Measurement of head scatter factor for different telecobalt machines using mini-phantom

Telecobalt radiotherapy machines have become an integral part for treatment of cancer in developing countries. Head scatter factor (Sc) is very essential for beam modelling in treatment planning system for accurate dose delivery in radiotherapy. The magnitude of Sc depends on design and materials of source head of telecobalt machines. Mini-phantoms were fabricated indigenously using water equivalent material polymethyl methaacrylate (PMMA) for Sc measurement as per ESTRO recommendation. It is a simple and reliable tool for Sc measurements. The effect of variation of Sc values for change in field sizes, interchange of collimator jaws and introduction of wedges for three different telecobalt machines of same manufacturer were measured. The collimator exchange effect and the effect of wedges on Sc are within the acceptable limit. Our measurements indicate that the variation of Sc for different telecobalt machines has some uncertainty and for clinical usage Sc data should be measured for each telecobalt machines separately.