TG-51: Clinical Reference Dosimetry

Overview

AAPM TG-51 is the current standard for clinical dosimetry for high-energy photon and electron fields. It is the direct predecessor of TG-21 with a focus on simplicity and human error reduction. This simplification was accomplish primarily through three important changes. First, the use of plastic phantoms in primary dosimetry was eliminated. This eliminated the need to transfer dose for the phantom to water using stopping powers and mass attenuation ratios. Second,  TG-51 used absorbed-dose-to-water calibration factor (\(N_{D,w}^{60_{Co}}\)), referred to in the addendum as the calibration coefficient, as directly in an accredited dosimetry calibration laboratory (ADCL). This avoided TG-21's need to transfer dose to chamber gas to dose to phantom using Bragg-Gray (BG) Cavity Theory. Third, the quality conversion factor (\(k_Q\)) greatly simplifies the process by rolling ratio of stopping powers as well as chamber corrections (wall, gradient, fluence, and central electrode) into a single quantify that may be looked up as a function of beam quality.

An addendum to TG-51 was published in March 2014 addressing flattening filter free (FFF) beams, updating some values based on new clinical data, an analysis of sources of error in TG-51, and providing guidance in choosing a reference class dosimeter. Changes introduced to the protocol are identified herein along with the original TG-51 values.

Farmer Chambers

Key Point: Farmer chambers are thimble ionization chambers widely used in reference dosimetry. Questions regarding farmer chamber design and operation are common on board exams.

  • Typical Sensitive Volume: 0.6cc (approximately cylindrical, 0.3cm radius, 2cm length)
  • Typical Response: 20nC/Gy
  • Effective Point of Measurement:
    • Photons: 0.6rcav (~0.18cm) upstream of central axis
    • Electrons: 0.5rcav (~0.15cm) upstream of central axis

Photon Dosimetry Procedure

1. Select an ADCL calibrated ion chamber and record ND,wCo-60

Recommendations for ion chamber performance are provided in TG-51 Addendum.

Reference Class Ionization Chamber Specifications

MeasureSpecification
Pleak<0.1% of chamber reading (0.999 < Ppol < 1.001)
Pion = 1 + Cinit + CgenDppCinit < 0.2%
Pion should be linear with pulse
Ppol<0.4% correction (0.996 < Ppol < 1.004)
Chamber Stability<0.3% change over calibration period of 2 years
Chamber Settling<0.5% change in chamber reading per MU from first irradiation to stabilization of the ion chamber

2. Determine beam quality conversion factor (kQ) using %DD(10)x.

Measure percent depth dose of the photon component of the beam at 10cm depth in a 10x10cm2 field (%DD(10)x). This measurement must be made at effective point of measurement with SSD=100cm.

Note that effective point of measurement is 0.6rcav for cylindrical chambers in photon beams.

kQ is found using the below table and is a function of both %DD(10)x and ionization chamber used.

Use of lead foil

      • TG-51 required 1mm lead foil be placed at 50cm (±5cm) or 30cm (±1cm) from the water surface for high energy fields (>10MV). This measurement, referred to as %DD(10)Pb, serves to filter out scatter electrons from the treatment head while introducing a known, and thus compensated for, quantity of electrons generated in the foil. %DD(10)Pb is corrected to %DD(10)x using the below equations.
      • TG-51 addendum removes the requirement to use lead foil for high energy beams and adds the requirement that FFF fields use lead foil. Note that use of lead foil will improve measurement accuracy.

kQ Key Points: The beam quality correction factor (kQ) corrects for more than just energy dependence of the ion chamber! kQ also accounts for:

  • Electron contamination from lead foil
  • Perturbations in electron fluence due to chamber design
  • Effective point of measurement
    • Chamber will later be placed at point of measurement rather than effective point of measurement.
kQ values at 10cm depth in accelerator photon beams. Source: AAPM TG-51 figure 4.

3. Find the correction factors and convert Mraw to M.

Note: All these measurements are taken at 10cm point of measurement in the 10x10cm2 reference field.

Pion is found by comparing readings with a high and low voltage applied to the ion chamber using the below equations. The subscripts L and H indicate low or high voltage, typically 150V and 300V respectively.

Pion Key Points:

Pion corrects for loss of ion collection efficiency due to recombination. Therefor, Pion cannot be lower than 1!

Pion generally increases with increased dose per pulse.

Pion values for farmer chamber:

  • 1.003 for 6X
  • 1.006 for 18X
  • 1.014 for electrons

PPol is found by measuring a with a positive and negative voltage. 

PTP is found using local atmospheric pressure for air communicating chambers and temperature. Here P is given in kPa and T is given in Celsius.

Pelec accounts for measurement error in the electrometer. It is given by the ADCL and will be 1 if the ion chamber and electrometer were calibrated together.

Pleak  accounts for leakage current of the ion chamber. It is typically measured with equipment in place, accelerator on, but no beam on. If the ion chamber meets the TG-51 addendum specifications for a reference-class ion chamber (i.e. Pleak < 0.1%) then Pleak may be taken as 1.000.

Prp is computed by the average of the radiation profile over the dimensions of the active part of the ion chamber then correcting to the reading at the point of measurement. At the time of writing, it remains common to assume Prp=1 for flattened fields.

PPol Key Points:

PPol corrects for differences in collection efficiency with different polarities.

PPol is usually the smallest correction factor that is routinely applied.

TG-51 and the TG-51 addendum vary in their acceptable range and handling of Ppol.

        • TG-51 states that if Ppol correction is <0.997 or >1.003, substitute for .
        • TG-51 addendum states that a reference class ion chamber must have 0.996≤ Ppol ≤ 1.004 at any energy and that the total variation in Ppol across all energies must be less than 0.5%.

4. Dose at 10cm depth is computed.

Measurement is made at 10cm point of measurement in a 10x10cm2 field using either an SSD or SAD setup.

5. %DD is used to compute dose at calibration depth (usually dmax).

When computing dose at calibration depth, one may choose to use either a measured %DD curve or the %DD curve modeled in one's treatment planning system. Using a measured %DD will optimize accuracy at dmax while using the TPS value of %DD(10) will optimize TPS calculation accuracy at depth. While either measurement technique is valid, it is generally accepted that TPS accuracy at depth is most clinically significant.

Electron Dosimetry Procedure

1. Select a calibrated ion chamber and record ND,wCo-60

Cylindrical Chambers may be used for electron fields where R50≥2.6cm (≥6MeV) and should calibrated at an ADCL.

Plane-Parallel Chambers must be used for electron fields  where R50<2.6cm (<6MeV) and are recommended for fields with R50<4.3cm (<10MeV). Plane-Parallel chambers should be cross calibrated in the user's beam with an ADCL calibrated cylindrical ion chamber. Read the plane-parallel chamber cross calibration procedure here (local link).

Key Point: Plane-parallel chambers must be cross calibrated against an ADCL calibrated cylindrical chamber in the users beam. They are not calibrated directly at an ADCL.

2. Determine the reference field size (usually 10x10cm)

For beams with R50≤8.5cm (<20MeV) a 10x10cm2 field may be used.

For R50>8.5cm (>20MeV) a 20x20cm2 or greater field must be used.

Reference Class Ionization Chamber Specifications

MeasureSpecification
Pleak<0.1% of chamber reading (0.999 < Ppol < 1.001)
Pion = 1 + Cinit + CgenDppCinit < 0.2%
Pion should be linear with pulse
Ppol<0.4% correction (0.996 < Ppol < 1.004)
Chamber Stability<0.3% change over calibration period of 2 years
Chamber Settling<0.5% change in chamber reading per MU from first irradiation to stabilization of the ion chamber

3. Determine the electron quality conversion factor (k'R50)

i. Measure an ionization curve using the field size determined in step 2 with SSD = 100cm using effective point of measurement. Use this curve to determine the depth of 50% of maximum ionization (I50).

Note: effective point of measurement for a cylindrical chamber in an electron beam is 0.5rcav and is the upstream face of the chamber for a plane-parallel chamber.

ii. I50 is converted to depth of 50% maximum dose (R50) using the below equation.

iii. k'R50 is found as a function of R50 and chamber model using table 5 from TG-51.

Calculated k'R50 values at dref Image credit: AAPM TG-51 figure 5.

Key Point: Conversion from depth ionization curve to depth dose curve is required for electron measurements but not for photon measurements.

Why?

Photon beams are liberating electrons continually along the beam path. As a result, although the electron fluence may vary with depth, the spectrum of electron energies is roughly constant. Since stopping power varies as a function of energy, this means that stopping power doesn't change with depth for electrons generated by a photon beam. Therefor depth dose and depth ionization curves both have the same shape.

Electron beams in contrast are constantly losing energy. This means that the stopping power for electron beams does vary with depth and the shape of a depth ionization curve is not the same as a depth dose curve. Failure to correct from this effect would result in a 3-5% error in PDD(dref)!

4. Determine the photon-electron conversion factor (kecal)

Kecal is needed to convert the ADCL supplied calibration factor (ND,wCo-60) from a photon reference to an electron-beam absorbed-dose calibration factor.

Kecal is determined from tabulated values presented in tables  III of TG-51.

Tabulated k<sub>ecal</sub> values for plane-parallel chambers. Image credit: AAPM TG-51 table II.
Tabulated kecal values for plane-parallel chambers. Image credit: AAPM TG-51 table II.
Tabulated kecal values for cylindrical chambers. Image credit: AAPM TG-51 table III.

5. Determine reference depth (dref)

dref is chosen as the measurement depth rather than 10cm (as in photon beams) because 10cm depth is in or beyond the region of rapid dose fall off for many clinical beams. dref is also preferred over simply using dmax because of significant scatter electron contribution at dmax in higher energy fields (<12MeV).

Typical %DD(dref) values:

<12E: dref ≅ 100%
16E: dref ≅ 99%
18E: dref ≅ 98%
20E: dref ≅ 95.5%

6. Determine the gradient correction factor (PgrQ) for cylindrical chambers

Measurement of Mraw should be taken at point of measurement.

Key Point: It's not necessary to physically measure Pgr! The same correction can be achieved by using effective point of measurement rather than point of measurement in subsequent measurements.

 

Typical PgrQ  values: 

Plane-parallel chambers: PgrQ = 1.00

Cylindrical chamber:

E < 12 MeV: PgrQ > 1.00
E = 12 MeV: PgrQ ≅ 1.00
E > 12 MeV: PgrQ < 1.00

Why this trend?

Because for low energies dref ≅ dmax placing the effective point of measurement for Mraw(dref) measurement in the buildup region.  At higher energies dref > dmax which means that shallower depths will have greater dose.

7. Find the correction factors and convert Mraw to M.

Note: All these measurements are taken at reference depth (drefpoint of measurement as determined in step 5.

Pion is found by comparing readings with a high and low voltage applied to the ion chamber using the below equations. The subscripts L and H indicate low or high voltage, typically 150V and 300V respectively.

Pion Key Points:

Pion corrects for loss of ion collection efficiency due to recombination. Therefor, Pion cannot be lower than 1!

Pion generally increases with increased dose per pulse.

Pion values for farmer chamber:

  • 1.003 for 6X
  • 1.006 for 18X
  • 1.014 for electrons

PPol is found by measuring a with a positive and negative voltage. 

PTP is found using local atmospheric pressure for air communicating chambers and temperature. Here P is given in kPa and T is given in Celsius.

Pelec accounts for measurement error in the electrometer. It is given by the ADCL and will be 1 if the ion chamber and electrometer were calibrated together.

Pleak  accounts for leakage current of the ion chamber. It is typically measured with equipment in place, accelerator on, but no beam on. If the ion chamber meets the TG-51 addendum specifications for a reference-class ion chamber (i.e. Pleak < 0.1%) then Pleak may be taken as 1.000.

Prp is computed by the average of the radiation profile over the dimensions of the active part of the ion chamber then correcting to the reading at the point of measurement. At the time of writing, it remains common to assume Prp=1 for flattened fields.

PPol Key Points:

PPol corrects for differences in collection efficiency with different polarities.

PPol is usually the smallest correction factor that is routinely applied.

TG-51 and the TG-51 addendum vary in their acceptable range and handling of Ppol.

        • TG-51 states that if Ppol correction is <0.997 or >1.003, substitute for .
        • TG-51 addendum states that a reference class ion chamber must have 0.996≤ Ppol ≤ 1.004 at any energy and that the total variation in Ppol across all energies must be less than 0.5%.

8. Dose at reference depth (dref) is computed.

Note: SSD may be chosen in the range of 90-110cm.

9. %DD is used to compute dose at calibration depth (usually dmax).

When computing dose at calibration depth, one may choose to use either a measured %DD curve or the %DD curve modeled in one's treatment planning system. Using a measured %DD will optimize accuracy at dmax while using the TPS value of %DD(dref) will optimize TPS calculation accuracy at depth. While either measurement technique is valid, it is generally accepted that TPS accuracy at depth is most clinically significant.

Plan-Parallel Ion Chamber Cross Calibration

Because Plane-Parallel chambers are sensitive to very small changes in their construction, it is recommended by TG-21, TG-39, and TG-51 that they be cross calibrated against an ADCL calibrated cylindrical chamber in a high energy electron field. Steps for this process are given below.

1. Determine dose at reference depth for a high energy electron field using the cylindrical chamber (Dw)Cyl.

2. Measure M at the reference depth for the cylindrical chamber.

3. Look up k'R50 for the Plane-Parallel chamber in figure 5 of TG-51 (internal link).

4. Look up kecal for the Plane-Parallel chamber using table II of TG-51 (internal link).

5. Compute (kecal ND,wCo-60)PP and use this factor in place of (kecal ND,wCo-60) for future plane-parallel chamber measurements.

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