RESUMO
PURPOSE: To compare beam characteristics of superficial X-rays with 6 MeV Electrons for the purpose of replacing superficial treatments with electron fields for skin lesions. METHODS: Electron beam cutouts were made with 12mm thickness cerrobend in diameters 2-5 cm to match superficial X-ray machine cones. Central axis depth doses and profiles were generated using 0.007 cc Exradin A-16 ion chamber, in PTW water phantom at 1mm steps, at 96 and 100cm SSD for electron beams, and at 15cm SSD for superficial X-rays. From the beam data, radiation penumbra 90-10%, 90% profile width, peripheral dose and percentage depth dose were obtained for comparison. RESULTS: The 90-10% radiation penumbra was ranging 7.4-13.7 mm, 12.6-17.6 mm for 6 MeV electrons at 96 and 100cm SSD respectively, and 4.2-11.4 mm for 2-5 cm superficial X-ray cones. For 90% treatment width, a margin ranging 5-7 mm, 6-9 mm is needed around the periphery of target for 6 MeV Electrons at 96 and 100cm SSD respectively, and 2-3 mm for superficial X-ray cones. For 96cm SSD, the peripheral dose from the geometrical field edge were 3.9% at 1cm and 0.5% at 2cm. It was 2.6% at 1cm and 0.7% at 2cm for 100cm SSD. For superficial, it was 6.2%, 7.5%, 9.1% at 1cm, and 3.4%, 4.3%, 5.6% at 2cm for 100 kV, 120 kV and 150 kV respectively. The electron surface dose was below 90%. CONCLUSION: 6 MeV Electron beam shows high superiority with rapid fall off of dose beyond target with lower peripheral dose compared to superficial X-rays. However, the electrons need higher margin around the target and also need appropriate bolus thickness to increase skin dose. The dose at depth beyond 2cm makes very significant advantage using electrons compared to superficial X-rays.
