PIECEWISE POLYNOMIAL APPROXIMATIONS TO THE ICRP 116 EFFECTIVE DOSE COEFFICIENTS: PHOTONS AND NEUTRONS.

We present fitting equations for estimating effective dose per unit fluence at any photon energy between 10 keV and 10 GeV and any neutron energy between 0.001 eV and 10 GeV. These new equations are based on the latest radiation protection quantities for external radiation exposure found in International Commission on Radiological Protection (ICRP) Publication 116 and incorporate the latest definition of effective dose as described in ICRP Publication 103. The ICRP 116 dose coefficients were fit to piecewise polynomial functions. A total of 8 irradiation geometries were considered: the six in ICRP 116 and two additional geometries presented elsewhere in the literature. The fitting functions generally reproduce the ICRP 116 data to within 3% or better. The functions were used to modify the Monte Carlo N-Particle radiation transport code version 6 (MCNP6) and were applied to a sample problem. The results are intended to be used as a basis for revising the American National Standards Institute/American Nuclear Society 6.1.1-1991 standard.

Patient-dependent beam-modifier physics in Monte Carlo photon dose calculations.

Model pencil-beam on slab calculations are used as well as a series of detailed calculations of photon and electron output from commercial accelerators to quantify level(s) of physics required for the Monte Carlo transport of photons and electrons in treatment-dependent beam modifiers, such as jaws, wedges, blocks, and multileaf collimators, in photon teletherapy dose calculations. The physics approximations investigated comprise (1) not tracking particles below a given kinetic energy, (2) continuing to track particles, but performing simplified collision physics, particularly in handling secondary particle production, and (3) not tracking particles in specific spatial regions. Figures-of-merit needed to estimate the effects of these approximations are developed, and these estimates are compared with full-physics Monte Carlo calculations of the contribution of the collimating jaws to the on-axis depth-dose curve in a water phantom. These figures of merit are next used to evaluate various approximations used in coupled photon/electron physics in beam modifiers. Approximations for tracking electrons in air are then evaluated. It is found that knowledge of the materials used for beam modifiers, of the energies of the photon beams used, as well as of the length scales typically found in photon teletherapy plans, allows a number of simplifying approximations to be made in the Monte Carlo transport of secondary particles from the accelerator head and beam modifiers to the isocenter plane.

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 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 method accounts for the isocenter-plane variations of the photon energy spectral distributions with increasing distance from the beam central axis for radiation produced in the bremsstrahlung 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 fluence 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% for an open 10x10 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 1 GB size of the underlying full-physics phase space file.