Incident contamination lepton doses measured using radiochromic film in radiotherapy

Measurement of lepton contamination is achieved across a radiotherapy photon beam and peripheral doses using radiochromic film. An extrapolation technique is used where several layers are suspended in air to measure incident contamination without the effects of phantom scatter. Surface dose was measured as 11% of D_max for 6 MV beams at central axis and 9% for 10 MV photons for a 10×10 cm field size. Peripheral lepton doses were found to decrease compared to central, however, were still measurable. Peripheral lepton dose was found to increase with field size and was 12% and 15%, 2 cm outside the geometric field edge of a 30×30 cm field size at 6 and 10 MV respectively. Radiochromic film is a suitable dosimeter for measurement of lepton contamination absorbed dose to surface layers of skin.