J Newton

Commissioning a small-field biological irradiator using point, 2D, and 3D dosimetry techniques

PURPOSE To commission a small-field biological irradiator, the XRad225Cx from Precision x-Ray, Inc., for research use. The system produces a 225 kVp x-ray beam and is equipped with collimating cones that produce both square and circular radiation fields ranging in size from 1 to 40 mm. This work incorporates point, 2D, and 3D measurements to determine output factors (OF), percent-depth-dose (PDD) and dose profiles at multiple depths. METHODS Three independent dosimetry systems were used: ion-chambers (a farmer chamber and a micro-ionisation chamber), 2D EBT2 radiochromic film, and a novel 3D dosimetry system (DLOS∕PRESAGE®). Reference point dose rates and output factors were determined from in-air ionization chamber measurements for fields down to ∼13 mm using the formalism of TG61. PDD, profiles, and output factors at three separate depths (0, 0.5, and 2 cm), were determined for all field sizes from EBT2 film measurements in solid water. Several film PDD curves required a scaling correction, reflecting the challenge of accurate film alignment in very small fields. PDDs, profiles, and output factors were also determined with the 3D DLOS∕PRESAGE® system which generated isotropic 0.2 mm data, in scan times of 20 min. RESULTS Surface output factors determined by ion-chamber were observed to gradually drop by ∼9% when the field size was reduced from 40 to 13 mm. More dramatic drops were observed for the smallest fields as determined by EBT∼18% and ∼42% for the 2.5 mm and 1 mm fields, respectively. PRESAGE® and film output factors agreed well for fields <20 mm (where 3D data were available) with mean deviation of 2.2% (range 1%-4%). PDD values at 2 cm depth varied from ∼72% for the 40 mm field, down to ∼55% for the 1 mm field. EBT and PRESAGE® PDDs agreed within ∼3% in the typical therapy region (1-4 cm). At deeper depths the EBT curves were slightly steeper (2.5% at 5 cm). These results indicate good overall consistency between ion-chamber, EBT2 and PRESAGE® measured OFs, PDDs, and profiles. CONCLUSIONS The combination of independent 2D and 3D measurements was found to be valuable to ensure accurate and comprehensive commissioning. Film measurements were time consuming and challenging due to the difficulty of film alignment in small fields. PRESAGE® 3D measurements were comprehensive and efficient, because alignment errors are negligible, and all parameters for multiple fields could be obtained from a single dosimeter and scan. However, achieving accurate superficial data (within 4 mm) is not yet feasible due to optical surface artifacts.

A method to correct for stray light in telecentric optical-CT imaging of radiochromic dosimeters

Radiochromic plastic and gel materials have recently emerged which can yield 3D dose information over clinical volumes in high resolution. These dosimeters can provide a much more comprehensive verification of complex radiation therapy treatments than can be achieved by conventional planar and point dosimeters. To achieve full clinical potential, these dosimeters require a fast and accurate read-out technology. Broad-beam optical-computed tomography (optical-CT) systems have shown promise, but can be sensitive to stray light artifacts originating in the imaging chain. In this work we present and evaluate a method to correct for stray light artifacts by deconvolving a measured, spatially invariant, point spread function (PSF). The correction was developed for the DLOS (Duke large field-of-view optical-CT scanner) in conjunction with radiochromic PRESAGE® dosimeters. The PSF was constructed from a series of acquisitions of projection images of various sized apertures placed in the optical imaging chain. Images were acquired with a range of exposure times, and for a range of aperture sizes (0.2-11 mm). The PSF is investigated under a variety of conditions, and found to be robust and spatially invariant, key factors enabling the viability of the deconvolution approach. The spatial invariance and robustness of the PSF are facilitated by telecentric imaging, which produces a collimated light beam and removes stray light originating upstream of the imaging lens. The telecentric capability of the DLOS therefore represents a significant advantage, both in keeping stray light levels to a minimum and enabling viability of an accurate PSF deconvolution method to correct for the residual. The performance of the correction method was evaluated on projection images containing known optical-density variations, and also on known 3D dose distributions. The method is shown to accurately account for stray light on small field dosimetry with corrections up to 3% in magnitude shown here although corrections of >10% have been observed in extreme cases. The dominant source of stray light was found to be within the imaging lens. Correcting for stray light extended the dynamic range of the system from ∼30 to ∼60 dB. The correction should be used when measurements need to be accurate within 3%.

Customising PRESAGE? for diverse applications

PRESAGE® is a solid radiochromic dosimeter consisting of a polyurethane matrix, a triarylmethane leuco dye, and a trihalomethane initiator. Varying the composition and/or relative amounts of these constituents can affect the dose sensitivity, post-irradiation stability, and physical properties of the dosimeter. This allows customisation of PRESAGE® to meet application-specific requirements, such as low sensitivity for high dose applications, stability for remote dosimetry, optical clearing for reusability, and tissue-like elasticity for deformable dosimetry. This study evaluates five hard, non-deformable PRESAGE® formulations and six deformable PRESAGE® formulations and characterizes them for dose sensitivity and stability. Results demonstrated sensitivities in the range of 0.0029 – 0.0467 ΔOD/(Gy·cm) for hard formulations and 0.0003 – 0.0056 ΔOD/(Gy·cm) for deformable formulations. Exceptional stability was seen in both standard and low sensitivity non-deformable formulations, with promising applications for remote dosimetry. Deformable formulations exhibited potential for reusability with strong post-irradiation optical clearing. Tensile compression testing of the deformable formulations showed elastic response consistent with soft tissues, with further testing required for direct comparison. These results demonstrate that PRESAGE® dosimeters have the flexibility to be adapted for a wide spectrum of clinical applications.

Preliminary commissioning investigations with the DMOS-RPC optical-CT Scanner

A midsized broad beam Optical-CT scanner is being developed for collaborative research between Duke and the Radiological Physics Center (RPC). The Duke Midsized Optical-CT Scanner (DMOS-RPC) is designed to be compatible with several of the RPC phantoms, including the head and neck, stereotactic SRS, and lung phantoms. Preliminary data investigating the basic performance of the scanner is described. Two 10 cm PRESAGE cylinders were irradiated with simple test plans. Projections of ~80 μm resolution of each dosimeter were collected at 1 degree intervals over a full 360 degrees both before and after irradiation. 3 dimensional reconstructions of attenuation coefficients throughout the dosimeter were computed with 1 mm(3) resolution. Scans were normalized to the calculated dose distribution and a 3D comparison was made with a commissioned treatment planning system. Initial results indicate DMOS-RPC can produce accurate relative dose distributions with high spatial resolution (up to 1 mm(3) in 3D) in less than 30 minutes (acquisition and reconstruction). A maximum dose of ~3.6Gy was delivered in these tests, and observed noise was ~2% for 1 mm(3) reconstructions. Good agreement is observed with the planning system in these simple distributions, indicating promising potential for this scanner.

Commissioning a CT-compatible LDR tandem and ovoid applicator using Monte Carlo calculation and 3D dosimetry.

PURPOSE To determine the geometric and dose attenuation characteristics of a new commercially available CT-compatible LDR tandem and ovoid (T&O) applicator using Monte Carlo calculation and 3D dosimetry. METHODS For geometric characterization, we quantified physical dimensions and investigated a systematic difference found to exist between nominal ovoid angle and the angle at which the afterloading buckets fall within the ovoid. For dosimetric characterization, we determined source attenuation through asymmetric gold shielding in the buckets using Monte Carlo simulations and 3D dosimetry. Monte Carlo code MCNP5 was used to simulate 1.5 × 10(9) photon histories from a (137)Cs source placed in the bucket to achieve statistical uncertainty of 1% at a 6 cm distance. For 3D dosimetry, the distribution about an unshielded source was first measured to evaluate the system for (137)Cs, after which the distribution was measured about sources placed in each bucket. Cylindrical PRESAGE(®) dosimeters (9.5 cm diameter, 9.2 cm height) with a central channel bored for source placement were supplied by Heuris Inc. The dosimeters were scanned with the Duke Large field of view Optical CT-Scanner before and after delivering a nominal dose at 1 cm of 5-8 Gy. During irradiation the dosimeter was placed in a water phantom to provide backscatter. Optical CT scan time lasted 15 min during which 720 projections were acquired at 0.5° increments, and a 3D distribution was reconstructed with a (0.05 cm)(3) isotropic voxel size. The distributions about the buckets were used to calculate a 3D distribution of transmission rate through the bucket, which was applied to a clinical CT-based T&O implant plan. RESULTS The systematic difference in bucket angle relative to the nominal ovoid angle (105°) was 3.1°-4.7°. A systematic difference in bucket angle of 1°, 5°, and 10° caused a 1% ± 0.1%, 1.7% ± 0.4%, and 2.6% ± 0.7% increase in rectal dose, respectively, with smaller effect to dose to Point A, bladder, sigmoid, and bowel. For 3D dosimetry, 90.6% of voxels had a 3D γ-index (criteria = 0.1 cm, 3% local signal) below 1.0 when comparing measured and expected dose about the unshielded source. Dose transmission through the gold shielding at a radial distance of 1 cm was 85.9% ± 0.2%, 83.4% ± 0.7%, and 82.5% ± 2.2% for Monte Carlo, and measurement for left and right buckets, respectively. Dose transmission was lowest at oblique angles from the bucket with a minimum of 56.7% ± 0.8%, 65.6% ± 1.7%, and 57.5% ± 1.6%, respectively. For a clinical T&O plan, attenuation from the buckets leads to a decrease in average Point A dose of ∼3.2% and decrease in D(2cc) to bladder, rectum, bowel, and sigmoid of 5%, 18%, 6%, and 12%, respectively. CONCLUSIONS Differences between dummy and afterloading bucket position in the ovoids is minor compared to effects from asymmetric ovoid shielding, for which rectal dose is most affected. 3D dosimetry can fulfill a novel role in verifying Monte Carlo calculations of complex dose distributions as are common about brachytherapy sources and applicators.PURPOSE To determine the geometric and dose attenuation characteristics of a new commercially available CT-compatible LDR tandem and ovoid (T&O) applicator using Monte Carlo calculation and 3D dosimetry. METHODS For geometric characterization, we quantified physical dimensions and investigated a systematic difference found to exist between nominal ovoid angle and the angle at which the afterloading buckets fall within the ovoid. For dosimetric characterization, we determined source attenuation through asymmetric gold shielding in the buckets using Monte Carlo simulations and 3D dosimetry. Monte Carlo code MCNP5 was used to simulate 1.5 × 109 photon histories from a 137 Cs source placed in the bucket to achieve statistical uncertainty of 1% at a 6 cm distance. For 3D dosimetry, the distribution about an unshielded source was first measured to evaluate the system for 137 Cs, after which the distribution was measured about sources placed in each bucket. Cylindrical PRESAGE® dosimeters (9.5 cm diameter, 9.2 cm height) with a central channel bored for source placement were supplied by Heuris Inc. The dosimeters were scanned with the Duke Large field of view Optical CT-Scanner before and after delivering a nominal dose at 1 cm of 5-8 Gy. During irradiation the dosimeter was placed in a water phantom to provide backscatter. Optical CT scan time lasted 15 min during which 720 projections were acquired at 0.5° increments, and a 3D distribution was reconstructed with a (0.05 cm)3 isotropic voxel size. The distributions about the buckets were used to calculate a 3D distribution of transmission rate through the bucket, which was applied to a clinical CT-based T&O implant plan. RESULTS The systematic difference in bucket angle relative to the nominal ovoid angle (105°) was 3.1°-4.7°. A systematic difference in bucket angle of 1°, 5°, and 10° caused a 1% ± 0.1%, 1.7% ± 0.4%, and 2.6% ± 0.7% increase in rectal dose, respectively, with smaller effect to dose to Point A, bladder, sigmoid, and bowel. For 3D dosimetry, 90.6% of voxels had a 3Dγ-index (criteria = 0.1 cm, 3% local signal) below 1.0 when comparing measured and expected dose about the unshielded source. Dose transmission through the gold shielding at a radial distance of 1 cm was 85.9% ± 0.2%, 83.4% ± 0.7%, and 82.5% ± 2.2% for Monte Carlo, and measurement for left and right buckets, respectively. Dose transmission was lowest at oblique angles from the bucket with a minimum of 56.7% ± 0.8%, 65.6% ± 1.7%, and 57.5% ± 1.6%, respectively. For a clinical T&O plan, attenuation from the buckets leads to a decrease in average Point A dose of ∼3.2% and decrease in D2cc to bladder, rectum, bowel, and sigmoid of 5%, 18%, 6%, and 12%, respectively. CONCLUSIONS Differences between dummy and afterloading bucket position in the ovoids is minor compared to effects from asymmetric ovoid shielding, for which rectal dose is most affected. 3D dosimetry can fulfill a novel role in verifying Monte Carlo calculations of complex dose distributions as are common about brachytherapy sources and applicators.

Dose Verification of Stereotactic Radiosurgery Treatment for Trigeminal Neuralgia with Presage 3D Dosimetry System

Achieving adequate verification and quality-assurance (QA) for radiosurgery treatment of trigeminal-neuralgia (TGN) is particularly challenging because of the combination of very small fields, very high doses, and complex irradiation geometries (multiple gantry and couch combinations). TGN treatments have extreme requirements for dosimetry tools and QA techniques, to ensure adequate verification. In this work we evaluate the potential of Presage/Optical-CT dosimetry system as a tool for the verification of TGN distributions in high-resolution and in 3D. A TGN treatment was planned and delivered to a Presage 3D dosimeter positioned inside the Radiological-Physics-Center (RPC) head and neck IMRT credentialing phantom. A 6-arc treatment plan was created using the iPlan system, and a maximum dose of 80Gy was delivered with a Varian Trilogy machine. The delivered dose to Presage was determined by optical-CT scanning using the Duke Large field-of-view Optical-CT Scanner (DLOS) in 3D, with isotropic resolution of 0.7mm(3). DLOS scanning and reconstruction took about 20minutes. 3D dose comparisons were made with the planning system. Good agreement was observed between the planned and measured 3D dose distributions, and this work provides strong support for the viability of Presage/Optical-CT as a highly useful new approach for verification of this complex technique.

Towards comprehensive characterization of Cs-137 Seeds using PRESAGE® dosimetry with optical tomography.

We describe a method to directly measure the radial dose and anisotropy functions of brachytherapy sources using polyurethane based dosimeters read out with optical CT. We measured the radial dose and anisotropy functions for a Cs-137 source using a PRESAGE® dosimeter (9.5cm diameter, 9.2cm height) with a 0.35cm channel drilled for source placement. The dosimeter was immersed in water and irradiated to 5.3Gy at 1cm. Pre- and post-irradiation optical CT scans were acquired with the Duke Large field of view Optical CT Scanner (DLOS) and dose was reconstructed with 0.5mm isotropic voxel size. The measured radial dose factor matched the published fit to within 3% for radii between 0.5–3.0cm, and the anisotropy function matched to within 4% except for θ near 0° and 180° and radii >3cm. Further improvements in measurement accuracy may be achieved by optimizing dose, using the high dynamic range scanning capability of DLOS, and irradiating multiple dosimeters. Initial simulations indicate an 8 fold increase in dose is possible while still allowing sufficient light transmission during optical CT. A more comprehensive measurement may be achieved by increasing dosimeter size and flipping the source orientation between irradiations.

Preliminary investigation and application of a novel deformable PRESAGE® dosimeter

Deformable 3D dosimeters have potential applications in validating deformable dose mapping algorithms. This study evaluates a novel deformable PRESAGE® dosimeter and its application toward validating the deformable algorithm employed by VelocityAI. The deformable PRESAGE® dosimeter exhibited a linear dose response with a sensitivity of 0.0032 ΔOD/(Gy/cm). Comparison of an experimental dosimeter irradiated with an MLC pencilbeam checkerboard pattern under lateral compression up to 27% to a non-deformed control dosimeter irradiated with the same pattern verified dose tracking under deformation. CTs of the experimental dosimeter prior to and during compression were exported into VelocityAI and used to map an Eclipse dose distribution calculated on the compressed dosimeter to its original shape. A comparison between the VelocityAI dose distribution and the distribution from the dosimeter showed field displacements up to 7.3 mm and up to a 175% difference in field dimensions. These results highlight the need for validating deformable dose mapping algorithms to ensure patient safety and quality of care.

Investigating the reproducibility of a complex multifocal radiosurgery treatment.

Stereotactic radiosurgery has become a widely used technique to treat solid tumors and secondary metastases of the brain. Multiple targets can be simultaneously treated with a single isocenter in order to reduce the set-up time to improve patient comfort and workflow. In this study, a 5-arc multifocal RapidArc treatment was delivered to multiple PRESAGE® dosimeters in order to explore the repeatability of the treatment. The three delivery measurements agreed well with each other, with less than 3% standard deviation of dose in the target. The deliveries also agreed well with the treatment plan, with gamma passing rates greater than 90% (5% dose-difference, and 2 mm distance-to-agreement criteria). The optical-CT PRESAGE® system provided a reproducible measurement for treatment verification, provided measurements were made immediately following treatment.

Three-dimensional dosimetry of a beta-emitting radionuclide using PRESAGE® dosimeters

Three-dimensional dose distributions from liquid brachytherapy were measured using PRESAGE(®) dosimeters. The dosimeters were exposed to Y-90 for 5.75 days and read by optical tomography. The distributions are consistent with estimates from beta dose kernels.