[Study of wedge factor dependence with depth, field size and SSD in [60] Co units]

In radiotherapy, wedge filters are used to modify iso-dose curves and hence optimize tumor dose distribution in patient. To deliver prescribed dose, timer or monitor unit should be involved the dose with no wedged field and wedge factor. Therefore, any variation of wedge factor due to treatment parameters will affect the deliver dose to patients. Effects of field size, SSD and depth of treatment on wedge factor were studied, using universal wedge filters with nominal angles of 30°, 45° and 60°, a hand made water phantom 21x21x10 Cm and a 0.6 cc Farmer ionizing chamber, at different SSD and depth from the water surface. Wedge factors were calculated as the ratio of charge [nC] of the wedge to open beam, measured by and an electrometer model NE2571, while the dosimeter was irradiated for 60s with Picker ATC9 [60]Co unit. Except for 60° wedge filter, with 95% certainty, there is no meaningful relation between wfs with field size and depth. For 60° wedge filter, variation of wf for situation used in this study is about 0.55% per Cm'Cm variation in field size and 0.28% per Cm variation in depth. An inverse linear relation between wf and SSD approved with 95% certainty. Variations of wf for 30°, 45° and 60° wedge filters, for situation used in this study are 0.11%, 0.18% and 0.33% per Cm variation in SSD respectively. Dependence of wfs to FS, depth and SSD for 60o wedge filter were formulated and shows that the field size and depth of treatment has a negligible effect on wedge factor. Correction of wf for SSD, which has a large domain of variation, is appreciable in clinical usage [more than 5%] and for 60o wedge filter can be calculated as: wfc=wf [1- 0.0033[SSD-80]] 65