Effects of electron contamination on output factor measurements of Cobalt GWGP -80 Teletherapy unit in Korle-Bu Teaching Hospital Accra, Ghana

A dose to any point in a medium can be analyzed into primary and scattered components. The scattered component can be further analyzed into collimator and phantom scatter. The phantom component can be effectively manipulated; thus the effective primary dose at a point is due to the primary dose and those scattered from collimator. Output factors which separate collimator and phantom scatter factors are required in dose calculations. Scatter factors used in these systems are based on reference depths equal to the depth of the maximum absorbed dose dmax on the beam central axis. These factors defined at the depth of maximum absorbed dose are sensitive to electron contamination and are difficult to measure and calculate. Percent Depth Dose and Tissue Mass Ratio which are dependent on Sp (phantom scatter factor) and Sc (collimator scatter factor) are difficult to measure; hence reference data from BJR (British journal of radiologists) supplement 25 are often used in evaluation. Sp and Sc are defined at the reference depth of dmax and actual measurement of these factors at this depth is not reliable as a result of the possible influence of electron contamination. Many suggestions on the influence of electron contamination have been made and approaches to reduce it, including the use of Helium bags, shadow trays, magnetic fields. Though each of these approaches have proved successes, in clinical situations some of the techniques are not the best. As the result with large fields (collimator setting) electron contamination decreases with distance from source hardware as electrons are absorbed and scattered, then EC increases with larger distance due to electrons originating from air as the beam size expands with distance. Petti et al, 1982 carried out calculations and measurement of electron contamination in clinical photon beams and concluded that electron contamination accounts for buildup dose. Duncan et al, 1984, also showed that buildup dose is due to electron contamination and not photon contamination. Therefore this study adopts the use of narrow cylindrical beam coaxial phantom(mini- phantom) for the measurement of the collimator scatter contribution to the dose at a reference depth of treatment of the isocentric SAD (Source to axis distance) setup. In combination with measurements in a full scatter water phantom, the phantom scatter contribution was derived. The aim of this study is to describe the separation of the total scatter correction factor into its component parts so as to accurately measure and deliver the prescribed dose to patient by analyzing the effect of the contaminant electrons on these output factors. Thus the success of this work is very crucial as per clinical standard practices. As a result, normalized output ratios were compared with ESTRO published values and NCR (S and G data). The percentage of variation between the measured and the literature values were about 0.23% for mini- phantom output factor measurement. Collimator exchange effect was measured for water and mini-phantom for different field size and was compared with ESTRO value. This was found to be 1% and 0.44% respectively. Phantom scatter correction factors were calculated for square and rectangular filed sizes; this was compared with ESTRO values, found to be 1% for square and rectangular filed size. Mini-phantom or in-air output factor measurements generally were done for all types of clinical photon beam dosimeters i.e. the small and big cavity specific farmer ionization chambers used in this project work. The measured values checked for chamber cavity and stem effect were found to be 0.2% for all available clinical field size combinations. These results therefore were compared and they were in good agreement with the published values. Therefore, this fabricated mini-phantom can also be used for beam parameter measurement of Co-60 machine. The measurement of head scatter is independent of the orientation of the axis of the cylindrical ion chamber. The results obtained were exactly the same as that of previous literature review (Storchi and Van Gasteren and ESTRO Booklet No: 6). Hence in-house fabricated mini-phantom can also be used for Co-60 beam data parameter measurement. Finally, the purpose of measuring the output factors separately was in order to accurately measure the output factors in non-reference treatment parameter condition specifically in this work using the non-reference field sizes. This leads to accurately deduce the level of production of contaminant electrons due to the photon interaction in the machine head collimator assembly and in phantom or patient surface volume of treatment considered. As a result, in-air electron contamination level was found to be averagely about 0.2% and a total of 2% of scatter factor dose to the output of the telecobalt photon beam from contaminant electrons was resulted. This was found to happen in clinically applied daily treatment planning system used in this external teletherapy machine as 1.2% of the contaminant electrons contribution was from the irradiated phantom volume at a reference depth of measurement. Hence, the effect of the resulting contaminant electrons from the scattered photons in-air and in water phantom for cobalt clinical photon beam generally can be neglected. This is because, its contribution to the treatment delivery setup as in ICRU (International Commission of Radiation units and Measurements) Report 24 recommended patient dose accuracy level is ± 5% with respect to prescribed dose delivery setup, which exactly complies with result 2% of electron contamination, found in this research work. (au)