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Physics for FRCR CO1 > 4.4 Radiation Dosimetry > Flashcards

4.4 Radiation Dosimetry Flashcards

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1
Q

What is the definition of Exposure (x)?

A

The electric charge freed by ionisation per unit mass of air.

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2
Q

What is the units of Exposure (x)?

A

Couloumbs/Kg

Charge per mass

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3
Q

What is the equation for Exposure (x)?

A

x = Q/M

Where Q is the charge produced when all electrons created by ionisation are completely stopped (measure the charge of electrons or ions)

and M is the mass of air

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4
Q

What type of radiation does Exposure (x) apply to?

A

Indirect only

i.e. only defined for photons

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5
Q

What is KERMA (k)?

A

Kinetic Energy Released Per Unit Mass

The initial kinetic energy of all the charged particles by the ionisation process - where the energy is released

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6
Q

What are the units of KERMA (k)?

A

Joules/Kg or Gy

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7
Q

What is the equation for KERMA (k)?

A

KERMA = Etr.M

Where Etr is mean energy released by ionising radiation
and M is mass of specific material

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8
Q

What type of radiation does KERMA (k) apply to?

A

Indirectly ionising radiation

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9
Q

What is Absorbed Dose (D)?

A

Energy absorbed per unit mass of material (any type)

This is where the energy is eventually absorbed (i.e. along the electron tracks)

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10
Q

What are the units of Absorbed Dose (D)?

A

Joules/kg or Gy

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11
Q

What is the equation for absorbed dose?

A

D = Eab/ M

Where Eab is energy absorbed
M is mass of marterial

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12
Q

What is Gy?

A

1 Grey = 1 Joule deposited per 1kg of material

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13
Q

What types of radiation does Absorbed Dose apply to?

A

Direct and indirect

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14
Q

How is KERMA found?

A

KERMA can’t be measured directly but exposure can be measured

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15
Q

What is the formula for Kair?

A

Kair = X x (W/e)

number of charges per unit mass x mean energy released per charge

W is a constant, e is the charge of electrons. W/e = 33.97J/C

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16
Q

How does KERMA relate to Absorbed Dose? (2 stage process)

A
  1. Unchanged photon kinetic energy transfers to electrons (e.g. via Compton scatter) - this is KERMA i.e. where the energy is released
  2. Electrons deposit energy in the medium along their tracks through ionisation and atomic excitiation = this is Absorbed dose - where the energy is absorbed
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17
Q

In relation to a target within material (e.g. a tumour within tissue, region of interest) which energy transfers are considered in KERMA?

A

If the photon transfers energy to an electron (ionisation) outside of the region of interest this is not considered in KERMA. However, if the electron then moves through and stops within the region of interest this is considered in absorbed dose. Ein

If the photon transfers energy to an electron (ionisation) inside the region of interest this is considered in KERMA. However, this does not contribute to the absorbed dose if the electron then travels and stops outside of the region of interest. Eout

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18
Q

What is the equation for the relation of KERMA to absorbed dose?

A

Absorbed dose = KERMA - Eout + Ein

When Eout is roughly equal to Ein this is Charged Particle Equilibrium - at this point Absorbed Dose = KERMA

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19
Q

How do we get from Exposure to KERMA inside a patient?

A

See notes for explanation

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20
Q

What is Charged Particle equilibrium?

A

Energy/electrons in = Energy/electrons out

This can only exist in homogeneous materials - i.e. not in patients

If CPE exists then absorbed dose = KERMA

CPE breaks down for high photon energies as the range of secondary electrons increases

Transient CPE does exist beyond the max range of secondary charged particles

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21
Q

What 3 conditions must be met for CPE to exist?

A
  1. The medium must be homogenous in atomic composition and density i.e. not patients
  2. Negligible attenuation of photon irradiation within a volume
  3. No inhomogenous electric/magnetic fields
22
Q

Describe KERMA and the photon depth dose curve

A

KERMA is an inverse straight line
- KERMA is max at the surface as photon fluence is max (most collisions)
- KERMA decreases with depth as the photons are attenuated

Absorbed dose rises quickly in the build up region until it intercepts the KERMA - Secondary charged particle fluence increases so absorbed dose increases until depth is equal to teh approximate range of the secondary particles (Dmax)

Beyond Dmax the absorbed dose is proportional to KERMA. The absorbed dose is always slightly higher than KERMA - this is because the dose at a depth is due to the secondary particles from further upstream - dose is deposited by electrons that have been created by up stream photons

23
Q

What are the features of an ideal dosimeter? (7)

A
  1. Accurate - close to the true value
  2. Precise - the results consistently demonstrate similar values
  3. Sensitive - high signal is seen even for low doses, in comparison to background noise
  4. Dose linearity - reading increases linearly with dose
  5. Response independent of energy
  6. Small size to allow sufficient spatial resolution
  7. The ability to represent dose in tissue (not detector material)
24
Q

What are the types of dosimetry?

A
  • Absolute
  • Reference
  • Relative
25
Q

What is Absolute dosimetry?

A

Direct measurements of dose only achieved at National Physics/Standards Laboratory

26
Q

What is Reference dosimetry?

A

Dose derived from a measurement using standardised set up and equipment which has been calibrated against the absolute measurement at the NSL

This is used to measure the outputs and calibrate the LINACs

This can be Thimble or Farmer dosimeters photons
Plate for electrons

27
Q

What is Relative dosimetry?

A

These look at how doses change - dose as compared to the maximum rather than the actual measurement of dose

e.g. PDDs, profiles, TMRs, output

28
Q

What are the Calibration standards?

A
  1. Primary standard
  2. Secondary standard
  3. Field instrument
29
Q

What is the Primary Calibration standard?

A

This is done at the National Physics Laboratory

Instruments of the highest quality - free air ionisation chamber or graphite calorimeter

30
Q

What is the Secondary calibration stardard?

A

High quality instruments often shared by departments or centres

These are calibrated against the primary standard every 2-3 years at the NPL which issues certificates

31
Q

What is Field instrument calibration?

A

These are routine measurements and consistency checks every 3 months - they are usually performed by measuring for a fixe time using a stable source of radiocativity e.g. Strontium-90

These are calibrated against the Secondary standard every 12 months

They measure dose and take into account any other factors such as temeprature, pressure, polarity, ion recombination corrections

32
Q

What are the different methods for dosimetry?

A
  • Ionisation methods e.g. ionisation chamber, geiger counter, and diodes
  • Thermoluminescence (TLD)
  • Calorimetry
33
Q

How do gas filled dosimeters work?

A
  1. Radiation beam passes through gas and ionises it
  2. Electrons and ions are generated - usually these would just recombine
  3. However, an electric charge is passed through the walls of the chamber so one side is an anode and one side is a cathode
  4. This separates the ions and electrons and allows the charges to be collected and measured
34
Q

What are the requirements for a successful Free Air Ionisation Chamber?

A
  • Distance between the collimator and collection volume must be greater than the range of electrons to achieve CPE
  • Distance between electrodes and collecting volume has to be less than the range of electrons to collect all of them
35
Q

How does a Geiger counter work?

A

Ionisation method

  1. Farmer Ionisation Chamber full of nobel gas with an anode electrode in the middle and cathode walls
  2. Incoming particle ionises nobel gas atoms
  3. Free electrons then hit other nobel gas particles causing an electrode cascade
  4. Ions travel to the cathode walls and gain an electron and become stable gas particles again
  5. The electrodes travel along the anode wire to the positive pole of the connected power supply
  6. High resistor forces majority of the electrons to flow through the counter which registers the current flowing through
36
Q

What are geiger coutners used for practically?

A

Radiation protectors

37
Q

What are the pros of geiger counters?

A
  1. Very sensitive
  2. Real-time
  3. Indication of intensity
38
Q

What are the cons of a geiger counter?

A
  1. Saturates at high doses of radiotherapy
  2. Only measures dose rate (not dose, energy or radiation type)
39
Q

How do diodes detect radiation?

A
  1. Small impure silicoln semi-conductors
  2. N-type produce free electrons, P-type produce ions
  3. If you put an n-type and a p-type together where they join electrons and ions will combine - this is the area of depletion
  4. No particles in this area can carry charge so no current will flow
  5. This leads to a small potential difference across the regions which can be detected
40
Q

What are diode dosimeters used for in practice?

A
  1. in-vivo dosimetry - put diodes on the patient
  2. Relative depth dose dosimetry
  3. Profile measurements

They can detect:
4. Machine output miscalculations
5. Incorrect treatment plan information transfer
6. Incorrect beam energy produced by the linac

41
Q

What are the pros of diode dosimeters?

A
  • High sensitivity
  • Instant reading
  • Can be small
  • Broad dynamic range
  • No high voltage applied
42
Q

What are the cons of diode dosimeters?

A
  • Over-estimates low energies
  • Slightly temperature dependent
  • Poor tissue equivalence
  • Sensitive to radiation damage
  • Lower dose region distal to detector if ‘buildup cap’
  • Needs connecting to an electrometer during dose delivery
43
Q

What is dose perturbation in diode dosimeters?

A

Diodes have ‘build up caps’ which ceate a lower dose region (shadow) distal to the detector
Magnitude of the shadow depends on the size of the build up cap
If the diode is used during only one fraction (for multi fraction treatments) the shadowing effect is negligible
If used during every fraction variation in the diode positioning will reduce the overall shadowing effect - moving it around will prevent underdosing at the point under the diode

44
Q

How do ThermoLuminescence detectors work?

A

Excitation

  1. Crystals are exposed to radiation
  2. Electrons in the crystals are excited and freed
  3. The electrons get trapped in the crystal lattice due to impurities and the energy is stored
  4. If the crystal is heated the energy is released as a light photon

The light released is proportional to the incident dose

45
Q

When are crystal dosimeters used?

A
  1. In-vivo dosimetry
  2. Personal monitoring
46
Q

What are the pros of crystal dosimeters?

A
  • Reusable
  • Very sensitive
  • Very small - can be used in vivo
  • Tissue equivalence
  • Wide dynamic range
47
Q

What are the cons of crystal dosimeters?

A
  • Poor accuracy - have to be careful calibrated
  • No instant readout
  • Can be read once before it needs wiping for reuse
  • Great care needed to acheive reproducability
48
Q

How do calorimeters work?

A
  1. Measure temperature rise of an absorbing medium
  2. Dose = temp rise x specific heat capacity (of water 4200J/kg/C)
49
Q

When are calorimeters used?

A

For the primary standard measurement at the NPL

50
Q

What are the pros of calorimeteres?

A
  • absolute - direct measure of dose for MV energies
  • various materials can be used - graphite because purer
  • not sensitive to radiation damage
  • independent of radiation type
51
Q

What are the cons of calorimeters?

A
  • Large dose required
  • Chemical defect must be accurately known - e.g. graphite chemical defect
  • Impracticle (bulky, sensitive, not ideal for hospitals)

Physics for FRCR CO1 (8 decks)

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