Abstract

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 6MV 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.