1. Chapter 8 (Part 1) Measurement of Absorbed Dose

2. Radiation Dosimetry Physical Dose exposure air kerma medium (tissue)
absorbed dose
(dose)

3. Energy Transfer Coefficient Energy Absorption Coefficient

4. Electron Equilibrium
Ein,c
If Ein,c = Eout,c (EE) Etr = Eab
Eout,c

5. Conditions for Electron Equilibrium
air
Ein,c
air
Eout,c 
<m-1 (photon mean free path), to keep photon fluence unchanged
>R (maximum electron range), to keep Ein,c=Eout,c

6. Kerma (Kinetic Energy Released in the Medium)
dEtr P Dm

7. Cema (collision kerma)
K = Kcol + Krad
dEen P Dm
bremsstrahlung

8. Exposure (air collision kerma = air cema)
Only to x and  radiations
For photon energy < 3 MeV
air Dm
DQen P
In order to measure X, electron equilibrium must be satisfied.

9. Graph showing schematically why the absorbed dose increases with depth and how electronic equilibrium is achieved when there is no attenuation of the primary.
Situation similar to that of a when attenuation of the primary occurs. Now electronic equilibrium is not produced .

10. If photon attenuation is negligible, then =1.
 = D/Kcol
Before the two curves meet, the electron buildup is less than complete, and <1.
If photon attenuation is negligible, then =1.
In the transient equilibrium region, >1.
D = · Kcol = ·(en/)·
 varies with energy, not medium. ( = for 60Co)
center of electron production

11. Absorbed Dose Calculation
medium

12. Absorbed Dose to Air
In the charged particle equilibrium (CPE)

13. Absorbed Dose to any Medium
In the CPE
A = a displacement factor

14. Continuous Slowing Down
Delta-ray
bremsstrahlung
Range straggling
Particle No.
Average range
Average range
Distance
Maximum range
Maximum range

15. Stopping Power dx
-dE
M, ze v
gas, r
ion pair (ip)

16. Stopping Power and Linear Energy Transfer
Delta-ray cutoff energy = D
Bremsstrahlung (Radiation)
S = Scol + Srad
= collisional SP + radiative SP
Scol = Scol,<D + Scol,>D
LET = Scol,<D
LET (linear energy transfer)
= restricted collisional stopping power

17. Bragg-Gray principle
medium
medium
gas
medium
gas
wall

18. Spencer-Attix formulation
L/ = the restricted mass collision stopping power
 = the cutoff energy
The Spencer-Attix formulation of the Bragg-Gray cavity theory

19. Spencer-Attix formulation Bragg-Gray formulation
of all generations of electrons
Dependent of cavity size by choosing a suitable value of 
Bragg-Gray formulation
of only the primary electrons
Independent of cavity size

20. Effective Point of Measurement
Plane Parallel Chambers
The front surface of the cavity
Cylindrical Chambers
0.85r from the center and toward the surface
Depth of effective center r X
Depth of geometry center x

21. Chamber shift correctionX

22. Calibration Protocol TG 21 of AAPM (Med Phys 1983;10:741)
Ngas = Dgas/M
Ngas (cavity-gas calibration factor)
Dose to the gas in the chamber per unit charge
Gy/C or Gy/scale division
Dgas
Absorbed dose to the gas in the chamber Gy M
Meter reading
Corrected for temperature and pressure
C or scale division

23. Absorbed Dose to the Medium
Dmed(Gy) : absorbed dose to the phantom medium
the relative mean, restricted, collision mass stopping power of the medium to the gas in the chamber
Pion : ion-recombination correction factor
Prepl : replacement correction factor
Pwall : wall correction factor

24. Nominal Accelerating Potential
A change in spectral quality changes the depth-dose curve.
The ratio of ionization in phantom
the nominal accelerating potential
SAD =100 cm, 1010 cm2
M20 cm/M10 cm(ionization ratio)
Ionization ratio
Nominal accelerating potential (MV)

25. Ratios of mean, restricted collision mass stopping powers of phantom materials to gas as a function of the ionization ratio and nominal accelerating potential.
Nominal accelerating potential (MV)
Ionization ratio
Water
Acrylic
Polystyrene

26. Ionization Recombination correction (Pion)
To correct recombination losses (collection efficiency)
Two sets of measurement
V1Q1
V2= V1/2 Q2
Q1/Q2 Pion
Q1/Q2
Pion at V1
Pulsed scanning beam
Pulsed radiation
Continuous radiation

27. Replacement Correction (Prepl)
An alternative approach
1. In build-up region
2. Depth  electron fluence
3. Prepl for electron fluence correction
1. In electronic equilibrium
2. Depth photon fluence (e-x)
3. Prepl for gradient correction
dmax

28. Prepl (Gradient Correction)
P< P’ = A P
Nominal accelerating potential
Pion
E Pion P’
Chamber sizePion

29. a Fraction of ionization due to electrons from the chamber wall a
Wall thickness
a 1
Nominal accelerating potential

30. Wall Correction factor (Pwall)
When the chamber wall and the Phantom are of the same composition, Pwall=1.
When the wall is of a composition different from the phantom,
(1-) = the fraction of ionization due to
electrons from the phantom

31. AAPM TG-21 Protocol Procedures
Ngas is obtained from the National Bureau of Standards (NBS) or an Accredited Dosimetry Calibration Laboratory (ADCL) at the time of the 60Co exposure calibration.
National Radiation Standard Laboratory (NRSL)
Nx=X/M
NxNgas (Ngas=Dgas/M)

32. Measurements are made in phantom and Dgas=MNgas
Transfer of dose from Plastic to water
Transfer of dose from water to muscle
MUDmuscle at dmax

33. Exposure calibration factor (Nx)
Nx=X/M
R/C or R/scale division
k = the charge produced in air per unit mass per unit exposure (2.5810-4Ckg-1R-1)
wall = D/Kcol in the wall =1.005

34. Awall
Awall takes account of attenuation and scattering of the primary 60 Co beam in the wall and buildup cap of the chamber

35. a
The fraction of the ionization due to electrons from the chamber wall irradiated by 60Co gamma rays

36. Thank you for your attention!