Dose Calculations

Dose Calculations Absorbed Dose from a charged particle beam [Image removed due to copyright considerations] • • ϕ A(−dE / dx) ∆x • ⎛ dE ⎞ ⎟⎟ = ϕ ⎜⎜ − D= ρ A∆x ρ dx ⎝ ⎠ • D = dose rate • ϕ = fluence rate (cm-2 s-1) ρ= density A = area [Image removed due to copyright considerations] Alpha and Low energy Beta emitters distributed in tissue. A radionuclide, ingested or inhaled, and distributed in various parts of the body is called an internal emitter. Many radionuclides follow specific metabolic pathways, acting as a chemical element, and localize in specific tissues. E.g., iodine concentrates in the thyroid radium and strontium are bone seekers tritium will distribute throughout the whole body in body water cesium tends to distribute throughout the whole body. If an internally deposited radionuclide emits particles that have a short range, then their energies will be absorbed in the tissue that contains them. Let: A = the activity concentration in Bq g-1, of the radionuclide in the tissue E = the average alpha or beta particle energy, in MeV per disintegration The rate of energy absorption per gram tissue is A E (MeV g-1 s-1). The absorbed dose rate is: MeV J g D = A E x 1.60 x10 −13 x 103 gs MeV kg = 1.60 x 10-10 A E Gy s-1 Point Source of Gamma Rays µ en CE µ en D=Ψ = ρ 4πr 2 ρ • • • D = Dose rate • Ψ = energy fluence rate (MeV/cm2 sec) C = activity (Bq) E = energy per decay (MeV) µen/ρ = mass energy-absorption coefficient of air (cm2g-1) (~ same for photons between ~60keV and 2MeV) Beam of Photons Dose = energy absorbed/mass ⎛ µ en ⎞ ⎜⎜ ⎟⎟ ( N )(E ) ( ρ x)( A) ρ ⎠ ⎛µ ⎞ Dose = ⎝ = ⎜⎜ en ⎟⎟ ( N )(E ) ( A)( ρ x) ⎝ ρ ⎠ (µen/ρ) = mass energy absorption coefficient (cm2/g) N = photon fluence (photons/cm2) E = energy per photon ρ = density x = thickness A = area Absorbed dose from neutrons • Elastic scatter (higher energies) • Capture (thermal neutrons) Thermal neutrons ΦNσ E D= ρ Φ = thermal neutron fluence (n/cm2) N = atom density (cm-3) σ = capture cross section (for each element) E = energy from capture reaction ρ = tissue density The major thermal neutron capture reactions in tissue 14 N(n,p)14C σ = 1.7 barns Q = 0.626 MeV Ep = 0.58 MeV, range in water ~ 8 µm EC = 0.04 MeV Energy is deposited locally 1 H(n,γ)2H σ = 0.33 barns 2.22 MeV gamma (µ/ρ) = 0.05 cm2/g (µen/ρ) = 0.025 cm2/g contribution to dose depends on the size of the “target” Principle elements in soft tissue of unit density Element Atoms cm-3 Capture cross section, σ H O C N 5.98 x 1022 2.45 x 1022 9.03 x 1021 1.29 x 1021 0.33 barns 0.00019 barns 0.0035 barns 1.70 barns Absorbed dose from fast neutrons Scattering: assume average energy lost is ½ Emax First collision dose • Representative of the absorbed dose when the mean free path is large compared to the target. • Expressed as dose delivered per individual neutron • Units are those of dose per neutron/cm2 (Gy cm2 ) D= N σ S Qave ρ N = atom density (cm-3) σS = scattering cross section (for each element) Qave = average energy transferred in collision (½ Emax) ρ = tissue density Must calculate dose for each element. E.g., Calculate the first collision dose for a 5 MeV neutron with tissue hydrogen. 5 MeV neutron σS = 1.61 barns 22 -3 N = 5.98 x 10 cm Mean energy per scattering collision, Qave = 2.5 MeV D = 3.88 x 10-11 Gy cm2