On the efficiency of photon beam treatment head simulations

The paper presents a theoretical derivation of the photon fluence simulation efficiency in photon beam treatment head simulations without and with bremsstrahlung splitting. The expressions obtained provide a rigorous explanation of the efficiency dependence on scoring zone size and splitting number and explain the larger efficiency of the directional bremsstrahlung splitting technique compared to uniform bremsstrahlung splitting. They are also very useful for determining the optimum splitting number in practical treatment head simulations and for understanding the variation of simulation efficiency with beam energy and field size.