The phenomenon of drop in output at larger field sizes for telecobalt units.

It is known that the output factors (OPFs) for external-beam radiotherapy units increase with field size due to increased scattered radiation from the collimator system. Saturation in the OPF value is generally reported beyond approximately 30 × 30 cm(2). For the first time, to the best of our knowledge, we report on a drop in OPF values, although marginal, measured for a telecobalt machine beyond the 38 × 38 cm(2) field size. We believe that reporting and explaining the results will lead to a better understanding of the scatter composition of the radiation from telecobalt machines. This also has the potential to impact the estimation of low dose regions in patients, in addition to being a purely scientific inquiry. We used Monte Carlo (MC) simulations to validate the measured values. The MC data showed that the decrease in OPF was due to decreased scatter from the machine head.