SURFACE DOSE MEASUREMENTS FOR HIGHLY OBLIQUE ELECTRON-BEAMS

Clinical applications of electrons may involve oblique incidence of be ams, and although dose variations for angles up to 60 degrees from nor mal incidence are well documented, no results are available for highly oblique beams. Surface dose measurements in highly oblique beams were made using parallel-plate ion chambers and both standard LiF:Mg, Ti a nd carbon-loaded LiF Thermoluminescent Dosimeters (TLD). Obliquity fac tors (OBF) or surface dose at an oblique angle divided by the surface dose at perpendicular incidence, were obtained for electron energies b etween 4 and 20 MeV. Measurements were performed on a flat solid water phantom without a collimator at 100 cm SSD. Comparisons were also mad e to collimated beams,The OBFs of surface doses plotted against the an gle of incidence increased to a maximum dose followed by a rapid dropo ff in dose. The increase in OBF was more rapid for higher energies. Th e maximum OBF occurred at larger angles for higher-energy beams and ra nged from 73 degrees for 4 MeV to 84 degrees for 20 MeV. At the dose m aximum, OBFs were between 130% and 160% of direct beam doses, yielding surface doses of up to 150% of D-max for the 20 MeV beam. At 2 mm dep th the dose ratio was found to increase initially with angle and then decrease as D-max moved closer to the surface. A higher maximum dose w as measured at 2 mm depth than at the surface. A comparison of ion cha mber types showed that a chamber with a small electrode spacing and la rge guard ring is required for oblique dose measurement. A semiempiric al equation was used to model the dose increase at the surface with di fferent energy electron beams. (C) 1996 American Association of Physic ists in Medicine.