Abstract:
The purpose of this research work is to evaluate the variation (or dependence) of the timer error as a function of the teletherapy treatment parameters such as treatment field size (FS) and treatment depth (d) for a SSD setup and compare the method used for others. Teletherapy timer error measurements were performed in a full scatter, large water phantom using a 0.6cc ionization chamber and an average photon energy of 1.25MeV from a cobalt-60 unit at an SSD of 100cm at various field size variations and treatment depth variation with gantry and collimator angles fixed at 0°. From the measurements taken, the timer error for the treatment field size variations of 5×5, 10×10, 15×15, 20×20, 30×30, 35×35 were found to be 0.654s, 0.648s, 0.622s, 0.633s, 0.639s and 0.643s respectively for a constant depth of 5cm; and the timer error for treatment depth variations of 5cm, 7cm, 10cm, 15cm and 20cm were found to be 0.648s, 0.585s, 0.612s, 0.665s and 0.6215s for a constant field size of 10×10. Comparing the two set-up techniques used with this work, it was noted that the field size variation and depth variation showed almost similar timer value to the order of . However, the treatment depth variations showed the lowest value as compared to treatment depths. The treatment depth of 15cm recorded the highest timer error of 0.665s and the treatment depth of 7cm showed the lowest value of the timer error of 0.585s. The timer error calculated for this experimental work with the variations in field size was found to vary in the range of 26.47% to 30.13% to the last teletherapy timer error value of 0.89s. The timer error for the variations in treatment depth was found to vary in the range of 25.27% to 34.31% to the last teletherapy timer error value of 0.89s. The conclusion of this work is that timer error depends on both field size and treatment depth and must be taken into account by the creation of a Lookup Table similar to Tissue maximum ratio (TMR) and Tissue Phantom ratio (TPR) Tables to account for these variations during treatment planning and /or manual dose calculations.
