A source occlusion independent method for reporting small field output factors

Abstract

The quantification of accurate output factors for small fields has been an area of active research over the last few years. Recently papers [1,2] have started to report output factors for certain field sizes along with correction factors for a variety of dosimeters based on the new formalism [3] proposed by the joint IAEA/AAPM task force. These correction factors were calculated based on Monte Carlo models that where calibrated by matching the focal spot size to that of the particular Linac used to perform the measurements. However, it is known that for very small field sizes (where the field size is small enough to cause an overlap of geometric penumbra) source occlusion can significantly affect the output factor [4]. The reported output factors for these field sizes are thus only valid for the spot sizes they were modelled for. Spot size is machine dependent as evidenced by the different spot sizes reported in the literature. For this reason it is still necessary and important for those wishing to use such fields in a clinical setting to determine their own output factors. The differences in on-axis collision kerma caused by source occlusion [4] make it unclear whether the correction factors reported would be valid under different circumstances and more work is needed to verify these factors. This makes determination of small field output factors from measurements alone more difficult and modelling may still be necessary on a machine-by-machine basis. A method is proposed here for determining spot-size independent output factors with corresponding spot size correction factors. An Elekta Precise model is implemented in Geant4 to produce a point source beam for each field size, a ‘universal’ output factor can be calculated. Then by modelling various spot sizes a relative output correction factor can be determined allowing the output factor to be scaled to an individual machine’s source properties.