EFFECTIVE SOURCE SIZE, YIELD AND BEAM PROFILE FROM MULTILAYERED BREMSSTRAHLUNG TARGETS

Modem conformal radiotherapy benefits from heterogeneous dose delivery using scanned narrow bremsstrahlung beams of high energy in combinati on with dynamic double focused multi-leaf collimation and purging magn ets. When using a purging magnet to remove electrons and positrons the target space is limited and unorthodox thin multi-layered targets are needed. A computational technique has therefore been developed to det ermine the forward yield and the angular distributions of the bremsstr ahlung beam as well as the size and location of the effective and the virtual photon point source for arbitrary multi-layer bremsstrahlung t argets. The Gaussian approximation of the diffusion equation for the e lectrons has been used and convolved with the bremsstrahlung productio n process. For electrons with arbitrary emittance impinging on targets of any multi-layer and atomic number combination, the model is well a pplicable, at least for energies in the range 1-100 MeV. The intrinsic bremsstrahlung photon profile has been determined accurately by decon volving the electron multiple scattering process from thin experimenta l beryllium target profiles. For electron pencil beams incident on a t arget of high density and atomic number such as tungsten, the size of the effective photon source stays at around a tenth of a millimetre. T he effective photon source for low-Z materials such as Be, C and Al is located at depths from 3-7 mm in the target, decreasing with increasi ng atomic number. The effective photon source at off-axis positions th en moves out considerably from the central axis, which should be consi dered when aligning collimators. For high-Z materials such as tungsten , the location of the effective photon source is at a few tenths of a millimetre deep. The virtual photon point source is located only a few tenths of a millimetre upstream of the effective photon source both f or high- and low-Z materials. For 50 MeV electrons incident on multi-l ayered full range targets the radial energy fluence distributions will have a full width at half maximum (FWHM) of 80 to 100 mm at 1 m from the target. The best target composition made of two layers when the sp ace is limited to 15 mm was found to be 9 mm Be followed by 6 mm W. A thin beryllium target (similar to 3 mm) results in a high-intensity br emsstrahlung lobe with a FWHM of about 35 mm at the isocentre. Interes tingly, the forward dose rate in such a beam is as high as 62% of the maximum achievable with an optimal target design, even if on average o nly 1 MeV is lost by the electrons.