Acquisition of the effective lateral energy fluence distribution for photon beam dose calculations by convolution models

A method is described for acquiring the lateral energy fluence distribution of therapeutic photon beams. The method is designed for characterization of clinical accelerators for the convolution/superposition methods of photon dose calculations. Radial dose profiles, measured in a water phantom at a reference depth for the largest field size of the machine, are used to construct a discrete Cartesian dose matrix. A corresponding pencil beam energy deposition kernel is calculated from a database of Monte Carlo calculated kernels for monoenergetic photons by using the energy bin data of an effective spectrum as weights. The lateral kernel distribution at the depth of measurement is filtered and resampled into a discrete matrix, similar to that of the measured dose distribution. The dose matrix is then deconvolved by the kernel matrix, using fast Fourier transforms and Wiener filtering, to yield the effective energy fluence distribution. The method is tested by using the acquired energy fluence for convolution dose calculations and comparing the result with measured dose distributions.