Build-up and surface dose measurements for different photon beams: oblique incidence and the use of spoilers

Knowing the dose distribution of high energy photon beams in the build-up region is essential to avoid under or over-dosage to cutaneous or subcutaneous tissues. Build-up and surface doses are influenced by beam energy, size and incidence. Since 90% of the reference dose is considered the minimum dose required for tumour control, in many cases a spoiler is needed to achieve adequate target volume coverage. The spoiler thickness is dependent on skin-spoiler separation (SSS) and the depth at which the 90% dose is prescribed. We used 8 and 18 MV photon beams from a Saturne II+ GE-CGR and 4 MV photon beams from a Clinac 600C Varian. Relative doses were measured by a markus plane parallel ionisation chamber in a polymethyl methacrylate (PMMA) square phantom (30x30 cm2). Measurements at different depths were made at fixed source-detector distance. Dose values were converted to percent depth dose and normalized at the maximum dose. Build-up doses were measured at standard treatment distance and in total body irradiation situation (18 MV, 1.5 cm spoiler). The exit dose was also measured to evaluate reduction of electron and photon backscatter, thus allowing us to calculate the dose distribution along the axis of opposed beams. The advantage of spoiler over bolus is that the increase in SSS reduces the surface dose and spares skin. Spoiler thickness is proportional to the secant of the incidence angle (0 deg. for perpendicular incidence). While wedge filters and the block tray have a negligible influence over build-up dose distribution within the field, together with the beam spoiler they produce an increase in the dose scattered outside the beam. This low penetrating scattered radiation can be reduced by the use of spoilers placed over superficial respect organs (e.g.: the eye lens)