TL If stimulation done by light, OSL
46
Applications of TLDs and form
In-vivo dosimetry Relative dosimetry EBR measurements Brachy measurements Can have powder, rods, chip, ribbon etc, small
47
What happens when TLD irradiated?
Radiation ionises electrons - produces electrons in conduction band Some fall back down to valence but some fall into impurity traps On application of heat the trapped electron is excited to conduction band When electron falls to valence band it emits light
48
How is signal read out of TLD?
PMT converts light to signal and amplifies so it can be read out
49
What is glow curve?
Plot of thermoluminescence intensity vs temperature Single glow peak: single energy trap depth
50
What is annealing and why is it done?
Heating at constant temperature over a fixed period Controls number of glow peaks - remove first low temperature peaks to make glow curve more stable and predictable
51
What are uses of heat in cycle of measurement for TLD?
Low temperature pre-readout anneal to remove low temp peaks Anneal during readout to induce production of light and signal High temperature post-readout removes residual signals
52
Energy dependence of TLDs
Energy dependent response, significant at lower energies, primarily because TLDs not tissue equivalent
53
Angular dependence of TLDs
No significant angular dependence
54
Dose linearity of TLDs
Dose response linear. Can become supralinear at higher doses, influence by type and amount of impurities
55
How do OSLDs work
Same principle as TLDs but use laser light source instead of heat to release trapped energy
56
Applications of OSLDs
In-vivo dosimetry Potentially more versatile than TLDs but not used as widely
57
Temperature dependence of OSLDs
Virtually none (immediate advantage of TLDs)
58
Angular dependence of OSLDs
None
59
Dose linearity of OSLDs
Linear in clinically relevant dose range 0-200cGy Supralinear at high doses
60
Dose rate dependence of OSLDs
None
61
Response sensitivity with accumulated dose of OSLDs
Sensitivity stable for accumulated doses up to 20Gy then begins to drop
62
Energy (beam quality) dependence of OSLDs
Some dependence on energy (but better than TLDs) Difference between photon and electron energies is because e energies give rise to more electrons in storage, affecting sensitivity
63
Advantages of TLDs
Small sizes - high resolution Various forms, different clinical applications Not expensive Many TLDs can be irradiated in single exposure Resuable
64
Disadvantages of TLDs
No instant readout (24 hours) Signal erased during readout Fiddly Energy and dose, temperature dependent, must be calibrated well
65
Advantages of OSLDs
Small size - high resolution No temp dependence No angular dependence No dose rate dependence Linear with dose in relevant range Reusable Quick readout time compared to TLDs No cables
66
Disadvantage of OSLDs
Some energy dependence, need correction factor Readout not instantaneous (8 minutes) Only linear up to about 4Gy More expensive than TLDs
67
What is a scintillator and what types can you have?
A material which fluoresces upon excitation - delay between excitation and light emittance is <10^-8s Inorganic (eg NaI, BaF), organic (eg plastics) Inorganic not suitable for dosimetry, high Z and density, plastics more tissue equivalent so acceptable
68
Size and application of scintillators
Small, 1x4mm Can be used in in-vivo dosimetry, brachy, small field dosimetry, radiosurgery
69
How do scintillators work?
Absorb energy from incident radiation Excitation occurs and emission of UV/visible light Light converted to signal and amplified by PMT Material in RT often polystyrene, PMMA
70
Temperature dependence of scintillators
None at ambient temperatures
71
Dose rate dependence of scintillators
Independent
72
Dose linearity of scintillators
Linear in clinical range
73
Reproducibility of scintillators
Stable response with good reproducibility
74
What is alanine and how does it work?
An amino acid which produces free radicals when exposed to ionising radiation When subjected to electron paramagnetic resonance (EPR), intensity of EPR spectrum proportional to concentration of free radicals Amplitude of signal proportional to dose
75
How does EPR work?
Free radical subjected to EM energy Gets excited Excitation energy matches frequency of B field, radical resonates Amplitude of frequency proportional to dose
76
Size and applications of alanine
Most commonly in pellets that are 2.5mm diameter x 5mm thick Limited clinical use over 1Gy Used by NPL for calibration at doses <15Gy
77
Energy dependence of alanine
Weakly energy dependent
78
Temperature dependence of alanine
Some temperature dependence
79
Response stability of alanine
Signal fades with time depending on dose, storage implications
80
What is Frick gel?
A gelatine, agarose or PVA phantom which has Fe2+ ions in Ferrous sulphate solution True 3D dosimeter
81
How does Fricke Gel work?
Upon irradiation, water decomposition happens and produces hydropoxy radicals Various reactions then lead to Fe2+ to Fe3+ (Ferrous to Ferric) ions, introducing change in paramagnetic properties Quantity of ferric ions depends on amount of energy absorbed, change in paramagnetic properties function of dose, measured using NMR
82
Advantages of alanine
Almost tissue equivalent Large dynamic range up to 1000Gy
83
Disadvantages of alanine
Weakly energy dependent, needs correction factors Signal fades with time dependent on dose, environmental and storage factors Not reusable Low sensitivity
84
Uses of water for measurement
Used for reference/baseline measurements
85
Using water equivalent phantoms
Water not always practical or convenient so can get equivalent water phantoms with similar densities and effective atomic numbers Designed to be durable, consistent and minimise accumulation of charge Generally used for daily verifications
86
Examples of equivalent water phantoms
Perspex Solid water Virtual water Plastic water Polystyrene

Radiotherapy Year 2 (16 decks)

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