ZERO-FIELD DOSE DATA FOR CO-60 AND OTHER HIGH-ENERGY PHOTON BEAMS IN WATER

A procedure of separating the primary- and scatter-dose components in therapeutic photon beams is examined. It is based on the observation t hat the scatter-dose component is proportional to the variable z=rd/(r +d), where r and d are beam radius and depth in phantom, respectively. It is, therefore, possible to express an absorbed dose in the form of a linear equation D(r)=P+Nz, where at a fixed depth d, both primary d ose P and coefficient N are constant. A method of linear extrapolation of an absorbed dose D(r) to ''zero-field'' size, i.e., r=0, is utiliz ed. Since Monte Carlo technique is capable of scoring separately the p rimary- and scatter-dose components, it is used to evaluate the accura cy of the linear extrapolation method within the range of Co-60 - 15-M V nominal photon energies. The results demonstrate that this method is sufficiently accurate to obtain the primary dose component in photon beams. For Co-60 gamma radiation in water, tabulated sets of measured depth-dose data are analyzed by the linear extrapolation method to rev iew ''zero-field'' dose values [percentage depth dose (PDD) and tissue -air ratio (TAR) tables of the British Journal of Radiology, Suppl. 17 ]. The ''zero-field'' PDD data are found to be accurate within limits of experimental uncertainties. Inconsistencies in the TAR table are il lustrated and discussed. Co-60 tables of relative doses, D(r,d)/P(d(ma x)), including ''zero-field'' values for both fixed SSD and isocentric geometries, are generated. Dose calculation in irregular fields is co nsidered. The linear extrapolation method is recommended as a standard procedure for separating primary dose from depth-dose data in high-en ergy photon beams.