Carbon fibre has become the material of choice for many radiotherapy applications, as it is lightweight, radio-translucent and rigid. However, as a result of high cost of carbon fibre, other composite materials have now become available that equally radio-translucent such as wood of 2cm² Ionization doses in the form of integral currents were measured by the electrometer at gantry angles of 180° and 0° degrees for the different radiation treatment depths systematically (non-randomly) selected for the study. A constant reference field size of 10cm x 10cm was used throughout the experiments. Treatment table top transmission factor was calculated by taking the ratio of table top (gantry angle at 180°) and open field (gantry at 0°) ionization data for the various treatment depths selected for the study. The couch top transmission factor varied from 0.975 to 0.945 with treatment depths ranging from 3cm-11cm for the 1.25Mev cobalt 60. The lowest transmission factor was observed for radiation treatment depth of 11cm (0.945) whiles the highest transmission factor was observed at a depth of 3cm (0.975).The couch top transmission factor decreases from the depth of 3cm to 11cm. A linear regression shows a strong inverse dependence of couch top transmission factor with treatment depth, i.e. r= -0.995, using r as regression constant (correlation coefficient). The coefficient of determination, i.e. r² = 0.991 calculated showed that approximately 99.1% of the variability in couch top transmission factor was linked with variability in treatment depth. Transmission factors for treatment table tops (solid radiation treatment table tops) used in dosimetry for radiotherapy procedures vary with treatment depths for isocentric set-up. Therefore, different couch factor should be used at different treatment depths for treatment time calculations of radiation beam that account for couch factor.

Keyword: Transmission factors, Treatment tabletop, Treatment depths, Isocentric set-up, Dosimetry, Target, Phantom,

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