Measurement of Absorbed Dose (6) CALIBRATION OF MEGAVOLTAGE BEAMS: TG-21 PROTOCOL A protocol for the determination of absorbed dose from high energy photon and electron beams

TG-21 Protocol (Med Phys 10:741-771, 1983) Exposure calibration factor (Nx) Cavity Gas calibration factor (Ngas) (In Co-60 beam) Part I (any radiation quality and energy) med med water x x x gas Dose to cavity gas Dgas dose to medium Dmed Dose to medium Dmed dose to water Dwater Part II Part III

Part I Exposure calibration factor (Nx) (In Co-60 beam) Cavity Gas calibration factor (Ngas)

Nx : 60Co Exposure Calibration Factor of a Chamber 60Co source M is the charge collected, corrected for temperature and pressure X is the exposure (in Roentgen) at the point of measurement × X M (22C, 760 mm Hg) NX is R/C (or R/reading), obtained in a primary or secondary standard laboratory.

Ngas : Cavity Gas Calibration Factor of a Chamber 60Co source Dgas, dose to cavity gas × X M (22C, 760 mm Hg) Q and M are, respectively, the charge produced and collected, corrected for temperature and pressure. They are related by the collection efficiency Q=M/Aion

Characteristics of Ngas Ngas only depends on the volume or the mass of the air in the chamber cavity. It is independent of the type or energy of the incident radiation. Once obtained, its value remains the same unless the physical properties are changed. If we know the volume (or mass) of the air in the chamber cavity, then we know Ngas. Unfortunately, the chamber volume cannot be accurately measured. Instead, Ngas is calculated from Nx.

Conversion from Nx to Ngas For commonly used ionization chambers TG-21: Table XVII

Conversion from NX to Ngas 60Co source Photon energy fluence electron fluence × X Dwall Dgas k = 2.58x10-4 C/kg-R Slide #9

Conversion from NX to Ngas (cont’d) Slide #8 Slide #10

Conversion from NX to Ngas (cont’d) cap wall Slide #9 is the fraction of ionization due to electrons from the wall. (1-) is the fraction of ionization due to electrons from the cap.

Conversion from NX to Ngas (cont’d) k = 2.58x10-4 C/kg-R bwall = 1.005 If wall-material = cap-material, a = 1.0, If wall-material ≠ cap-material, obtain a from TG-21: Fig.1

Conversion from NX to Ngas (cont’d) use TG-21: Table I

Conversion from NX to Ngas (cont’d) Aion provided by the standard lab Awall use TG-21: Table II or Table III

Conversion from air-kerma calibration factor Nk to exposure calibration factor Nx x

Part II med med x x gas Dose to cavity gas Dgas dose to medium Dmed

Photons & electrons any energy Dose-to-cavity gas Photons & electrons any energy Dgas Chamber cavity gas calibration factor Chamber reading needs corrections

Corrections to Chamber Reading Reading for an unsealed chamber generally needs to be corrected for temperature, pressure, polarity effect, and ion-recombination correction factor Pion(the inverse of collection efficiency Aion). Pion can be obtained using the two-voltage method: Q1 is the charge collected at voltage V1. Q2 is the charge collected at voltage V2= V1 /2. Pion can be obtained through the ratio Q1/ Q2. (TG-21: Fig.4)

TG-21: Fig.4

Conversion from Dose-to-cavity gas to Dose-to-medium Photons & electrons any energy (thin wall, or wall material same as medium) × Dgas Dmed

Conversion from Dose-to-air to Dose-to-medium (cont’d) Dependent on beam quality & energy

Ratios of mean, restricted mass collision stopping powers Photon beams (TG-21: Fig.2, Table IV)

Characterization of Beam Quality (photons) For photon beams, energy is characterized by the ionization ratio of a 10x10 cm field: d = 20 cm d = 10 cm I20 I10 OR

Electron beams (TG-21: Table V-VII for water/air, polystyrene/air, acrylic/air) Ratios of mean, restricted mass collision stopping powers

Characterization of Beam Quality (electrons) For electron beams, energy is characterized by: 100% ionization 50% Depth in water (cm) d50

Replacement correction factor: Prepl For parallel-plate chambers: Prepl = 1.00 point of measurement x x gas med med

Replacement correction factor: Prepl (Photon beams) For cylindrical chambers: d x x gas med med point of measurement Photon beams (gradient corrections) If d = dmax, Prepl = 1.0 If d > dmax, use TG-21: fig.5

Replacement correction factor: Prepl (Electron Beams) For cylindrical chambers: dmax point of measurement x x gas Electron beams (fluence corrections) med med d = dmax, use TG-21: Table VIII

Conversion from Dose-to-air to Dose-to-medium Photons (Wall material different from medium) Thick wall: All ionization produced by electrons arising in the wall wall wall × × Dgas Dwall Dmed med med med

Conversion from Dose-to-air to Dose-to-medium Photons (Wall material different from medium) a is the fraction of total ionization produced by electrons arising in the wall, (1- a) the fraction arising in the medium

Wall Correction Factor: Pwall For electron beams: Pwall = 1.00 For photon beams: ‘a’ is the fraction of total ionization produced by electrons arising in the wall, (1- a) the fraction arising in the medium. use TG-21, Fig.7

Wall Correction Factor: Pwall For electron beams: Pwall = 1.00 For photon beams:

(note: chamber is not involved in this part.) Part III x x med water Dose to medium Dmed dose to water Dwater (note: chamber is not involved in this part.)

Conversion from Dose-to-medium to Dose-to-water (photons) Same source-to-detector distance, same field size, But scale depth by: SF = dmed/dwater (TG-21: Table XIII) dwater dmed × × Dmed Dwater med water Under conditions of electron equilibrium:

Conversion from Dose-to-medium to Dose-to-water (photons)

Ratios of mass energy absorption coefficients (photons) Use TG-21: Table XII

Excess scatter correction (ESC) (photons) for polystyrene: ESC = 1.0 for acrylic: use TG-21, Table XIV

Dose to water at dmax: Dwater(dmax) (photons) SSD setup SAD setup d isocenter dmax × dmax isocenter × d water × water

Conversion from Dose-to-medium to Dose-to-water (electrons) dmax dmax × × Dmed Dwater plastic water

Conversion from Dose-to-medium to Dose-to-water (electrons)

Ratios of mean, unrestricted collision mass stopping powers (electrons) Use TG-21: Table XV Ratios of electron fluences at dmax: For acrylic: For polystyrene: Use TG-21: Table XVI

TG-21 Summary photon electron Dwater(dmax) dose to water Dgas dose to chamber cavity gas Dmed dose to phantom medium Dwater dose to water 1.00 (polystyrene) Table XIV (acrylic) Table XII photon Fig.5 (photon) Table VIII (electron) electron Fig.7, fig.2 or table IV, table IX (photon) 1.00 (electron) Fig.2, Table IV (photon) Tables V-VII (electron) Table XVI (polystyrene) 1.00 (acrylic) 1.030 (polystyrene) 1.033 (acrylic) Fig.4

Miscellaneous

For Output Calibration point of measurement Inner surface of the front plate of Parallel-plate chamber Center of cylindrical chamber (the effect of displacement is included in Prepl)

Equipment Needed Ion chamber and electrometer, need calibration every 2 years. calibration traceable to standard laboratory (primary standard lab: NIST, secondary standard labs). cylindrical chamber for photons of all energies and electrons with energies  10 MeV. plane-parallel chamber for electrons of all energies (mandatory for energies < 10 MeV). system to measure air pressure and water temperature

8.4 The Bragg-Gray Cavity Theory (C- effective point of measurement) For parallel plate chambers, the effective point of measurement is at the inner face of the front plate Effective point of measurement For cylindrical chambers, the effective point of measurement is displaced 0.85r from the center Effective point of measurement 0.85r

8.4 The Bragg-Gray Cavity Theory (C- effective point of measurement) Tracklength of each electron entering through ds  amount of ionization produced Number of electrons entering the circle through ds Total amount of ionization produced due to electrons entering through ds Area perpendicular to electron fluence ds  r x d 2x Xeff = 8r/3p = 0.85r

Depth Dose measurement with a cylindrical chamber 100 Before shifting 80 60 dose After shifting 40 20 5 10 15 20 Depth in water (cm)

Depth Dose measurement with chamber For photon beam, depth-ionization = depth-dose Photon energy spectrum does not change much with depth. Consequently, the energy spectrum of the secondary electrons (generated by the photons) also does not change much with depth. Thus, the stopping power ratio remains (nearly) the same with depth. For electron beam, depth-ionization ≠ depth-dose Electron energy decreases with depth in water ( ~ 2 MeV/cm ), therefore the stopping power ratio changes with depth. Stopping power ratio at different energy/depth needs to be applied to convert ionization to dose. (note: if diode is used for electron depth-dose measurement, no stopping power ratio correction is needed, because does not change with energy)