Purpose: Diode detectors over-respond in small fields and their over-response needs to be corrected for field output factors (FOFs). The goal of this study is to compare field output correction factors (FOCFs) for the Sun Nuclear Edge detector among various measurement setups and among four different energies for stereotactic cones.
Methods: FOFs for Varian stereotactic cones (4, 5, 7.5, 10, 12.5, 15 and 17.5 mm in diameter) (Varian Medical Systems, Palo Alto, CA) referenced to a field size of 10 × 10 cm2 were measured on Varian TrueBeam STx using a plastic scintillation detector (W2, Standard Imaging Inc., Middleton, WI) and the Sun Nuclear Edge detector (Sun Nuclear Corp., Melbourne, FL). Measurements were performed for four different energies (6MV, 6MV flattening filter free (FFF), 10MV and 10MV FFF) at depths of 1.5 cm, 5 cm and 10 cm in source-to-skin distance (SSD) and source-to-axis distance (SAD) setups. The daisy-chain method was used only for the Edge. FOCFs were determined by taking a ratio of FOFs for the W2 to those for the Edge.
Results: First, absolute differences in FOCFs among three depths range from 0.002 to 0.011 (6MV), from 0.001 to 0.008 (6MV FFF), from 0 to 0.004 (10MV) and from 0.01 to 0.013 (10MV FFF) for the SSD setup and from 0.002 to 0.004 (6MV), from 0.002 to 0.006 (6MV FFF), from 0.001 to 0.003 (10MV) and from 0.012 to 0.02 (10MV FFF) for the SAD setup. Second, FOCFs for the SAD setup are usually larger than for the SSD setup and differences are largest for the 4 mm cone. Third, FOCFs for 6MV FFF are largest in both setups.
Conclusion: FOCFs for the Edge detector can be different depending on the measurement setup and energy. Therefore, FOCFs for each setup and energy should be considered for accurate FOFs.