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Radiotherapy > RT3 - Clinical use of dosimetry equipment > Flashcards

RT3 - Clinical use of dosimetry equipment Flashcards

1
Q

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

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

Ion chamber vs Diode:

Asbolute Dose

A

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
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3
Q

Ion chamber vs Diode:

PDDs

A

Ion Chamber

  • Ideal, flat energy response

Diode

  • High resolution (small active volume) is useful (particularly in build-up region) but response varies with energy
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4
Q

Ion chamber vs Diode:

Ouput factors

A

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
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5
Q

Ion chamber vs Diode:

Penumbra (beam profile)

A

Ion chamber

  • Acceptable but low res due to large collecting volume

Diode

  • Steep dose gradients need high res, use a diode for beam profile
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6
Q

Principle of Thermoluminescence

A
  • 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
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7
Q

Advantages of TLDs

A
  • Linear response with dose over wide range
  • Sensitivity is nearly energy independent
  • Small size (2mm) = high res
  • No leads/connections needed
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8
Q

Disadvantages of TLDs

A
  • 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
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9
Q

Uses of TLDs

A
  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
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10
Q

What is TBI?

How do you do dosimetry?

A

Total Body Irradiation

  • TLD sachets at several location on patient
  • Additional sachets irradiated at known dose for clibration
  • Results compared to expected
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11
Q

2 Types of film dosimetry?

A
  1. Radiographic
  2. Radiochromic (GafC)
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12
Q

Advantages of Film?

A
  • Good spat res
  • Convenient for QA and commisioning
  • Calibrated for absolute dose
  • Geometry is good for dose mapping (thin, flat, large area)
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13
Q

Disadvantgaes of Film?

A
  • Wet processing, not very consistent
  • Sensitivity is energy dependent (PE proportial to z^3)
  • dose is sensitive to scan parameters
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14
Q

How does Radiographic film work?

A
  • Silver halide crystals become opaque when irradiated
  • Detected optically
  • Optical density vs Dose = charichteristic curve
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15
Q

How does Radiochromic film (GafC) work?

A
  • 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
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16
Q

Uses of Gafc

A
  1. Star shots (isocentric test)
  2. PDD for shallow depths
  3. Light vs rad field size
  4. Picket fence tests
17
Q

Chemical Dosimetry principles

A
  • 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
18
Q

Two types of chemical dosimetry

A
  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
19
Q

Why are chambers better than TLD film and chemical methods?

A

Other methods are:

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

Only realy useful as checks in QA or audit.

Radiotherapy (14 decks)

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