RT3 - Clinical use of dosimetry equipment

Question 1

What could you use for clinical dosimetry in RT (x5)

Answer 1

1. Ion chamber
2. Semiconductor Diode
3. TLD
4. Film
5. Chemical

Question 2

Ion chamber vs Diode:

Asbolute Dose

Answer 2

Ion Chamber

• Density & atomic number close to water, minimum perturbation of energy fluence

Diode

• Higher density Z leads to significant perturbation of energy fluence
• Ocer-response to low energies

Question 3

Ion chamber vs Diode:

PDDs

Answer 3

Ion Chamber

• Ideal, flat energy response

Diode

• High resolution (small active volume) is useful (particularly in build-up region) but response varies with energy

Question 4

Ion chamber vs Diode:

Ouput factors

Answer 4

Ion Chamber

• Spectral variation over field sizes not issue
• physical size of collecting volume is bad for small field sizes (loss of electronic equilibrium)
• Smaller volume reduces sensitivity

Diode

• Size of active component menas you can measure small fields
• Sensitive
• Variation in spectrum small enough to ignore over small range of field sizes

Question 5

Ion chamber vs Diode:

Penumbra (beam profile)

Answer 5

Ion chamber

• Acceptable but low res due to large collecting volume

Diode

• Steep dose gradients need high res, use a diode for beam profile

Question 6

Principle of Thermoluminescence

Answer 6

• Impurities in material’s latice prodeuces additional energy levels (a few eV below conduction band). Meta-stable to trap electrons
• Radiation excites electrons into conduction band, relax back but some get trapped
• Heating material results in trapped electrons premoted to conduction band and relaxing back to valence emitting light
• Light collected ∝ absorbed dose

Question 7

Advantages of TLDs

Answer 7

• Linear response with dose over wide range
• Sensitivity is nearly energy independent
• Small size (2mm) = high res
• No leads/connections needed

Question 8

Disadvantages of TLDs

Answer 8

• Need to be calibrated
• Fade with time
• Annealing is needd to return electrons to traps
• Affected by history of thermal and radiation
• Difficult to keep constant over long periods

Question 9

Uses of TLDs

Answer 9

1. Commisioning & QA
   
   • small fields, surface & high dose/rates, internal phantom dose
2. Patient Dosimetry
   
   • Total body irradiation (TBI)
   • Skin
   • Eyes
3. Personal Dosimetry
   
   • Multi-chip badges

Question 10

What is TBI?

How do you do dosimetry?

Answer 10

Total Body Irradiation

• TLD sachets at several location on patient
• Additional sachets irradiated at known dose for clibration
• Results compared to expected

Question 11

2 Types of film dosimetry?

Answer 11

1. Radiographic
2. Radiochromic (GafC)

Question 12

Advantages of Film?

Answer 12

• Good spat res
• Convenient for QA and commisioning
• Calibrated for absolute dose
• Geometry is good for dose mapping (thin, flat, large area)

Question 13

Disadvantgaes of Film?

Answer 13

• Wet processing, not very consistent
• Sensitivity is energy dependent (PE proportial to z^3)
• dose is sensitive to scan parameters

Question 14

How does Radiographic film work?

Answer 14

• Silver halide crystals become opaque when irradiated
• Detected optically
• Optical density vs Dose = charichteristic curve

Question 15

How does Radiochromic film (GafC) work?

Answer 15

• Exposure causes reaction between polymers, transparent layer becomes opaque in proportion to dose
• Self-developing & scanned (direction is impoertant)
• Sensitivity independent of energy
• Expensive
• Tissue equiv
• Can go in water

Question 16

Uses of Gafc

Answer 16

1. Star shots (isocentric test)
2. PDD for shallow depths
3. Light vs rad field size
4. Picket fence tests

Question 17

Chemical Dosimetry principles

Answer 17

• Ionising radiation = cascade of secondary electrons, tranfers energy to molecules, ionisation excitation and breaking of chemical bonds
• Reactive species produce stable products that you measure

Question 18

Two types of chemical dosimetry

Answer 18

1. Fricke Solutions - Radiation oxidises fe2+ to fe3+. Absorption specrophotometry used to assess ratio proprtional to dose. 3D dose distribution.
2. Alanine - Solid-state cylindrical pellets, irradiation = free radicals, dose determine from EPR spectrum. Used for individual patient dosimetry

Question 19

Why are chambers better than TLD film and chemical methods?

Answer 19

Other methods are:

• Fiddly & time consuming
• Need calibrations
• Cosly
• Full of errors

Only realy useful as checks in QA or audit.