Development and validation of a beam model applicable to small fields

A beam model for dose verification of narrow photon beams in water is presented. This model includes the two main effects affecting dose distributions for small fields: the spatial extension of the radiation source and the lateral non-equilibrium conditions of the energy transfer in the absorber. The spatial extension of the radiation source is determined experimentally using two methods: a 'slit-method' for the inner part and the measurement of relative output factors free in air for the outer part. The lateral non-equilibrium is taken into account using dose deposition kernels obtained with Monte Carlo calculations and subsequent convolution techniques. The model was validated in water for a wide range of field sizes in a 6 MV photon beam. Good agreement between measured and calculated dose profiles and output factors was found for all the cases studied. The model is considered to be especially useful for narrow-beam IMRT such as typically used in TomoTherapy units. Thus, it can be used as a supplemental dosimetry check for dose delivery performance and as an independent treatment verification tool for cases involving small radiation fields.