A novel method for the determination of field output factors and output correction factors for small static fields for six diodes and a microdiamond detector in megavoltage photon beams.

PURPOSE

The goal of this work is to provide a large and consistent set of data for detector-specific output correction factors, , for small static fields for seven solid-state detectors and to determine field output factors, , using EBT3 radiochromic films and W1 plastic scintillator as reference detectors on two different linear accelerators and four megavoltage photon beams. Consistent measurement conditions and recommendations given in the International Code of Practice TRS-483 for small-field dosimetry were followed throughout the study.

METHODS

were determined on two linacs, Elekta Versa HD and Varian TrueBeam, for 6 and 10 MV beams with and without flattening filter and for nine fields ranging from 0.5 × 0.5 cm to 10 × 10 cm . Signal readings obtained with EBT3 radiochromic films and W1 plastic scintillator were fitted by an analytical function. Volume averaging correction factors, determined from two-dimensional (2D) dose matrices obtained with EBT3 films and fitted to bivariate Gaussian function, were used to correct measured signals. were determined empirically for six diodes, IBA SFD, IBA Razor, PTW 60008 P, PTW 60012 E, PTW 60018 SRS, and SN EDGE, and a PTW 60019 microDiamond detector.

RESULTS

Field output factors and detector-specific are presented in the form of analytical functions as well as in the form of discrete values. It is found that in general, for a given linac, small-field output factors need to be determined for every combination of beam energy and filtration (WFF or FFF) and field size as the differences between them can be statistically significant (P < 0.05). For different beam energies, the present data for are found to differ significantly (P < 0.05) from the corresponding data published in TRS-483 mostly for the smallest fields (<1.5 cm). For the PTW microDiamond detector, statistically significant differences (P < 0.05) between values were found for all investigated beams on an Elekta Versa HD linac for field sizes 0.5 × 0.5 cm and 0.8 × 0.8 cm . Significant differences in between beams of a given energy but with and without flattening filters are found for measurements made in small fields (<1.5 cm) at a given linac. Differences in are also found when measurements are made at different linacs using the same beam energy filtration combination; for the PTW microDiamond detector, these differences were found to be around 6% and were considered as significant.

CONCLUSIONS

Selection of two reference detectors, EBT3 films and W1 plastic scintillator, and use of an analytical function, is a novel approach for the determination of for small static fields in megavoltage photon beams. Large set of data for seven solid-state detectors and four beam energies determined on two linacs by a single group of researchers can be considered a valuable supplement to the literature and the TRS-483 dataset.