1 A protocol for the determination of absorbed dose from high energy photon and electron beams AAPM TG-21 Protocol (Med Phys 10: , 1983) 和信治癌中心醫院 醫學物理科 徐椿壽

2 TG-21 is based on the Cavity Gas Calibration Factor, N gas. What is N gas ? When a chamber is exposed to radiation (photon or electron), the dose to the chamber cavity gas, D gas, is proportional to the charge produced in gas, Q: D gas  Q or D gas = N gas Q dose to cavity gas charges produced in cavity gas  N gas  D gas /Q (Gy/C or cGy/nC) photon or electron beam chamber medium gas Slide #21

3 N gas is usually not a directly measurable quantity, it is converted (or calculated) from the exposure calibration factor N x, which is obtained from the standard laboratory. NxNx N gas NxNx NkNk Or, the standard laboratory may provide the air-kerma calibration factor N k, which is converted to N x, then to N gas. provided by standard laboratory

4 TG-21 Protocol (Med Phys 10: , 1983) Exposure calibration factor (N x ) Cavity Gas calibration factor (N gas ) x x x med water gas Part I Part II Part III Calibration of beam output (any radiation quality and energy) Calibration of chamber (In Co-60 beam) Dose to cavity gas D gas Dose to medium D med Dose to water D water

5 Part I Exposure calibration factor (N x ) (In Co-60 beam) Cavity Gas calibration factor (N gas )

6 N x : 60 Co Exposure Calibration Factor of a Chamber 60 Co source X M (22  C, 760 mm Hg) N X (R/C or R/reading) is obtained in the primary or secondary standard laboratory. M is the charge collected, corrected for temperature and pressure × X is the exposure (in Roentgen) at the point of measurement build-up cap Slide #8

7 x Co-60 Conversion from air-kerma calibration factor N k to exposure calibration factor N x k = 2.58  C/kg-R (W/e) = joule/C

8 N gas : Cavity Gas Calibration Factor of a Chamber 60 Co source 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/A ion D gas, dose to cavity gas × Slide #6

9 Characteristics of N gas N gas 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 N gas. Unfortunately, the chamber volume cannot be accurately measured. Instead, N gas is calculated from N x.

10 Conversion from N x to N gas For commonly used ionization chambers TG-21: Table XVII

11 Conversion from N x to N gas More complete table for commonly used cylindrical ionization chambers Gastorf et al, Med Phys 13, (1986) Table II

12 Gastorf et al, Med Phys 13, (1986) Table II (cont’d) Conversion from N x to N gas More complete table for commonly used cylindrical ionization chambers

13 If your chamber is listed in Gastorf’s Table II, If your chamber is not listed in Gastorf’s Table II, the you’ll have to convert from N x to N gas  proceed to Part II.

14 X × Conversion from N X to N gas 60 Co source Photon energy fluence D wall electron fluence D gas k = 2.58  C/kg-R Slide #15

15 Conversion from N X to N gas (cont’d) Slide #14 Slide #16

16 Conversion from N X to N gas (cont’d) wall cap  is the fraction of ionization due to electrons from the wall. (1-  ) is the fraction of ionization due to electrons from the cap. Slide #15

17 Conversion from N X to N gas (cont’d) k = 2.58  C/kg-R  wall = If wall-material = cap-material,  = 1.0, If wall-material ≠ cap-material, obtain  from TG-21: Fig.1

18 Conversion from N X to N gas (cont’d) use TG-21: Table I

19 Conversion from N X to N gas (cont’d) A ion provided by the standard lab A wall use TG-21: Table II or Table III

20 Part II x x med gas Dose to cavity gas D gas Dose to medium D med

21 Dose to cavity-gas D gas Photons & electrons any energy Chamber reading needs corrections Chamber cavity-gas calibration factor Slide #2

22 Corrections to Chamber Reading Reading for an unsealed chamber generally needs to be corrected for polarity effect, temperature, pressure, and ion-recombination correction factor P ion (the inverse of collection efficiency A ion ). P ion can be obtained using the two-voltage method: Q 1 is the charge collected at voltage V 1. Q 2 is the charge collected at voltage V 2 = V 1 /2. P ion can be obtained through the ratio Q 1 / Q 2. (TG-21: Fig.4)

23 TG-21: Fig.4

24 Conversion from Dose-to-cavity gas to Dose-to-medium D med D gas Photons & electrons any energy × (thin wall, or wall material same as medium) To exclude low-energy electrons (E<  ) that enter and then stop in the cavity  is the electron energy required to cross the cavity, typically ~10 kev) (L/  ) is the restricted mass collision stopping power with  as the cutoff energy To exclude high-energy  -rays (E>  ) that are produced and then escape from the cavity

25 Conversion from Dose-to-air to Dose-to-medium (cont’d) Dependent on beam quality & energy Slide #34 Slide #33

26 Ratios of mean, restricted mass collision stopping powers Photon beams (TG-21: Fig.2, Table IV) Slide #36

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

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

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

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

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

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

33 Conversion from Dose-to-air to Dose-to-medium D wall D gas Photons (Wall material different from medium) D med × Thick wall: All ionization produced by electrons arising in the wall wall med × Slide #34 Slide #25 Under CPE: Assume  wall =  med

34  is the fraction of total ionization produced by electrons arising in the wall, (1-  ) the fraction arising in the medium Conversion from Dose-to-air to Dose-to-medium Photons (Wall material different from medium) Slide #25 Slide #33

35 Wall Correction Factor: P wall For electron beams: P wall = 1.00 For photon beams: ‘  ’ 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

36 Wall Correction Factor: P wall For electron beams: P wall = 1.00 For photon beams: Slide #26

37 Part III x med water x Dose to medium D med Dose to water D water (note: chamber is not involved in this part!)

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

39 Conversion from Dose-to-medium to Dose-to-water (photons) (Excess scatter correction)

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

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

42 Dose to water at d max : D water (d max ) water × d max water × d SSD setup SAD setup d isocenter (photons) ×

43 Conversion from Dose-to-medium to Dose-to-water (electrons) D water D med water plastic × × d max

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

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

46 TG-21 Summary D water (d max ) dose to water D gas dose to chamber cavity gas D med dose to phantom medium D water dose to water photon electron Fig.4 Fig.5 (photon) Table VIII (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) (polystyrene) (acrylic) 1.00 (polystyrene) Table XIV (acrylic) Table XII Part-II Part-III

47 Miscellaneous

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

49 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 0.85r

50  dd ds x r 2x  X eff = 8r/3  = 0.85r Area perpendicular to electron fluence Number of electrons entering the circle through ds Tracklength of each electron entering through ds  amount of ionization produced Total amount of ionization produced due to electrons entering through ds Effective point of measurement

51 Depth in water (cm) dose Before shifting After shifting Depth Dose measurement with a cylindrical chamber

52 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)