Achieving accurate small field dosimetry is challenging. This study investigates the utility of a radiochromic plastic PRESAGE ® read with optical-CT for the acquisition of radiosurgery field commissioning data from a Novalis Tx system with a high-definition multileaf collimator (HDMLC). Total scatter factors (S c, p ), beam profiles, and penumbrae were measured for five different radiosurgery fields (5, 10, 20, 30 and 40 mm) using a commercially available optical-CT scanner (OCTOPUS, MGS Research). The percent depth dose (PDD), beam profile and penumbra of the 10 mm field were also measured using a higher resolution in-house prototype CCD-based scanner. Gafchromic EBT ® film was used for independent verification. Measurements of S c, p made with PRESAGE ® and film agreed with mini-ion chamber commissioning data to within 4% for every field (range 0.2-3.6% for PRESAGE ® , and 1.6-3.6% for EBT). PDD, beam profile and penumbra measurements made with the two PRESAGE ® /optical-CT systems and film showed good agreement with the high-resolution diode commissioning measurements with a competitive resolution (0.5 mm pixels). The in-house prototype optical-CT scanner allowed much finer resolution compared with previous applications of PRESAGE ® . The advantages of the PRESAGE ® system for small field dosimetry include 3D measurements, negligible volume averaging, directional insensitivity, an absence of beam perturbations, energy and dose rate independence.
This work presents an investigation into the use of PRESAGE ™ dosimeters with an optical-CT scanner as a 3D dosimetry system for quantitative verification of respiratory-gated treatments. The CIRS dynamic thorax phantom was modified to incorporate a moving PRESAGE ™ dosimetersimulating respiration motion in the lungs. A simple AP/PA lung treatment plan was delivered three times to the phantom containing a different but geometrically identical PRESAGE ™ insert each time. Each delivery represented a treatment scenario: static, motion (free-breathing) and gated. The dose distributions, in the three dosimeters, were digitized by the optical-CT scanner. Improved optical-CT readout yielded an increased signal-to-noise ratio by a factor of 3 and decreased reconstruction artifacts compared with prior work. Independent measurements of dose distributions were obtained in the central plane using EBT film. Dose distributions were normalized to a point corresponding to the 100% isodose region prior to the measurement of dose profiles and gamma maps. These measurements were used to quantify the agreement between measured and ECLIPSE ® dose distributions. Average gamma pass rates between PRESAGE ™ and EBT were >99% (criteria 3% dose difference and 1.2 mm distance-to-agreement) for all three treatments. Gamma pass rates between PRESAGE ™ and ECLIPSE ® 3D dose distributions showed excellent agreement for the gated treatment (100% pass rate), but poor for the motion scenario (85% pass rate). This work demonstrates the feasibility of using PRESAGE ™ /optical-CT 3D dosimetry to verify gating-enabled radiation treatments. The capability of the Varian gating system to compensate for motion in this treatment scenario was demonstrated.
The RadBall dosimeter is a novel device for providing 3-D information on the magnitude and distribution of contaminant sources of unknown radiation in a given hot cell, glovebox, or contaminated room. The device is presently under evaluation by the National Nuclear Lab (NNL, UK) and the Savannah River National Laboratory (SRNL, US), for application as a diagnostic device for such unknown contaminants in the nuclear industry. A critical component of the technique is imaging the dose distribution recorded in the RadBall using optical-CT scanning. Here we present our initial investigations using the Duke Mid-sized Optical-CT Scanner (DMOS) to image dose distributions deposited in RadBalls exposed to a variety of radiation treatments.
RadBall™ is a novel technology that can locate unknown radioactive hazards within contaminated areas, hot cells, and gloveboxes. The device consists of a colander-like outer tungsten collimator that houses a radiation-sensitive polymer semisphere. The collimator has a number of small holes; as a result, specific areas of the polymer are exposed to radiation, becoming increasingly more opaque in proportion to the absorbed dose. The polymer semisphere is imaged in an optical computed tomography scanner that produces a high resolution three-dimensional map of optical attenuation coefficients. A subsequent analysis of the optical attenuation data, using a reverse ray tracing technique, provides information on the spatial distribution of gamma-ray sources in a given area, forming a three-dimensional characterization of the area of interest. The RadBall™ technology and its reverse ray tracing technique were investigated using known radiation sources at the Savannah River Site's Health Physics Instrument Calibration Laboratory and unknown sources at the Savannah River National Laboratory's Shielded Cells facility.
Purpose: To measure surface doses from small fields from flattened and unflattened 6MV and 10MV photon beams using radiochromic film.Methods: GAFCHROMIC EBT2 film was used in a solid water phantom to make surface dose measurements from photon beams generated by a Varian TrueBeam Stx linac. Four energies were included: 6MV and 10MV, as well as two corresponding flattening filter free beams (6FFF and 10FFF). The field sizes considered where 10, 5, 4, 3, 2, 1 cm2. One 3×3 cm piece of EBT2 film was irradiated at the surface and at dmax in a solid water phantom for every energy and field size. Films were also placed at dmax and irradiated to obtain a calibration curve. All films were scanned in an EPSON V700 flatbed scanner and analyzed according to established protocols using in‐house developed software. Results: The measurements of surface dose for each field were normalized by the dose measured at dmax. Surface doses ranged from 10% for a 1 cm2 6MV field to 25% for a 10 cm2 6MV field. The ranges for other energies were 16–33%, 5–17%, and 10– 17% for 6FFF, 10MV, and 10FFF, respectively. As expected, the surface dose for both 6MV and 6FFF fields are higher than surface doses from 10MV and 10FFF fields. The surface dose relative to dmax is shown to be a monotonically increasing function of field size. Unflattened beams exhibit a higher surface dose in comparison to flattened beams and this discrepancy is larger for 6MV and 6FFF fields Conclusions: This work makes use of a simple technique for measuring surface doses from small fields relevant to SRS and SBRT, taking advantage of the high resolution offered by radiochromic film. The surface dose measured for a range of standard and small field sizes for flattened and unflattened beams has been presented.
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