Determination of small-field output factors for beam-matched linear accelerators using various detectors and comparison of detector-specific output correction factors using IAEA Technical Report Series 483 protocol.

BACKGROUND

Beam matching is widely used to ensure that linear accelerators used in radiotherapy have equal dosimetry characteristics. Small-field output factors (OF) were measured using different detectors infour beam-matched linear accelerators and the measured OFs were compared with existing treatment planning system (TPS) Monte Carlo algorithm calculated OFs.

MATERIALS AND METHODS

Three Elekta Versa HD and one Elekta Infinitylinear accelerators with photon energies of 6 MV flattening filter (FF), 10 MVFF, 6 MV flattening filter free (FFF) and 10 MVFFF were used in this study. All the Linac'swere beam-matched, Dosimetry beam data were ± 1% compare with Reference Linac. Ten different type of detectors (four ionizationchambers and six diode detectors) were used for small-field OF measurements. The OFs were measured for field sizes of 1 × 1 to 10 × 10 cm, and normalized to 10 × 10 cm field size. The uncorrected and corrected OFs were calculated from these measurements. The corrected OF was compare with existing treatment planning system (TPS) Monte Carlo algorithm calculated OFs.

RESULTS

The small-field corrected and Uncorrected OF variations among the linear accelerators was within 1% for all energies and detectors. An increase in field size led to a reduction in the difference between OFs among the detectors, which was the case for all energies. The RSD values decreased with increasing field size. The TRS 483 provided Detector-specificoutput-correction factor (OCF) reduced uncertainty in small-field measurements.

CONCLUSION

It is necessary to implement the OF-correction of small fields in a TPS. Special care must be taken to incorporate the corrected small-field OF in a TPS.