Output and ([Formula: see text]) correction factors measured and calculated in very small circular fields for...

The purpose of this work was to obtain [Formula: see text] factors for microDiamond and EFD-3G detectors in very small (less than 5 mm) circular fields. We also investigated the impact of possible variations in microDiamond detector design schematics on the calculated [Formula: see text] factors. Output factors (OF's) of 6 MV beams from TrueBeam linac collimated with 1.27-40 mm diameter cones were measured with EBT3 films, microDiamond and EFD-3G detectors as well as calculated (in water) using Monte Carlo (MC) methods. Based on EBT3 measurements and MC calculations [Formula: see text] factors were derived for these detectors. MC calculations were performed for microDiamond detector in parallel and perpendicular orientations relative to the beam axis. Furthermore, [Formula: see text] factors were calculated for two microDiamond detector models, differing by the presence or absence of metallic pins. The measured OFs agreed within 2.4% for fields  ⩾10 mm. For the cones of 1.27, 2.46, and 3.77 mm maximum differences were 17.9%, 1.8% and 9.0%, respectively. MC calculated output factors in water agreed with those obtained using EBT3 film within 2.2% for all fields. MC calculated [Formula: see text] factors for microDiamond detector in fields  ⩾10 mm ranged within 0.975-1.020 for perpendicular and parallel orientations. MicroDiamond detector [Formula: see text] factors calculated for the 1.27, 2.46 and 3.77 mm fields were 1.974, 1.139 and 0.982 with detector in parallel orientation, and these factors were 1.150, 0.925 and 0.914 in perpendicular orientation. Including metallic pins in the microDiamond model had little effect on calculated [Formula: see text] factors. EBT3 and MC obtained [Formula: see text] factors agreed within 3.7% for fields of  ⩾3.77 mm and within 5.9% for smaller cones. Including metallic pins in the detector model had no effect on calculated [Formula: see text] factors. Our results show that microDiamond and EFD-3G detectors can be used in very small (1.27-3.77 mm) fields once [Formula: see text] corrections determined in this work are applied. Expected uncertainty of such measurements will be in the range of 8%-2.5%.