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Measurements
Low beam current and/or Compton scatter methods
Not practical for clinical use
Monte Carlo methods
Mohan et al MedPhys 12 p 592 1985 widely used for testing
BEAM (EGS4/nrc) standard tool
Other codes also used, PENELOPE, GEANT, etc.
Still not practical for routine use
MC data to be standard part of linac purchase procedure?
Analytical modelling from cross sections
Target designs requires use of ‘thick target theory’, i.e. must model the
electron transport prior to bremsstrahlung interactions
Unfolding from transmission through attenuators
Based on ‘in air’ measurements
Requires good control of attenuator purity
Most methods use some support of spectral shape constrains
Unfolding from depth dose distributions in water
Requires access to monoenergetic depth dose data (Monte Carlo)
Unfolding methods needs spectral shape constrains
require trimming of the
resulting spectrum so
that measured dose
matches calculated
dose
Beam energy spectra
- methods to determine spectra for clinical beams