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

We present a method for condensing the photon energy and angular distributions obtained from Monte Carlo simulations of medical accelerators. This method represents the output as a series of correlated histosrams and as such is well-0suited for inclusion as the photon-source package for Monte Carlo codes used to determine the dose distributions in photon teletherapy. The method accounts for lhc isocenler-plane variations of the photon energy spectral distributions with increasing distance from the beam central axis for radiation produced in the brenisstrahiung target as well as for radiation scattered 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 indicates that energy Ouence errors are less than 1% of the maximum energy fluence for a range of open-field sizes. Comparison of jaw-edge penumbrae shows that the angular distributions of the photons are accurately reproduced. The Monte Carlo sampling efficiency (the fraction of generated photons which clear the collimator jaws) of the algorithm is approximately 83% lor an open 10 X 10 field, rising to approximately 96% for an open 40x40 field. Data file sizes for a typical medical accelerator, at a given energy, are approximately 150 kB, compared to the I GB size of the underlying full-physics phase space file. ©1999 American Association of Physicists in Medicine. [S0094-2405(99)02307-X]