Chapter 8 Part 1 Measurement of Absorbed Dose

Chapter 8 (Part 1) Measurement of Absorbed Dose

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

Energy Transfer Coefficient Energy Absorption Coefficient

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

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

Kerma (Kinetic Energy Released in the Medium) Dm P d. Etr

Cema (collision kerma) K = Kcol + Krad P d. Een Dm bremsstrahlung

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

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

n = 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. ( = 1. 005 for 60 Co) n center of electron production

Absorbed Dose Calculation medium

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

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

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

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

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

Bragg-Gray principle medium gas wall

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

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

Effective Point of Measurement n Plane Parallel Chambers n n The front surface of the cavity Cylindrical Chambers n 0. 85 r from the center and toward the surface X r x Depth of effective center Depth of geometry center

Chamber shift correction X

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

Absorbed Dose to the Medium n n n n 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

Nominal Accelerating Potential A change in spectral quality changes the depth-dose curve. n The ratio of ionization in phantom the nominal accelerating potential n SAD =100 cm, 10 10 cm 2 M 20 cm/M 10 cm(ionization ratio) Nominal accelerating potential (MV) n Ionization ratio

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

Ionization Recombination correction (Pion) n To correct recombination losses (collection efficiency) Two sets of measurement n V 1 Q 1 n V 2= V 1/2 Q 2 n Q 1/Q 2 Pion Pulsed scanning beam Pion at V 1 n Pulsed radiation Continuous radiation Q 1/Q 2

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

Prepl (Gradient Correction) A n P< P’ = A P Pion Nominal accelerating potential E Pion P’ Chamber size Pion

a n Fraction of ionization due to electrons from the chamber wall a Wall thickness a 1 Nominal accelerating potential

Wall Correction factor (Pwall) n n 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, n (1 - ) = the fraction of ionization due to electrons from the phantom

AAPM TG-21 Protocol Procedures 1. Ngas is obtained from the National Bureau of Standards (NBS) or an Accredited Dosimetry Calibration Laboratory (ADCL) at the time of the 60 Co exposure calibration. n n n National Radiation Standard Laboratory (NRSL) Nx=X/M Nx Ngas (Ngas=Dgas/M)

2. Measurements are made in phantom and Dgas=M Ngas 3. 4. Transfer of dose from Plastic to water 5. Transfer of dose from water to muscle 6. MU Dmuscle at dmax

Exposure calibration factor (Nx) n Nx=X/M n R/C or R/scale division n k = the charge produced in air per unit mass per unit exposure (2. 58 10 -4 Ckg-1 R-1) wall = D/Kcol in the wall =1. 005 n

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

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

Thank you for your attention!