This study reports a single-institution experience with beam data acquisition and film-based validation for a novel self-shielded sterotactic radiosurgery unit and investigates detector dependency on field output factors (OFs), off-axis ratios (OARs), and percent depth dose (PDD) measurements within the context of small-field dosimetry.
Methods:The delivery platform for this unit consists of a 2.7-MV S-band linear accelerator mounted on coupled gimbals that rotate around a common isocenter (source-to-axis distance [SAD] = 450 mm), allowing for more than 260 noncoplanar beam angles. Beam collimation is achieved via a tungsten collimator wheel with eight circular apertures ranging from 4 mm to 25 mm in diameter. Three diodes (PTW 60012 Diode E, PTW 60018 SRS Diode, and Sun Nuclear EDGE) and a synthetic diamond detector (PTW 60019 micro Diamond [µD] detector)were used for OAR, PDD, and OF measurements. OFs were also acquired with a PTW 31022 PinPoint ionization chamber. Beam scanning was performed using a 3D water tank at depths of 7, 50, 100, 200, and 250 mm with a source-tosurface distance of 450 mm. OFs were measured at the depth of maximum dose (d max = 7 mm) with the SAD at 450 mm. Gafchromic EBT3 film was used to validate OF and profile measurements and as a reference detector for estimating correction factors for active detector OFs. Deviations in field size, penumbra, and PDDs across the different detectors were quantified.Results: Relative OFs (ROFs) for the diodes were within 1.4% for all collimators except for 5 and 7.5 mm, for which SRS Diode measurements were higher by 1.6% and 2.6% versus Diode E. The µD ROFs were within 1.4% of the diode measurements. PinPoint ROFs were lower by >10% for the 4-mm and 5-mm collimators versus the Diode E and µD. Corrections to OFs using EBT3 film as a reference were within 1.2% for all diodes and the µD detector for collimators 10 mm and greater and within 2.0%, 2.8%, and 1.1% for the 7.