Dosimetry

Question 1

What is the equation for exposure and its unit?

Answer 1

X = dq/dm [Coloumbs per kg]

Question 2

What is equation for dose in air from exposure?

Answer 2

D_air = (W_air/e) X
where W_air is average energy to produce ion pair - 33.85eV

Question 3

Why is thimble chamber necessary for MV?

Answer 3

Need charged particle equilibrium. Minimum dimension between boundaries of dm and surrounding medium must be > range of electrons. This is impractical for MV energies - need 4m.

Question 4

Composition of thimble chamber

Answer 4

Smaller volume of air in thimble like conducting (graphite) cap with insulating axial collecting electrode. Walls similar Z to air.

Question 5

How does ionisation chamber work?

Answer 5

High polarising voltage between the wall and the central electrode
Beam interacts with air and creates ion pairs.
Ions created move to electrode if opposite charge
The collected charge is measured with an electrometer
Charge is proportional to dose - can calculate this

Question 6

Why use parallel plate chamber

Answer 6

Sometimes need to measure a high dose gradient field (electron beam, kV beam, build up region of MV beam). Thimble chamber is too big - parallel plate chamber is thin in the direction of the gradient for better resolution

Question 7

Why do we need to use saturation region

Answer 7

At low V, measured charge is proportional to current. At saturation voltage, current is saturated - nearly all ions created are collected.

Question 8

Most common farmer chamber for linacs

Answer 8

0.6cc Farmer chamber

Question 9

What are ionisation chambers good for

Answer 9

Absolute dose, PDD, output factors, penumbra (but last two may need smaller resolution)

Question 10

How do semiconductor diodes work

Answer 10

p-type semiconductor doped with excess holes
n-type doped with excess electrons
Combined to produce p-n junction
Incident radiation results in ionisation and electrons/holes created move towards a side, creating a current that is proportional to dose rate.

Question 11

General characteristics of Semiconductor diode

Answer 11

Energy dependence
Significant angular dependence
Significant temperature dependence
High spatial resolution
High sensitivity
Immediate readout
Stopping power makes them good for electron PDDs
Sensitivity can change with dose (usually flattens)

Question 12

Uses of semiconductor diode

Answer 12

Absolute dose (but high Z means over response to lower energies)
PDD (high resolution of very small volume but response varies with energy)
Outputs (small field measurements are possible, good sensitivity in small fields, variation of energy spectrum may be small enough to ignore)
Penumbra (steep dose gradients require high resolution)

Question 13

How does thermoluminescence dosimetry work

Answer 13

Crystals have conduction and valence energy bands
Lattice impurities produce additional energy levels below conduction band
This can trap electrons
Radiation excites electrons to conduction band, some will fall back down and get trapped
Released via heating, emitting a photon, and amount of light is proportional to dose
Anneal them to ensure all electrons are removed from traps before using again

Question 14

Advantages and disadvantages of TLDs

Answer 14

Advantages:
Linear response over wide energy range ~0.001 to ~10Gy
Sensitivity almost energy independent
Small size means high resolution
No leads/connections

Disadvantages:
Must be calibrated
‘Fade’ with time
Careful annealing necessary to ensure it returns to initial state
Affected by previous history

Question 15

Uses of TLDs

Answer 15

Commissioning and QA - small fields, high dose gradients
Backscatter internal dose
Patient dosimetry and personal dosimetry
TBI

Question 16

Film dosimetry methods

Answer 16

Radiographic, radiochromic (GafC)

Question 17

Advantages/disadvantages of film dosimetry methods

Answer 17

Advantages:
Good spatial resolution
Convenient for some commissioning
Can be calibrated for absolute dose measurement (with caution)
Geometry well suited for dose mapping

Disadvantages:
Silver halide needs wet processing (must be well controlled and consistent and available)
Silver halide film sensitivity energy dependent
Radiochromic dose sensitive to scan parameters

Question 18

How does radiographic film work

Answer 18

Layer of gel contains Ag halide crystals
Development leaves opaque microscopic grains of silver, detected optically, in quantity related to dose

Question 19

How doe GafC work?

Answer 19

Self developing
Exposure causes reactions between polymers, original transparent layer becomes opaque proportionally with dose
More expensive but no processing costs

Question 20

What is film used?

Answer 20

In linacs: gantry/collimator star shots, PDD, light vs radiation field size
HDR applicator commissioning
SXT commissioning
Patient dosimetry

Question 21

Chemical dosimetry

Answer 21

Absorption of ionising radiation results in secondary electrons, resulting in ionisation, excitation and breaking of chemical bonds. Can lead to chemical species as products - detection of these is basis of chemical dosimetry

Question 22

Fricke solutions

Answer 22

Radiation oxidises 4% frrous sulphate solution to ferric sulphate Fe2+ –> Fe3+ - ratio of these is measured by absorption spectrophotometry and change is proportional to dose.
Uses: departmental or national audits, low energy electron beam and Ir-192 dosimetry, 3D dose distribution visualisation

Question 23

Alanine

Answer 23

Solid state cylindrical pellets. Irradiation produces long-lived free radical species. Absorbed dose determined by electron paramagnetic resonence spectrum of free radicals.
Uses: departmental or national audit, linac commissioning, individual patient dosimetry