Dependence of Some Transmission Factors on Field Size and Treatment Depth in External Beam Radiation Therapy (EBRT) Using the Theratron Equinox 100 Cobalt 60 Machine

ABSTRACT The use of beam modifiers in today’s radiotherapy is very important as it attenuates the beam and reduces the dose to the patient; therefore the need to know the amount of attenuation (in terms of a transmission factor) they provide during treatment. The purpose of this research work is to evaluate the variation (or dependence) of the transmission factors (TFs) of block tray and physical wedges (of different angles) as a function of treatment depth and field size using both iso-centric setups, SAD and SSD; and thus compare the results from the two setup techniques. Wedge and tray TF measurements were performed in a full scatter, large water phantom using a 0.04cc ionization chamber and an average photon energy of 1.25MV from a cobalt-60 unit at an SAD/SSD of 100cm at various depths and field sizes with gantry and collimator angles fixed at 0o . From the measurements carried out, the wedge TF of the 15o , 30o , 45o , and 60o wedges were found to be 0.775±0.005, 0.650±0.010, 0.505±0.015, and 0.280±0.015 respectively; and the tray TF was found to be 0.960±0.003. Also, the results obtained showed that both the wedge TF and the tray TF has a strong linear dependence on treatment depth; however, the variation of the 15o wedge TF and the tray TF with depth is less significant (less than 2%). Maximum percentage variation for the 15o wedge for the SAD setup was 1.1% and 1.59% for the SSD setup; and that for the tray was 0.60% for the SAD setup and 0.12% for the SSD setup. Also, the variation of the 15o , 30o and 45o wedge TF with field size was less significant (less than 2%); and a weaker dependence was observed with field size as compared to the treatment depth. However, the 60o wedge showed a significant variation (maximum of 2.22% and 2.88% for the  SAD and SSD setups respectively) as an increase in field size was accompanied by an increase in its wedge TF. Also though the tray TF graphically showed a strong linear dependence on field size the percentage variation was far less significant (maximum of 0.29% for the SAD setup and 0.18% for the SSD setup). It can therefore be concluded from this study that the wedge TF depends strongly on treatment depth and must be taken into account during treatment planning and/or manual dose calculations, whilst a single tray TF can be used for any field size and at any treatment depth.