L 8: Measurement of absorbed dose

Question 1

TG-21

Answer 1

• This AAPM calibration protocol (published in 1983) has been superseded by the AAPM TG-51 protocol.
• TG-21 requires chamber calibration in terms of air kerma or Nx (exposure calibration factor for cobalt-60 beam).
• Ngas (dose to cavity air per unit charge of ionization) is calculated from Nx and other factors related to chamber design.
• Phantoms used were PAW- Polystyrene, Acrylic plastics & Water.

Question 2

TG-51

Important

Answer 2

• Published in 1999
• It is the clinic reference for high energy photons and electron beams.
• Photon beam energy 1.25Mv to 50 Mv
• Electron beam energy 4-50MeV
• Only phantom is water
• The major difference between TG-51 and TG-21 is the chamber calibration, which is based on absorbed dose to water instead of exposure in air.
• ND,w 60Co is the absorbed dose to-water calibration factor for the chamber determined in a cobalt-60 beam under reference conditions.
• Beam quality for photon beams is specified by percent depth dose for the photon component of the beam at 10 cm depth in water [%dd(10)x].
• Beam quality for the purpose of electron beam calibration is specified by the depth of 50% dose in water (R50).
• Photons = Sensitivity of monitor chambers is adjusted to give Dmax/MU
  close to unity for a 10 × 10-cm field size at SSD = 100 cm (SSD-type calibration) or SAD = 100 cm (SAD-type calibration).
• Calibration of an electron beam is performed at a reference depth dref given by dref = 0.6 R50 – 0.1cm. It is then converted to dose at dmax by using percent depth dose at dref. Calibration is set to give Dmax/MU close to unity for a 10 × 10-cm field size (reference applicator) at SSD = 100 cm.
• So reference depth for photons = 10cm; electrons = 0.6R50-0.1 cm
• The difference in measured dose between TG-21 and TG-51 is less than 2% for photons but can be as much as 5% for electron beams.

Question 3

Important formula for TG-51

Answer 3

Reference depth
Photons = 10cm
Electrons = 0.6R50-0.1cm
Pion: should have less than 0.5% error, should not exceed 1.05

Question 4

IAEA TRS-398

Answer 4

• Used in Europe
• Published in 2000
• There are minor differences between TG-51 and IAEA TRS-398; for example, beam quality specification in TRS-398 is by TPR20,10 instead of %dd(10)x.

Question 5

Absolute dosimeters

Answer 5

• Absolute dosimetry means that the dose is determined from the first principles—without reference to another dosimeter.
• The free-air ionization chamber, specially designed spherical chambers of known volume (e.g., at NIST), the calorimeter, and the ferrous sulfate (Fricke) dosimeter are examples of absolute dosimeters.
• They are also called primary standards.

Question 6

Secondary dosimeters

Answer 6

• Secondary dosimeters require calibration against a primary standard. Examples are thimble chambers and plane-parallel ion chambers.
• TLDs, diodes, and film are also secondary dosimeters but are used primarily for relative dosimetry.
• They require calibration against a calibrated ion chamber as well as appropriate corrections for energy dependence (e.g., with depth) and other conditions that may affect their dose response characteristics.

Question 7

TLDs

Answer 7

• The most commonly used TLD consists of LiF with a trace amount of impurities (magnesium).
• It is available in many forms and sizes for use in special dosimetry
• situations (e.g., powder capsules, extruded rods or chips, and crystals embedded in Teflon or silicon disks). It is reusable if properly annealed and recalibrated in terms of its dose–response curve.
• TLD response is almost independent of energy in the megavoltage range of photon and electron beams used clinically.

Question 8

Diodes

Answer 8

• Silicon p–n junction diodes are well suited for relative dosimetry of electron beams, output constancy checks, and in vivo patient dose monitoring.
• Their higher sensitivity, instantaneous response, small size (~0.2 to 0.3 mm3), and ruggedness offer special advantages over ionization chambers in certain situations.
• Their major limitations as dosimeters include energy dependence in photon beams,directional dependence, thermal effects, and radiation-induced damage with prolonged use. Modern diodes minimize these effects.
• Unlike ion chambers, diodes do not require high-voltage bias to collect ions.

Question 9

Radiographic film

Answer 9

• Sensitivity of film depends on the size of emulsion grains (crystals of silver bromide) and the quality and type of radiation.
• Optical density is given by** log10(I0/It), **where I0 is the amount of light incident on film and It is the amount of light transmitted through film. It is measured by a densitometer having a light source and a tiny aperture (~1 mm diameter or less).
• Film is well suited for relative dosimetry of electron beams (shows practically no energy dependence). In photon beams, however, it shows significant energy dependence and therefore it is used mostly for portal imaging and quality assurance procedures such as checking beam alignment, isocentric accuracy, and beam flatness.
• For measuring dose distributions, photon energy dependence must be taken into account.

Question 10

Radiochromic film

Answer 10

• Major advantages include almost tissue equivalence, high spatial resolution, large dynamic range (10−2 to 106 cGy), low energy dependence, insensitivity to visible light, and no need for processing.
• It is well suited for dosimetry of brachytherapy sources where the doses and dose gradients close to the sources are very high.

Question 11

How do you define beam quality for <3MeV

Answer 11

Kvp
Filteration
Half value layer

Question 12

How do you define beam quality for >3MeV

Answer 12

% depth dose
TAR
TMR