ON-AXIS AND OFF-AXIS PRIMARY DOSE COMPONENT IN HIGH-ENERGY PHOTON BEAMS

The depth dose of the primary dose component, on axis and off axis of six different x-ray beams, has been determined from transmission measu rements in narrow beam geometry with and without flattening filter usi ng a Perspex column of a cross section large enough to ensure electron ic equilibrium. In order to derive the primary photon fluence, a corre ction for the scatter from the column has been applied according to th e following method: A number of spectra taken from the literature have been used for computing a scatter coefficient S(c) at different depth s by convolution of dose spread arrays. Using the relationship between S(c) and the single attenuation coefficient mu(i) to represent each e ntire spectrum, it has been possible to correct the experimental trans mission curves iteratively, until the corresponding values of mu were stabilized and representative of the primary. The measured attenuation coefficients were found to have a linear increase as a function of th e distance from the central axis for all the energies and types of lin ear accelerators. For the same nominal energy, this increase is differ ent from one accelerator to another. The same phenomenon was observed for the attenuation coefficients obtained without the flattening filte r in the same experimental conditions. The results are tentatively int erpreted considering the angular variation of bremsstrahlung energy sp ectra with and without a flattening filter as calculated by a Monte Ca rlo method and they are consistent and useful to take accurately into account the softening of the beam as the off-axis distance increases.