Correction factors for diode and diamond detectors in the measurement of small field output factors, using film dosimetry as reference

The accurate measurement of field output factors at small field sizes typically encountered in SRS and IMRT is challenging. In this study we measured detector output ratios for small fields using a range of detectors and orientations, we then calculated correction factors for each detector using film dosimetry as reference to obtain field output factors. The calculated correction factors were compared to the ones from TRS 483. Detectors used were: EBT3 Gafchromic film, PTW semiflex 31010 chamber, PTW pinpoint 31016 chamber, PTW diamond (used in two different orientations) and PTW SRS 60018 diode. Photon beam energies were 6 MV flat and 7 MV FFF with square field sizes from 0.5 to 10.0 cm side. Semiflex and pinpoint chambers agreed well with film down to 2.0 cm field size but under-responded by up to 40% at 0.5 cm field size. At 0.5 cm field size, diamond with stem parallel to beam axis under-estimated field output factors by up to 6.2%; diamond with stem perpendicular to beam axis over-estimated field output factors by up to 1.5% and diode over-estimated field output factors by up to 6.5%. Calculated correction factors, in general, agreed with and confirmed those given in TRS 483 film is an acceptable alternative to Monte Carlo simulation for deriving correction factors for solid state detectors. For field sizes smaller than 1.0 cm, diamond detector with stem perpendicular to beam axis was the most adequate for field output factor measurement, although not recommended in TRS 483.