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

Abstract

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.

Technical note: Absolute dose measurements of a vault‐free radiosurgery system

“…13 The water tank setup and procedure for PDD alignment has been previously described. 13 A distance factor was then required to convert the source-to-surface (SSD) setup to the predefined SAD calibration (Equation 1).…”

“…A PDD curve previously acquired with a PTW MP3-XS water tank and PTW 60012 diode was used to obtain the 50 mm and 100 mm water-equivalent values of 67.3 and 40.6%, respectively. 13 A distance factor was also required to convert the SSD setup to the predefined SAD calibration (Equation 1). Measurements using this approach were taken after system calibration using the modified TG-21 approach as described above but were not used for output adjustment.…”

“…For a 25-mm collimator at 5 cm depth, the dose profile would change by <0.3%. 13 The original vendor-provided phantom had a hemispheric shape (Figure 3a,b), placing the center of the TN31010 IC at the nominal machine calibration point of 450-mm SAD and 6.5 mm depth (7 mm water equivalent). For a 3.0 MV energy and the inner TN31010 IC diameter of 5.5 mm, however, TG-21 recommends a 50-mm calibration depth in liquid water.…”

“…A per-centage depth dose (PDD) curve acquired with a PTW MP3-XS water tank and PTW 60012 diode was used to get the 5 cm water-equivalent value of 67.3%. 13 The water tank setup and procedure for PDD alignment has been previously described. 13 A distance factor was then required to convert the source-to-surface (SSD) setup to the predefined SAD calibration (Equation 1).…”

“…Small-field dosimetry aspects of this novel unit have been previously published. 13 At the time of this work, there was no standard calibration protocol and none provided by the manufacturer. However, an acrylic phantom was provided that was intended to get the output within ±5%.…”