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) dEtr P Dm

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

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.

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 .

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. ( = 1.005 for 60Co) center of electron production

Absorbed Dose Calculation medium

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

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

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

Stopping Power dx -dE M, ze v 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 medium gas medium gas wall

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

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

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

Chamber shift correction X

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

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

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)

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

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

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

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

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

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

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)

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

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

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

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

Thank you for your attention!