Correlated histogram representation of Monte Carlo derived medical accelerator photon-output phase space

We present a method for condensing the photon energy and angular distributi ons obtained from Monte Carlo simulations of medical accelerators. This met hod represents the output as a series of correlated histograms and as such is well-suited for inclusion::as the photon-source package for Monte Carlo codes used to determine the dose distributions in photon teletherapy. The m ethod accounts for the isocenter-plane variations of the photon energy spec tral distributions with increasing distance from the beam central axis for radiation produced in the bremsstrahlung target as well as for radiation sc attered by the various treatment machine components within the accelerator head. Comparison of the isocenter energy fluence computed by this algorithm with that of the underlying full-physics Monte Carlo photon phase space in dicates that energy fluence errors are less than 1% of the maximum energy f luence for a range of open-field sizes. Comparison of jaw-edge penumbrae sh ows that the angular distributions of the photons are accurately reproduced . The Monte Carlo sampling efficiency (the fraction of generated photons wh ich clear the collimator jaws) of the algorithm is approximately 83% for an open 10 x 10 field, rising to approximately 96% for an open 40 x 40 held. Data file sizes for a typical medical accelerator, at a given energy, are a pproximately 150 kB, compared to the 1 GB size of the underlying full-physi cs phase space file. (C) 1999 American Association of Physicists in Medicin e. [S0094-2405(99)02307-X].