Edge effects in 3D dosimetry: characterisation and correction of the non-uniform dose response of PRESAGE®.

Previous work has shown that PRESAGE® can be used successfully to perform 3D dosimetric measurements of complex radiotherapy treatments. However, measurements near the sample edges are known to be difficult to achieve. This is an issue when the doses at air-material interfaces are of interest, for example when investigating the electron return effect (ERE) present in treatments delivered by magnetic resonance (MR)-linac systems. To study this effect, a set of 3.5 cm-diameter cylindrical PRESAGE® samples was uniformly irradiated with multiple dose fractions, using either a conventional linac or an MR-linac. The samples were imaged between fractions using an optical-CT, to read out the corresponding accumulated doses. A calibration between TPS-predicted dose and optical-CT pixel value was determined for individual dosimeters as a function of radial distance from the axis of rotation. This data was used to develop a correction that was applied to four additional samples of PRESAGE® of the same formulation, irradiated with 3D-CRT and IMRT treatment plans, to recover significantly improved 3D measurements of dose. An alternative strategy was also tested, in which the outer surface of the sample was physically removed prior to irradiation. Results show that for the formulation studied here, PRESAGE® samples have a central region that responds uniformly and an edge region of 6-7 mm where there is gradual increase in dosimeter response, rising to an over-response of 24%-36% at the outer boundary. This non-uniform dose response increases in both extent and magnitude over time. Both mitigation strategies investigated were successful. In our four exemplar studies, we show how discrepancies at edges are reduced from 13%-37% of the maximum dose to between 2 and 8%. Quantitative analysis shows that the 3D gamma passing rates rise from 90.4, 69.3, 63.7 and 43.6% to 97.3, 99.9, 96.7 and 98.9% respectively.

An investigation of a PRESAGE® in vivo dosimeter for brachytherapy.

Determining accurate in vivo dosimetry in brachytherapy treatment with high dose gradients is challenging. Here we introduce, investigate, and characterize a novel in vivo dosimeter and readout technique with the potential to address this problem. A cylindrical (4 mm × 20 mm) tissue equivalent radiochromic dosimeter PRESAGE® in vivo (PRESAGE®-IV) is investigated. Two readout methods of the radiation induced change in optical density (OD) were investigated: (i) volume-averaged readout by spectrophotometer, and (ii) a line profile readout by 2D projection imaging utilizing a high-resolution (50 micron) telecentric optical system. Method (i) is considered the gold standard when applied to PRESAGE® in optical cuvettes. The feasibility of both methods was evaluated by comparison to standard measurements on PRESAGE® in optical cuvettes via spectrophotometer. An end-to-end feasibility study was performed by a side-by-side comparison with TLDs in an (192)Ir HDR delivery. 7 and 8 Gy was delivered to PRESAGE®-IV and TLDs attached to the surface of a vaginal cylinder. Known geometry enabled direct comparison of measured dose with a commissioned treatment planning system. A high-resolution readout study under a steep dose gradient region showed 98.9% (5%/1 mm) agreement between PRESAGE®-IV and Gafchromic® EBT2 Film. Spectrometer measurements exhibited a linear dose response between 0-15 Gy with sensitivity of 0.0133 ± 0.0007 ΔOD/(Gy ⋅ cm) at the 95% confidence interval. Method (ii) yielded a linear response with sensitivity of 0.0132 ± 0.0006 (ΔOD/Gy), within 2% of method (i). Method (i) has poor spatial resolution due to volume averaging. Method (ii) has higher resolution (∼1 mm) without loss of sensitivity or increased noise. Both readout methods are shown to be feasible. The end-to-end comparison revealed a 2.5% agreement between PRESAGE®-IV and treatment plan in regions of uniform high dose. PRESAGE®-IV shows promise for in vivo dose verification, although improved sensitivity would be desirable. Advantages include high-resolution, convenience and fast, low-cost readout.

Investigating end-to-end accuracy of image guided radiation treatment delivery using a micro-irradiator.

There is significant interest in delivering precisely targeted small-volume radiation treatments, in the pre-clinical setting, to study dose-volume relationships with tumour control and normal tissue damage. For these studies it is vital that image guidance systems and target positioning are accurately aligned (IGRT), in order to deliver dose precisely and accurately according to the treatment plan. In this work we investigate the IGRT targeting accuracy of the X-RAD 225 Cx system from Precision X-Ray using high-resolution 3D dosimetry techniques. Small cylindrical PRESAGE® dosimeters were used with optical-CT readout (DMOS) to verify the accuracy of 2.5, 1.0, and 5.0 mm X-RAD cone attachments. The dosimeters were equipped with four target points, visible on both CBCT and optical-CT, at which a 7-field coplanar treatment plan was delivered with the respective cone. Targeting accuracy (distance to agreement between the target point and delivery isocenter) and cone alignment (isocenter precision under gantry rotation) were measured using the optical-CT images. Optical-CT readout of the first 2.5 mm cone dosimeter revealed a significant targeting error of 2.1 ± 0.6 mm and a cone misalignment of 1.3 ± 0.1 mm. After the IGRT hardware and software had been recalibrated, these errors were reduced to 0.5 ± 0.1 and 0.18 ± 0.04 mm respectively, within the manufacturer specified 0.5 mm. Results from the 1.0 mm cone were 0.5 ± 0.3 mm targeting accuracy and 0.4 ± 0.1 mm cone misalignment, within the 0.5 mm specification. The results from the 5.0 mm cone were 1.0 ± 0.2 mm targeting accuracy and 0.18 ± 0.06 mm cone misalignment, outside of accuracy specifications. Quality assurance of small field IGRT targeting and delivery accuracy is a challenging task. The use of a 3D dosimetry technique, where targets are visible on both CBCT and optical-CT, enabled identification and quantification of a targeting error in 3D. After correction, the targeting accuracy of the irradiator was verified to be within 0.5 mm (or 1.0 mm for the 5.0 mm cone) and the cone alignment was verified to be within 0.2 mm (or 0.4 mm for the 1.0 mm cone). The PRESAGE®/DMOS system proved valuable for end-to-end verification of small field IGRT capabilities.

The effect of motion on IMRT - looking at interplay with 3D measurements.

Six base of skull IMRT treatment plans were delivered to 3D dosimeters within the RPC Head and Neck Phantom for QA verification. Isotropic 2mm 3D data was obtained using the DLOS-PRESAGE system and compared to an Eclipse (Varian) treatment plan. Normalized Dose Distribution pass rates were obtained for a number of criteria. High quality 3D dosimetry data was observed from the DLOS system, illustrated here through colormaps, isodose lines, profiles, and NDD 3D maps. Excellent agreement with the planned dose distributions was also observed with NDD analysis revealing > 90% NDD pass rates [3%, 2mm], noise < 0.5%. This paper focuses on a detailed exploration of the quality and use of 3D dosimetry data obtained with the DLOS-PRESAGE system.

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.

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.

Comprehensive quality assurance for base of skull IMRT.

Six base of skull IMRT treatment plans were delivered to Presage dosimeters within the RPC Head and Neck Phantom for quality assurance (QA) verification. Isotropic 2mm 3D data were acquired by optical-CT scanning with the DLOS system (Duke Large Optical-CT Scanner) and compared to the Eclipse (Varian) treatment plan. Normalized Dose Distribution (NDD) pass rates were obtained for a number of criteria. High quality 3D dosimetry data was observed from the DLOS system, illustrated here through colormaps, isodose lines, and profiles. Excellent agreement with the planned dose distributions was also observed with NDD analysis revealing > 90% pass rates (with criteria 3%, 2mm), and noise < 0.5%. The results comprehensively confirm the high accuracy of base-of-skull IMRT treatment in our clinic.

How accurate is image guided radiation therapy (IGRT) delivered with a micro-irradiator?

There is significant interest in delivering precisely targeted small-volume radiation treatments, in the pre-clinical setting, to study dose-volume relationships with tumor control and normal tissue damage. In this work we investigate the IGRT targeting accuracy of the XRad225Cx system from Precision x-Ray using high resolution 3D dosimetry techniques. Initial results revealed a significant targeting error of about 2.4mm. This error was reduced to within 0.5mm after the IGRT hardware and software had been recalibrated. The facility for 3D dosimetry was essential to gain a comprehensive understanding of the targeting error in 3D.

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.

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.

Locating, quantifying and characterising radiation hazards in contaminated nuclear facilities using a novel passive non-electrical polymer based radiation imaging device.

This paper provides a summary of recent trials which took place at the US Department of Energy Oak Ridge National Laboratory (ORNL) during December 2010. The overall objective for the trials was to demonstrate that a newly developed technology could be used to locate, quantify and characterise the radiological hazards within two separate ORNL hot cells (B and C). The technology used, known as RadBall(®), is a novel, passive, non-electrical polymer based radiation detection device which provides a 3D visualisation of radiation from areas where effective measurements have not been previously possible due to lack of access. This is particularly useful in the nuclear industry prior to the decommissioning of facilities where the quantity, location and type of contamination are often unknown. For hot cell B, the primary objective of demonstrating that the technology could be used to locate, quantify and characterise three radiological sources was met with 100% success. Despite more challenging conditions in hot cell C, two sources were detected and accurately located. To summarise, the technology performed extremely well with regards to detecting and locating radiation sources and, despite the challenging conditions, moderately well when assessing the relative energy and intensity of those sources. Due to the technology's unique deployability, non-electrical nature and its directional awareness the technology shows significant promise for the future characterisation of radiation hazards prior to and during the decommissioning of contaminated nuclear facilities.

WE-E-BRB-04: Quantitative Dose Tracking Enabled Through a Novel Deformable 3D Dosimeter.

PURPOSE: To evaluate and investigate the feasibility of a new method for validating dose tracking algorithms in deforming tissues using a novel deformable 3D dosimeter. METHODS: A novel deformable 3D Presage dosimeter is reported consisting of a stretchy polyurethane matrix doped with radiochromic leuco-dye. Two deformable cylindrical dosimeters (6 cm diameter, 5 cm long) were manufactured and irradiated with a checkerboard arrangement of 5 mm square pencil beams created by MLC fields. One dosimeter was irradiated under lateral compression by 33% (6 cm down to 4 cm diameter) to simulate a deformed organ. A second control dosimeter was irradiated with the same checkerboard pattern but without deformation applied. High-resolution 3D dose distributions (isotropic 1 mm resolution) were obtained by optical-CT imaging. Physical dose deformation was quantified by comparing checkerboard pencil beam shapes and positions in the deformed and control dosimeters. RESULTS: Deformation of dose in the deformed dosimeter was clearly visible in all 3 dimensions. The deformed checkerboard dose pattern showed expansion of 16% - 46% along the axis of compression, with higher expansion observed in the central regions of the dosimeter. Perpendicular to the compression axis, the dose pattern contracted by 7% - 13%. Peak dose changes of -6% and +30% were observed parallel and perpendicular to the compression axis respectively. Dose response was linear from 0-8 Gy. CONCLUSIONS: Dose tracking was successfully quantified in a novel deforming 3D dosimeter. This capability has potential as a powerful new method for validating deformable dose tracking and registration algorithms. NCI R01CA100835.

SU-E-T-132: Investigation of Photon and Proton Overlapping Fields in PRESAGE- Dosimeters.

PURPOSE: To evaluate the effects of overlapping dose volumes for varying field arrangements in two formulations of PRESAGE®: one intended for, and irradiated with, proton beams and the other photon beams. METHODS: For each treatment modality (photon, proton), three overlapping field setups were performed. These included a stationary dosimeter irradiated over six fractions, a dosimeter shifted laterally to the field to deliver a dose plateau in two fractions, and a dosimeter rotated on its axis to deliver a two-field (for protons) and four-field (for photons) box treatment overlapping in the center of the dosimeter. All subsequent fractions were given within ten minutes and never less than one minute apart. Two cylindrical PRESAGE® dosimeters approximately 7.5 cm in length by 7.5 cm in diameter were irradiated for each setup. The dosimeters were paired, with one dosimeter given total dose by a single fraction while the other followed one of the overlapping field setups. The dosimeters were analyzed using an optical CT scanner and exported to the CERR environment where the doses were compared between paired dosimeters. RESULTS: Dose profile comparisons showed relative dose agreement between paired dosimeters within 5% along the SOBP region of the proton formulation. In the case of the fractionated proton irradiation, there was an over-response while other setups resulted in under-responses. Dose agreement between the photon dosimeter treated with six fractions showed a dose under-response within 11% and never less than 5%. Future measurements will include the remaining field setups. CONCLUSIONS: The proton formulation of PRESAGE® showed good dose agreement between single and multiple field irradiations. While the photon formulation had slightly less agreement, additional field setup comparisons may show improved results. These results will aid future measurements of overlapping field treatment plans delivered to PRESAGE® for treatment verification for proton and photon 3D dosimetry.

SU-E-T-95: Investigation of 3D Dosimetry for an Anthropomorphic Spine Phantom.

PURPOSE: To evaluate 3D dosimetry for a spinal cord treatment plan delivery using the Radiological Physics Center's (RPC) anthropomorphic spine phantom. METHODS: The RPC's spine phantom currently uses radiochromic film and thermoluminescent dosimeters (TLD) to evaluate spinal metastases treatments. A second dosimetry insert for the phantom was created to hold a PRESAGE® 3D dosimeter which matched the location of the TLD and film in the original insert. The phantom was CT imaged with each insert and an IMRT treatment plan was developed. The IMRT plan was delivered to the phantom twice; once with each insert. The film and PRESAGE® were scanned on a CCD microdensitometer and optical-CT system, reconstructed to a 2 mm slice width, respectively. The measured dose distributions were compared to the treatment plan calculated dose distribution using RPC in-house developed software or the Computational Environment for Radiotherapy Research (CERR). Film and PRESAGE® dose profiles were taken across several planes and compared for agreement. The distance to agreement (DTA) between the measured data and treatment plan, within the high dose gradient region, was quantified. RESULTS: The PRESAGE® and plan dose profiles agreed to within 2and 1 mm in the AP and SI directions, respectively. The film and plan also agreed to within 2 mm across all profiles. CONCLUSIONS: The PRESAGE® 3D dosimeter, based on these preliminary data, shows potential as a dosimeter for the RPC's phantom irradiation studies. Future work will add markers to the PRESAGE® insert to allow for a reproducible registration in CERR and a an optical-CT system, reconstructed to a 2 mm slice width dose calibration protocol will be created. CA 100835.

SU-D-213AB-05: Commissioning a CT Compatible LDR T&O Applicator Using Analytical Calculation with ID and 3D Dosimetry.

PURPOSE: To determine the characteristics of a new commercially available CT-compatible LDR Tandem and Ovoid (T&O) applicator using 3D dosimetry. METHODS: We characterized source attenuation through the asymmetric gold shielding in the buckets by measuring dose with diode and 3D dosimetry and compared to an analytical line integral calculation. For 3D dosimetry, a cylindrical PRESAGE dosimeter (9.5cm diameter, 9.2cm height) with a central 6mm channel bored for source placement was scanned with the Duke Large field of view Optical CT-Scanner (DLOS) before and after delivering a nominal 7.7Gy at a distance of 1 cm using a Cs-137 source loaded in the bucket. The optical CT scan time lasted approximately 15 minutes during which 720 projections were acquired at 0.5° increments, anda 3D dose distribution was reconstructed with a 0.5mm3 isotropic voxel size. The 3D dose distribution was applied to a CT-based T&O implant to determine effect of ovoid shielding on the dose delivered to ICRU 38 Point A as well as D2cc of the bladder, rectum, bowel, and sigmoid. RESULTS: Dose transmission through the gold shielding at a radial distance of 1-3cm from midplane of the source was 86.6%, 86.1, and 87.0% for analytical calculation, diode, and 3D dosimetry, respectively. For the gold shielding of the bucket, dose transmission calculated using the 3D dosimetrymeasurement was found to be lowest at oblique angles from the bucket witha minimum of ∼51%. For the patient case, attenuation from the buckets leadto a decrease in average Point A dose of ∼4% and decrease in D2cc to bladder, rectum, sigmoid, and bowel of 2%, 15%, 2%, and 7%, respectively. CONCLUSIONS: The measured 3D dose distribution provided unique insight to the dosimetry and shielding characteristics of the investigated applicator, the technique for which can be applied to commissioning of other brachytherapy applicators. John Adamovics is the owner of Heuris Pharma LLC. Partially supported by NIH Grant R01 CA100835-01.

A 'quad-phantom' film dosimeter for use as a multi-planar verification tool for PRESAGE/optical-CT.

INTRODUCTION: To develop and characterize the accuracy and reproducibility of a quad-phantom dosimeter which will serve as an independent verification tool during commissioning of a PRESAGE/optical-CT 3D dosimetry system. METHODS: A 16cm × 12cm cylindrical quad-phantom was constructed from four pieces of solid polyurethane mimicking the PRESAGE material. Films were placed and anchored in orthogonal planes and the quad-phantom was fastened tightly together and placed in a water-filled Styrofoam container for irradiation. A simple, two-field plan consisting of 6×6cm anterior-posterior and right-lateral 6MV photon beams (400cGy) was delivered three times (fresh films inserted for each) with a Varian Clinac 600C. Image registration was performed in the Computational Environment for Radiological Research (CERR) and dose profiles and gamma analysis was performed in CERR and MATLAB. RESULTS #ENTITYSTARTX00026; DISCUSSION: Excellent reproducibility was observed during the irradiations, with ~2.3% standard deviation between all pixels. Using a 3%, 3mm gamma criteria, excellent dosimetric accuracy was observed, with 98.8% and 96.3% passing rates in the sagittal and axial planes, respectively. CONCLUSION: The preliminary results indicate that the quad-phantom can serve as a reproducible and accurate system for high resolution dosimetry in orthogonal planes and should serve as an effective verification tool for PRESAGE/optical-CT in more challenging clinical scenarios.

Determination of the depth dose distribution of proton beam using PRESAGE™ dosimeter.

PRESAGE™ dosimeter dosimeter has been proved useful for 3D dosimetry in conventional photon therapy and IMRT [1-5]. Our objective is to examine the use of PRESAGE™ dosimeter for verification of depth dose distribution in proton beam therapy. Three PRESAGE™ samples were irradiated with a 79 MeV un-modulated proton beam. Percent depth dose profile measured from the PRESAGE™ dosimeter is compared with data obtained in a water phantom using a parallel plate Advanced Markus chamber. The Bragg-peak position determined from the PRESAGE™ is within 2 mm compared to measurements in water. PRESAGE™ shows a highly linear response to proton dose. However, PRESAGE™ also reveals an underdosage around the Bragg peak position due to LET effects. Depth scaling factor and quenching correction factor need further investigation. Our initial result shows that PRESAGE™ has promising dosimetric characteristics that could be suitable for proton beam dosimetry.

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.

Initial experience with optical-CT scanning of RadBall Dosimeters.

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.

A comprehensive evaluation of the PRESAGE/optical-CT 3D dosimetry system.

This work presents extensive investigations to evaluate the robustness (intradosimeter consistency and temporal stability of response), reproducibility, precision, and accuracy of a relatively new 3D dosimetry system comprising a leuco-dye doped plastic 3D dosimeter (PRESAGE) and a commercial optical-CT scanner (OCTOPUS 5x scanner from MGS Research, Inc). Four identical PRESAGE 3D dosimeters were created such that they were compatible with the Radiologic Physics Center (RPC) head-and-neck (H&N) IMRT credentialing phantom. Each dosimeter was irradiated with a rotationally symmetric arrangement of nine identical small fields (1 x 3 cm2) impinging on the flat circular face of the dosimeter. A repetitious sequence of three dose levels (4, 2.88, and 1.28 Gy) was delivered. The rotationally symmetric treatment resulted in a dose distribution with high spatial variation in axial planes but only gradual variation with depth along the long axis of the dosimeter. The significance of this treatment was that it facilitated accurate film dosimetry in the axial plane, for independent verification. Also, it enabled rigorous evaluation of robustness, reproducibility and accuracy of response, at the three dose levels. The OCTOPUS 5x commercial scanner was used for dose readout from the dosimeters at daily time intervals. The use of improved optics and acquisition technique yielded substantially improved noise characteristics (reduced to approximately 2%) than has been achieved previously. Intradosimeter uniformity of radiochromic response was evaluated by calculating a 3D gamma comparison between each dosimeter and axially rotated copies of the same dosimeter. This convenient technique exploits the rotational symmetry of the distribution. All points in the gamma comparison passed a 2% difference, 1 mm distance-to-agreement criteria indicating excellent intradosimeter uniformity even at low dose levels. Postirradiation, the dosimeters were all found to exhibit a slight increase in opaqueness with time. However, the relative dose distribution was found to be extremely stable up to 90 h postirradiation indicating excellent temporal stability. Excellent interdosimeter reproducibility was also observed between the four dosimeters. Gamma comparison maps between each dosimeter and the average distribution of all four dosimeters showed full agreement at the 2% difference, 2 mm distance-to-agreement level. Dose readout from the 3D dosimetry system was found to agree better with independent film measurement than with treatment planning system calculations in penumbral regions and was generally accurate to within 2% dose difference and 2 mm distance-to-agreement. In conclusion, these studies demonstrate excellent precision, accuracy, robustness, and reproducibility of the PRESAGE/optical-CT system for relative 3D dosimetry and support its potential integration with the RPC H&N credentialing phantom for IMRT verification.