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Small-field beam data acquisition, detector dependency, and film-based validation for a novel self-shielded stereotactic radiosurgery system.

PURPOSE
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, 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-to-surface distance of 450 mm. OFs were measured at the depth of maximum dose (dmax = 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 7.5-mm, 5-mm, and 4-mm collimators, respectively. The maximum difference in full width at half maximum (FWHM) between the Diode E and the other active detectors was for the 25-mm collimator and was 0.09 mm (µD), 0.16 mm (SRS Diode), and 0.65 mm (EDGE). Differences seen in PDDs beyond the depth of dmax were <1% across the three diodes and the µD. FWHM and penumbra measurements made using EBT3 film were within 1.34% and 3.26%, respectively, of the processed profile data entered into the treatment planning system.


CONCLUSIONS
Minimal differences were seen in OAR and PDD measurements acquired with the diodes and the µD. ROFs measured with the three diodes were within 2.6% and within 1.4% versus the µD. Gafchromic Film measurements provided independent verification of the OAR and OF measurements. Estimated corrections to OFs using film as a reference were <1.6% for the Diode E, EDGE, and µD detector.