Dose evaluation of photon beam in asymmetric and irregular irradiation fields

For the purpose to improve the precision of output dose evaluation in the asymmetric and irregular irradiation fields, a consideration for components of the collimator scattering (Sc) and of phantom scattering (Sp) was made on the literatures and actual experimental data. Varian Clinac 2100C-generated 6 and 15 MV X-rays were used. For measurement of FA (component of irradiation field), an AE-132a, 0.6 cc JARP dosimeter was used and for the phantom, tough water and Mix-Dp types were used. Following evaluation of the present A/P ratio method, calculation based on separation of Sc and Sp was performed for seeking the best method to calculate FA. Khan's concept for separation of Sc and Sp, the separation of scattering radiation, in FA (=Sc,p) was considered more useful for incorporating various effects into calculation although the measurement of Sc requires standardization in future. (K.H.)