Survey of 5 mm small-field output factor measurements in Australia

Varian eclipse stereotactic 5 mm cone data commissioning

Conical collimators are effective and readily available accessories for the field shaping of small stereotactic fields, however the measurements required to accurately characterise the smallest radiation fields are difficult, prone to large errors, and there is little published commissioning data to compare measurements against. The aim of this investigation was to commission the cone dose calculation algorithm of a Varian Eclipse treatment planning system for a Varian 5 mm cone attached to a Varian TrueBeam linear accelerator beam-matched to the Varian Golden Beam Data (GBD). Tissue maximum ratios (TMRs) and off-axis ratios (OARs) were measured using PTW 60019 microDiamond and PTW 60018 SRS Diode detectors for a flattening filter free 6MV beam. The output factor (OF) was measured with the microDiamond and EBT-XD film. Results were compared to the GBD for this cone and an OF measured by the Australian Clinical Dosimetry Service during an independent audit. Film dosimetry was used to evaluate Eclipse dose calculations in a solid water phantom and end-to-end accuracy with an anthropomorphic head phantom. Output correction factors were derived from IAEA TRS-483. Gamma analysis was used to compare measured TMRs and OARs, and to compare Eclipse dose planes with film dosimetry results. Comparisons between measured and GBD TMRs passed gamma analysis within the specified criteria, while differences between distances to agreement for OARs measured with different detectors was attributed to different volume averaging characteristics. The OFs measured with the microDiamond and film agreed within measurement uncertainty. It was decided to configure Eclipse with the microDiamond measured OF and the SRS Diode measured TMR and OAR data. This was validated with various comparisons carried out to confirm both measurement accuracy and Eclipse configuration.