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Absolute Dose Measurement. TG-51 : Photons. D Q,water is the absorbed dose to water at the point of measurement of the ion chamber placed under reference conditions for a clinical beam with beam quality Q. TG-51 : Photons.

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tg 51 photons
TG-51 : Photons

DQ,water is the absorbed dose to water at the point of measurement of the ion chamber placed under reference conditions for a clinical beam with beam quality Q.

tg 51 photons1
TG-51 : Photons
  • D,wNCo is the absorbed-dose-to-water calibration factor traceable to national primary standards for the ion chamber used.
photon beam quality q
Photon Beam Quality (Q)
  • The beam quality Q is specified by the %dd(10)x.
    • %dd – Percent Depth Dose
    • (10)x – Photon component at a 10-cm depth for a field size of 10x10 cm2 on the surface of a phantom at SSD of 100 cm.
  • Photon energies < 10 MeV, %dd(10)x is the fractional depth dose at 10 cm depth, %dd(10).
  • Depth ionization curve can be treated as depth dose.
  • Be sure to shift the depth dose upstream by 0.6rcav.
    • rcav is the radius of the air cavity of the ion chamber.
photon beam quality q2
Photon Beam Quality (Q)
  • Photon energies > 10 MeV, %dd(10)x is obtained by taking depth-dose measurements using a 1-mm lead foil placed 30 or 50 cm from the phantom surface and applying appropriate equation in the protocol.
    • %dd(10)x = [0.8905 + 0.00150 . %dd(10)Pb] . %dd(10)Pb
      • Foil at 50 cm, %dd(10)Pb ≥ 73%
    • %dd(10)x = [0.8116 + 0.00264 . %dd(10)Pb] . %dd(10)Pb
      • Foil at 50 cm, %dd(10)Pb ≥ 71%
    • If %dd(10)Pb is less than thresholds, %dd(10)x = %dd(10)Pb
photon beam quality q3
Photon Beam Quality (Q)
  • No lead foil available?
    • %dd(10)x = 1.267%dd(10) – 20.0
      • For 75% < %dd(10) ≤ 89%
    • %dd(10)x = %dd(10)
      • For %dd(10) ≤ 75%
    • Not recommended, but acceptable if unavoidable.
ion chamber reading m
Ion Chamber Reading (M)
  • Point of measurement – the point at which absorbed dose is measured.
    • Cylindrical ion chambers – on the central axis of the cavity at the center of the active volume of the cavity.
    • Plane-parallel chamber – at the front (upstream side) of the air cavity at the center of the collecting region.
  • Mraw is the uncorrected ion chamber reading with the point of measurement at a depth d in water, for a given number of monitor units.
ion chamber reading m1
Ion Chamber Reading (M)
  • M : the fully corrected ion chamber reading.
  • M = Pion. PTP. Pelec. PPol. Mraw
    • Pion : recombination correction factor
    • PTP : temperature/pressure correction factor
    • Pelec : electrometer correction factor
      • Calibrated at ADCL for a specific scale
    • PPol : polarity correction factor
recombination p ion
Recombination (Pion)
  • Pion : recombination correction factor
  • Pion is dependent on chamber design, pulse rate, and bias voltage.
    • Usually pulse rate is set for a particular dose rate, but measurements should be redone if pulse rate is changed for that dose rate.
  • Ion chambers with Pion > 1.05 should not be used because the uncertainty in the Pion value.
  • Bias voltage should not be above 300 V, again because theory starts to breaks down.
recombination p ion1
Recombination (Pion)
  • For continuous beams (i.e., 60Co)
  • For pulsed or scanned beams
temperature pressure p tp
Temperature/Pressure (PTP)
  • Standard conditions
    • Temperature: 22° C
    • Pressure 101.33 kPa (760 mmHg)
  • Wait 5-10 minutes after readings stabilize to assume equilibrium with environment.
  • Humidity in range of 20% to 80% not a factor.
polarity p pol
Polarity (PPol)
  • Polarity effects vary with beam quality and other conditions such as cable position.
  • Measured with reference voltage of opposing bias.
quality conversion factor k q
Quality Conversion Factor (kQ)
  • kQ is the quality conversion factor which converts the calibration factor for a 60Co beam to that for a photon beam of quality Q.
  • kQ has been determined for numerous commercial ion chambers and varies with beam quality.
  • kQ = 1.000 for 60Co by definition.
  • kQ have not been determined for plane-parallel chambers because of insufficient information about wall correction factors in photon beams other than 60Co.
tg 51 electrons
TG-51 Electrons
  • M – raw measured ionization reading
  • kQ – beam quality conversion factor
  • D,wNCo60 – absorbed-dose to water calibration factor
beam quality conversion factor
Beam Quality Conversion Factor
  • grPQ – gradient correction factor – the component of kQ that is dependent on the ionization gradient at the POM.
  • k50 – component of kQ in an electron beam that is independent of the ionization gradient at the POM (point of measurement).
  • R50k' – electron quality conversion factor
  • kecal – photon-electron conversion factor
depth of 50 dose r 50
Depth of 50% dose (R50)
  • First measure ionization curve
    • Remember to shift depth dose upstream by 0.5rcav when using a cylindrical ion chamber.
  • Locate depth of 50% ionization (I50)
detectors for tg 51 electrons
Detectors for TG-51 Electrons
  • R50 ≤ 2.6 cm (6 MeV or less)
    • Must use plane-parallel chamber
  • R50 ≤ 4.3 cm (10 MeV or less)
    • Plane-parallel chamber is preferred
  • R50 > 4.3 cm
    • Cylindrical or plane-parallel chamber can be used
reference condition
Reference Condition
  • Reference depth dref = 0.6R50 – 0.1 cm
  • POM of chamber is placed at dref
  • SSD = 100 cm
  • For R50 ≤ 8.5 cm, use a 10x10-cm2 field
  • For R50 > 8.5 cm, use a 20x20-cm2 field
gradient correction factor gr p q
Gradient Correction Factor grPQ
  • grPQ = 1.00 for plane-parallel chambers
  • Close to unity for cylindrical chambers in an electron beam of 10 MeV or less.
  • For cylindrical chambers:
r50 k plane parallel chambers
R50k' – plane parallel chambers
  • Figure 6 and 8 gives r50k' as a function of R50 for various plane-parallel chambers.

Figure 6. Low-energy electron beams

Figure 8. High-energy electron beams

r50 k plane parallel chambers1
R50k' – plane parallel chambers
  • Analytical expression of curves in Figs. 6 and 8.
r50 k cylindrical chambers
R50k' – cylindrical chambers
  • Figure 7 plots r50k' vs. R50 for various cylindrical chambers.
r50 k cylindrical chambers1
R50k' – cylindrical chambers
  • Analytical expression of curves in Figs. 6 and 8.
photon electron conversion factor
Photon-electron conversion factor
  • kecal determined for commercial chambers using beam quality R50 = 7.5 cm.
  • Cylindrical chambers
    • Table III lists kecal value for cylindrical chambers.
  • Plane-parallel chambers
    • Table II lists kecal value for plane-parallel chambers
    • It is recommended to cross-calibrate plane-parallel chambers with cylindrical chambers in a high-energy electron beam using equation 21 (item 8 on worksheet).
absorbed dose at d max
Absorbed dose at dmax
  • If dref does not occur at dmax on the depth dose curve, the reference output dose must be corrected to dmax.
    • Will happen for high-energy beams.
    • Example: dref = 95%, D(dmax) = D(dref)/0.95
  • Use the depth-dose curve, not the depth-ionization curve.
    • Will require TG-25 protocol to determine stopping power ratios and Prepl as a function of depth if measured with ion chamber.
    • Can be measured directly with diode detector.