Reduction of electron contamination using a filter for 6 MV photon beam

Secondary electrons generated by interaction between primary X-ray beam and block tray in megavoltage irradiation, result in excess soft radiation dose to the surface layer. To reduce the surface dose from the electron contamination, electron filters were attached under the tray when a customized block was used. Cu, Al or Cu/Al combined plate with different thickness was used as a filter and the surface dose reduction was measured for each case. The measurement to find optimal filter was performed with 10cm x 10cm field size and 78.5cm source to surface distance. The measurement points are positioned with 2mm intervals from surface to maximum build-up point. To acquire the effect of field size dependence on optimal electron filter, the measurement was performed from 4cm x 4cm to 25cm x 25cm field sizes. The surface dose was slowly increased by increasing irradiation field but rapidly increased beyond 15cm x 15cm field size. Al plate was found to be inadequate filter because of the failure to have surface dose kept lowering than the dose of deep area. Cu 0.5mm plate and Cu/Al=0.28mm/1.5mm combined plate were found to be optimal filters. By using these 2 filters, the absorbed dose to the surface layer was effectively reduced by 5.5%, 11.3%, and 22.3% for the field size 4cm x 4cm, 10cm x 10cm, and 25cm x 25cm, respectively. The surface dose attributable to electron contamination had a dependence on field size. The electron contamination was increased when tray was used. Specially the electron contamination was in the surface layer was greater when the larger field was used. 0.5mm Cu plate and Cu/Al=0.28mm/1.5mm combined plates were selected as optimal electron filters. When the optimal electron filter was attached under the tray, excessive surface dose was decreased effectively. The effect of these electron filters was better when a larger field was used. (author)