Small Field Correction Factors for the IBA Razor

The IBA Razor diode supersedes the IBA SFD and is intended for use in small fields. However, its behaviour in small fields has not yet been quantified. In this work, we examine the response of the Razor diode against the air core scintillation dosimeter (FOD) and Gafchromic film in photon beams from three Varian linac beams. Fields between 4mm and 30mm in width were measured, both with and without a flattening filter and at two energies. The Razor exhibited an over-response of up to 4.5% for MLC collimated fields and 7.1% for stereotactic cone collimated fields. The presence of the flattening filter altered the over-response by up to 1.5%. The small field correction factors are tabulated and agree with the mathematical relation of Liu et al. (2014). Four samples of the Razor were used, two having received a significant prior dose. The correction factors for the four samples differed and may depend on their dose history.