Dose measurements in the build-up region for cobalt-60 therapy units

The dose in the build-up region for four different Cobalt-60 therapy units was measured. It was found that, for large collimator openings and relatively short SSDs, a new dose peak occurs at a depth very much smaller than 0.5 cm. The dose at this peak is a function of collimator openings and SSDs and, in some extreme case, could be 15% or more higher than the dose at the conventional peak dose of 0.5 cm. The new dose peak is probably due to electrons produced in the source capsule and the part of the collimator close to the source; it can be almost eliminated by a filter placed just below the collimator.