Tin foil as bolus material for therapeutic electron beams from the Varian Clinac 2100C/D

Tin foils of sub-millimetre thickness have been investigated as bolus material for therapeutic electron beams from the Varian Clinac 2100C/D linear accelerator. Measurements with ionisation chamber and radiographic film in Plastic Water or water were performed under tin foil bolus to determine surface dose, therapeutic ranges, output factor correction, penumbra and dose outside the field edge. Appropriate thicknesses of tin foil for 90% dose at the surface were found to be approximately 0.3 mm for 6 MeV, and 0.4 mm for 9 MeV and 12 MeV. Enhanced therapeutic interval with tin foil bolus over water-equivalent bolus has previously been reported, but was found not to be evident for 12 MeV and for a small (4 x 4 cm2) 9 MeV field. The penumbra width of fields with tin foil and water-equivalent bolus were found to be within 2 mm, while the doses at 1 cm outside the field edge were within 1.5% of peak dose. Output factor corrections for fields with tin foil were measured as within 2% of unity. Air gaps between the tin foil and phantom surface up to 5 mm were observed to have minimal effect on output correction factor, relative surface dose, and therapeutic range.