A novel extrapolation method using OSL detectors for very small field output factor measurement for stereotactic radiosurgery

Appropriate methods for the determination of very small X-ray beam output factors are essential to ensure correct clinical outcomes for stereotactic radiosurgery. To date, substantial work has been performed in identifying and quantifying suitable dosimeters for relative output factor (ROF) measurements including recent IAEA published recommendations. In this work, we provide a novel method using optically stimulated luminescent dosimeters (OSLDs) with different effective sizes of the readout area to determine ROFs. This involves applying an extrapolation technique to assess ROFs for 6MV SRS X-ray beams with field diameters ranging from 4 to 30 mm as defined by the Brainlab SRS cones. By combining the use of multiple sized OSLDs and water droplets to remove air gaps located around the OSLD detectors, both volume averaging and density variation effects were minimised to estimate ROFs for an extrapolated zero volume detector. The measured results showed that for a 4 mm diameter cone, the ROF was 0.660 ± 0.032 (2SD) as compared to 0.661 ± 0.01 and 0.651 ± 0.018 for the PTW 600019 microDiamond detector and Gafchromic EBT3 film respectively. Whilst the uncertainties were larger than conventional detectors, the technique shows promise and improvements in accuracy may be obtained by higher quality manufacturing techniques. Based on these results, using OSLDs with different effective sizes of readout area and an extrapolation technique shows promise for use as an independent verification tool for very small X-ray field ROFs in the clinical department.