Optical CT Scanner Research Articles

Improved sensitivity and ambient light stability of radiochromic plastic dosimeters through composite azo compounds

The aim of this study is to evaluate and compare the dose sensitivity and ambient light resistance of radiochromic plastic dosimeters (RPDs) incorporating mixed azo compound with those incorporating individual azo compounds.RPDs were fabricated using a mixture of polyurethane, leuco dye, initiator, and solvent.In this study, three azo compounds (tartrazine, sunset yellow, and allura red) were incorporated into RPDs either individually or in combination. Thus, we produced five dosimeter types: one original RPD (A) without azo compounds, three RPDs (B, C, D) with individual azo compounds, and one RPD (E) with mixed azo compounds. Initially, to compare dose response and sensitivity, one group of each sample was delivered to doses ranging from 0.2 Gy to 3 Gy with 6 MV photon beams at 600 cGy/min. To assess ambient light resistance, another group was exposed to light for varying durations. Following irradiation and light exposure, optical density (OD) was obtained using a cone-beam optical CT scanner for each group. Wavelength spectra for each sample were acquired using a spectrometer. This study encompassed analysis and comparison of dose response curves, sensitivity, and netOD-time graphs among samples. Additionally, the absorption wavelength spectra of the samples were compared with the light spectrum emitted by a light bulb.The dose response curves were represented by linear functions, with the slope indicating the sensitivity of each sample. The sensitivity for samples A-E were 0.0759, 0.0988, 0.0902, 0.0809, and 0.1013 ΔOD/Gy, respectively. Samples B-E demonstrated increases in sensitivity of 30.1%, 18.8%, 6.5%, and 33.4%, respectively, compared to sample A. Sample E, which incorporated mixed azo compounds, demonstrated the highest sensitivity and remarkable resistance to ambient light. The results suggested that all azo compounds influenced the sensitivity of the RPDs, and that mixed azo compounds does not reduce the sensitivity of the RPDs. The ambient light resistance was dictated by the overlap between the light bulb's wavelength and the azo dyes' absorption range, which for mixed azo compounds spanned widely from 300 nm to 600 nm. The mixed azo compounds, as opposed to a single azo compound, allowed for the absorption of a wider range of light wavelengths, leading to an increased resistance to ambient light.Our study demonstrated that the inclusion of azo compounds enhances the sensitivity of RPDs. Specifically, the use of mixed azo compounds provided the highest sensitivity and exhibited superior resistance to ambient light. This improved performance can be attributed to the broader absorption wavelength spectrum achievable with mixed azo compounds, enabling absorption across a wider range of light wavelengths. These findings highlight the potential utility of mixed azo compounds in the fabrication of RPDs, enhancing their function as highly sensitive and resistant dosimeters to ambient light.

Iterative image reconstruction algorithm analysis for optical CT radiochromic gel dosimetry

Background. Modern radiation therapy technologies aim to enhance radiation dose precision to the tumor and utilize hypofractionated treatment regimens. Verifying the dose distributions associated with these advanced radiation therapy treatments remains an active research area due to the complexity of delivery systems and the lack of suitable three-dimensional dosimetry tools. Gel dosimeters are a potential tool for measuring these complex dose distributions. A prototype tabletop solid-tank fan-beam optical CT scanner for readout of gel dosimeters was recently developed. This scanner does not have a straight raypath from source to detector, thus images cannot be reconstructed using filtered backprojection (FBP) and iterative techniques are required. Purpose. To compare a subset of the top performing algorithms in terms of image quality and quantitatively determine the optimal algorithm while accounting for refraction within the optical CT system. The following algorithms were compared: Landweber, superiorized Landweber with the fast gradient projection perturbation routine (S-LAND-FGP), the fast iterative shrinkage/thresholding algorithm with total variation penalty term (FISTA-TV), a monotone version of FISTA-TV (MFISTA-TV), superiorized conjugate gradient with the nonascending perturbation routine (S-CG-NA), superiorized conjugate gradient with the fast gradient projection perturbation routine (S-CG-FGP), superiorized conjugate gradient with with two iterations of CG performed on the current iterate and the nonascending perturbation routine (S-CG-2-NA). Methods. A ray tracing simulator was developed to track the path of light rays as they traverse the different mediums of the optical CT scanner. Two clinical phantoms and several synthetic phantoms were produced and used to evaluate the reconstruction techniques under known conditions. Reconstructed images were analyzed in terms of spatial resolution, signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), signal non-uniformity (SNU), mean relative difference (MRD) and reconstruction time. We developed an image quality based method to find the optimal stopping iteration window for each algorithm. Imaging data from the prototype optical CT scanner was reconstructed and analysed to determine the optimal algorithm for this application. Results. The optimal algorithms found through the quantitative scoring metric were FISTA-TV and S-CG-2-NA. MFISTA-TV was found to behave almost identically to FISTA-TV however MFISTA-TV was unable to resolve some of the synthetic phantoms. S-CG-NA showed extreme fluctuations in the SNR and CNR values. S-CG-FGP had large fluctuations in the SNR and CNR values and the algorithm has less noise reduction than FISTA-TV and worse spatial resolution than S-CG-2-NA. S-LAND-FGP had many of the same characteristics as FISTA-TV; high noise reduction and stability from over iterating. However, S-LAND-FGP has worse SNR, CNR and SNU values as well as longer reconstruction time. S-CG-2-NA has superior spatial resolution to all algorithms while still maintaining good noise reduction and is uniquely stable from over iterating. Conclusions. Both optimal algorithms (FISTA-TV and S-CG-2-NA) are stable from over iterating and have excellent edge detection with ESF MTF 50% values of 1.266 mm−1 and 0.992 mm−1. FISTA-TV had the greatest noise reduction with SNR, CNR and SNU values of 424, 434 and 0.91 × 10−4, respectively. However, low spatial resolution makes FISTA-TV only viable for large field dosimetry. S-CG-2-NA has better spatial resolution than FISTA-TV with PSF and LSF MTF 50% values of 1.581 mm−1 and 0.738 mm−1, but less noise reduction. S-CG-2-NA still maintains good SNR, CNR, and SNU values of 168, 158 and 1.13 × 10−4, respectively. Thus, S-CG-2-NA is a well rounded reconstruction algorithm that would be the preferable choice for small field dosimetry.

End-to-end quality assurance for Volumetric Modulated Arc Therapy with Fricke-Xylenol orange-Gelatin gel dosimeters and dual-wavelength cone-beam optical CT scanner

The whole treatment process undergone by patients in clinics with Volumetric Modulated Arc Therapy (VMAT) can be tested by implementing 3D end-to-end (E2E) quality assurance with gel dosimetry. In this work, a 3D E2E test was performed in a head phantom for the verification of a VMAT treatment, using FXG (Fricke-Xylenol orange-Gelatin) gel dosimetry and a newly developed dual-wavelength reading method on a cone-beam optical CT scanner. This dosimetric method intends to enable accurate measurements in the out-of-field zone and in the tumor volume, with an effective dose range up to 10 Gy. CT images of the phantom with a gel flask were used to create a treatment plan with a brain tumor of complex shape. A very good agreement between 90 %, 80 %, 60 % and 40 % isodose curves and high 3D γ passing rates (2%/2mm) of 98.6 % and 96.7 % between measured and computed dose maps showed that E2E tests can be successfully implemented with this novel dosimetric method.

Modified optical computed tomography scanner for low temperature scanning

Large radiochromic hydrogels, such as ferrous xylenol formulations require several hours to warm from the storage temperature of 278 K to 293 K for optical CT scanning and irradiation. This warm-up time can be avoided if gels are scanned at 278 K, resulting in a more practical 3D dose measurement. In this study a Vista 16 optical CT scanner was modified to allow scanning at 278 K, below the temperature where condensation forms on the aquarium windows. The refractive index matching liquid was first cooled to 276K in order to cool the aquarium as it was filled. The modification involved pumping cool, dry air into the aquarium scanner compartment to provide a positive pressure, preventing condensation on the aquarium windows. A warming rate of 2 K per hour was achieved with passive cooling which relied on the liquid-filled aquarium’s thermal mass. The radiochromic reaction rate decreased with temperature, requiring an increased post-irradiation wait time from 1 hour at 293 K to 1.5 hours at 278 K. In addition, initial sample transmission was greater by avoiding the auto-darkening that occurred as the samples were warmed from 278 K to 293 K over several hours. This increase in transmission resulted in a larger dynamic range for the dosimeter system.

Bare spherical gel dosimeter with optical computed tomography scanning

Radiochromic polyvinyl alcohol-iodide (PVA-I) hydrogel dosimeter with crosslinking by glutaraldehyde (GTA) has been reported. Because of the transparent gel’s mechanical strength, a study to cast samples in spherical shapes was initiated. The bare sample, 47 mm in diameter was mounted on a funnel and held by suction in a custom, 96 mm diameter, cylindrical vessel for 3D optical CT scanning. A solution of the same formulation without GTA provided refractive index matching inside the vessel for optic scanning and eliminated concentration gradients. The gel sphere remained in the vessel after draining the solution for the ‘in-air’ irradiation. A 20 Gy, maximum dose to the gel sphere was delivered by a single, 6 MV, x-ray beam. Comparison of central axis depth doses for the gel reconstruction and Monte Carlo calculation revealed similar results, indicating accurate 3D dosimetry within 1 mm of the gel surface will be possible. The gel recorded slightly greater dose in build-up region and slightly lower dose in the exit hemisphere relative to the calculation.

3D Dosimetry for Electron Flash Radiotherapy: Assessment of Radiochromic Dosimeter Phantoms with Optical CT Scanning as a 3D Dosimetry System

Many of the dosimeters used in conventional radiation therapy exhibit dose rate dependence which prohibits their use in ultra-high-dose-rate (FLASH) radiation therapy. Radiochromic plastic dosimeter PRESAGE® has been used for 3D dosimetry for many years. We hypothesized that these phantoms would show dose-rate independence throughout both the conventional and FLASH RT regimes, indicating these phantoms exhibit qualities useful for relative 3D dosimetry in FLASH electron beams. FLASH experiments were performed using a commercially available linear accelerator, converted to deliver an ultra-high-dose-rate 10 MeV electron beam. The LINAC delivered approximately 0.7 Gy/pulse for FLASH irradiations. Dose rate was varied from about 40 Gy/s to 240 Gy/s by changing the repetition rate. PRESAGE phantoms were irradiated en face at six FLASH dose rates: 40 Gy/s, 80 Gy/s, 120 Gy/s, 160 Gy/s, 200 Gy/s, and 240 Gy/s. EBT film and scintillator measurements were used to verify dose delivered. The optical response of the PESAGE phantom versus delivered dose was evaluated with various known doses. A novel parallel-beam optical CT scanner, utilizing fiber optic taper for collimated images, was developed for fast, high resolution, and accurate readout of 3D dosimeters. Percent depth dose curves for various FLASH dose rates and conventional dose rate beams were generated and compared based on the optical response versus dose measurements. Percent depth dose curves from Monte Carlo calculation of the presage phantom were also compared. As shown in Table 1, the percent depth dose as a function of depth for six FLASH dose rates (240-40 Gy/s) are nearly identical, indicating that optical response of PRESAGE is dose-rate independent. The optical density of PRESAGE phantom was confirmed to be linear with absorbed dose for all FLASH dose rates, consistent with the observation at regular treatment dose rates. PRESAGE phantoms show dose-rate independence in electron beams for a wide range of dose rates from conventional to ultra-high-dose-rates, indicating these phantoms can be useful for relative 3D dose measurements in FLASH electron beams. Future experiments will be undertaken as part of the commissioning of a commercially available FLASH radiotherapy unit.

Iterative image reconstruction with polar coordinate discretized system matrix for optical CT radiochromic gel dosimetry.

Gel dosimeters are a potential tool for measuring the complex dose distributions that characterize modern radiotherapy. A prototype tabletop solid-tank fan-beam optical CT scanner for readout of gel dosimeters was recently developed. This scanner does not have a straight raypath from source to detector, thus images cannot be reconstructed using filtered backprojection (FBP) and iterative techniques are required. Iterative image reconstruction requires a system matrix that describes the geometry of the imaging system. Stored system matrices can become immensely large, making them impractical for storage on a typical desktopcomputer. Here we develop a method to reduce the storage size of optical CT system matrices through use of polar coordinate discretization while accounting for the refraction in optical CTsystems. A ray tracing simulator was developed to track the path of light rays as they traverse the different mediums of the optical CT scanner. Cartesian coordinate discretized system matrices (CCDSMs) and polar coordinate discretized system matrices (PCDSMs) were generated by discretizing the reconstruction area of the optical CT scanner into a Cartesian pixel grid and a polar coordinate pixel grid, respectively. The length of each ray through each pixel was calculated and used to populate the system matrices. To ensure equal weighting during iterative reconstruction, the radial rings of PCDSMs were asymmetrically spaced such that the area of each polar pixel was constant. Two clinical phantoms and several synthetic phantoms were produced and used to evaluate the reconstruction techniques under known conditions. Reconstructed images were analyzed in terms of spatial resolution, signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), signal nonuniformity (SNU), and Gamma map passpercentage. A storage size reduction of 99.72% was found when comparing a PCDSM to a CCDSM with the same total number of pixels. Images reconstructed with a PCDSM were found to have superior SNR, CNR, SNU, and Gamma (1mm, 1%) pass percentage compared to those reconstructed with a CCDSM. Increasing spatial resolution in the radial direction with increasing radial distance was found in both PCDSM and CCDSM reconstructions due to the outer regions refracting light more severely. Images reconstructed with a PCDSM showed a decrease in spatial resolution in the azimuthal directions as radial distance increases, due to the widening of the polar pixels. However, this can be mitigated with only a slight increase in storage size by increasing the number of projections. A loss of spatial resolution in the radial direction within 5mm radially from center was found when reconstructing with a PCDSM, due to the large innermost pixels. However, this was remedied by increasing the number of radial rings within the PCDSM, yielding radial spatial resolution on par with images reconstructed with a CCDSM and a storage size reduction of 99.26%. Discretizing the image pixel elements in polar coordinates achieved a system matrix storage size reduction of 99.26% with only minimal reduction in the imagequality.

Prognostic factors of traumatic optic neuropathy based on multimodal analysis-Especially the influence of postoperative dressing change and optic nerve blood supply on prognosis.

To investigate the critical prognostic factors of patients with traumatic optic neuropathy (TON) treated with endoscopic transnasal optic canal decompression (ETOCD) and to perform multimodal analysis based on imaging examinations of optical coherence tomography angiography (OCTA) and CT scan. Subsequently, a new prediction model was established. The clinical data of 76 patients with TON who underwent decompression surgery with the endoscope-navigation system in the Department of Ophthalmology, Shanghai Ninth People's Hospital from January 2018 to December 2021 were retrospectively analyzed. The clinical data included demographic characteristics, reasons for injury, interval between injury and surgery, multimode imaging information of CT scan and OCTA, including orbital fracture, optical canal fractures, vessel density of optic disc and macula, and the times of postoperative dressing change. Binary logistic regression was used to establish a model for best corrected visual acuity (BCVA) after treatment as a predictor of TON outcome. Postoperative BCVA improved in 60.5% (46/76) patients and did not improve in 39.5% (30/76) patients. The times of postoperative dressing change had a significant impact on the prognosis. Other factors affecting the prognosis were microvessel density of the central optic disc, the cause of injury, and the microvessel density above the macula. The area under the raw current curves of the predictive model was 0.7596. The times of dressing changes after the operation, i.e., continuous treatment, is the key factor affecting prognosis. The microvessel density in the center of the optic disc and superior macula, quantitatively analyzed by OCTA, is the prognostic factor of TON and may be used as a prognostic marker of TON.

Feasibility of a Novel 3D Dosimetry System for Commissioning of Single Isocenter Multitarget SRS Treatment

<h3>Purpose/Objective(s)</h3> Verification of Single Isocenter Multitarget SRS treatments (SIM-SRS) is uniquely challenging due to the combination of high dose, steep dose gradients and the requirement for very high spatial accuracy. Here we demonstrate the feasibility of a novel 3D dosimetry system for uniquely comprehensive dosimetry validation and commissioning of this exceptionally challenging technique. <h3>Materials/Methods</h3> The 3D dosimetry system utilizes a new commercially available radiochromic gel dosimeter and a state-of-art in-house telecentric optical-CT readout system DLOS (Duke Large Field of View telecentric Optical CT Scanner). The system was used to quantify the accuracy in 3D and in high resolution (isotropic 1mm) of two SIM-SRS treatments: a simpler two target treatment, and a more complex 5 target treatment. The SIM-SRS plans were created on patient anatomy and then recalculated on a CT of a 15cm diameter dosimeter, with isocenter placed such that each target was located within the dosimeter. Both treatments were delivered to the dosimeters which are gellan gum based radiochromic dosimeters containing a water-soluble tetrazolium salt which reduces into an insoluble formazan dye (with associated color change) under ionizing radiation. Optical-CT readout of the dosimeters pre and post irradiation enabled a high-resolution comprehensive evaluation of the accuracy of delivery. The DLOS is a bi-telecentric system which enables highest accuracy optical-CT readout through strong scatter and stray light rejection. The measured optical density distribution was spatially registered to the dose from the planning system and converted to dose via a calibration curve. Independent verification of treatment accuracy was also performed with a commercial Monte Carlo dose calculation algorithm. <h3>Results</h3> For the 2-target plan, the measured dose agreed with the planning system with a Gamma-Index passing rate of 99.88%, 99.27%, and 92.75%, using a 3%/3mm, 3%/2mm, 5%/1mm criteria, respectively. For the 5-target plan, the Gamma-Index passing rate was 99.59%, 97.35%, and 93.82%, using a 3%/3mm, 3%/2mm, 5%/1mm criteria, respectively. In comparison, the dose calculated using Monte Carlo agreed with the planning system with a Gamma-Index passing rate of 100.0% using 3%/1mm for both the 2 target and 5 target plans. Mean dose for the PTVs of the 2-target plan was within –0.71% and –1.53%. Mean dose for the PTVs of the 5-target plan was within (1.08±0.97)%. <h3>Conclusion</h3> This study demonstrates the feasibility of a new 3D dosimetry tool for verification of advanced radiation treatments. Application to SIM-SRS treatments has demonstrated excellent agreement between measured and predicted dose in the phantom. The combination of DLOS optical-CT with Clearview dosimeters was found to be important in obtaining accurate 3D dosimetry. The method introduced here is anticipated to be applicable to a wide range of clinically challenging treatment techniques.

Self-healing of slag-cement ultra-high performance steel fiber reinforced concrete (UHPFRC) containing sisal fibers as healing conveyor

This paper presents the results of a research on the influence of sisal fibers on the self-healing capacity of slag-cement UHPFRC reinforced with steel fibers. In order to evaluate the composite healing capacity, specimens were submitted to pre-cracking by tensile test and then to a 3 months of wetting and drying cycles treatment. Treated specimens were resubmitted to tensile test and evaluated by Optical and Electronic microscopy and CT Scan. Results indicate that the sisal fiber works as a healing conveyor improving by about 20% and 15% the tensile stress and post cracking energy of treated specimens, respectively. Sisal fibers played a major role densifying the interface, working as a vehicle for the healing agents into small interface cracks. All specimens with cracks under 80 μm could completely self-seal mainly by calcium carbonate. Wider cracks could not be completely sealed, though the specimens exhibited a recovery of the mechanical behavior essentially due to late slag-cement hydration.

Application of an in-house developed complementary metal-oxide-semiconductor-based optical computed tomography (CMOS-OCT) imaging system for stereotactic radiosurgery dosimetry using a PRESAGE® dosimeter

PRESAGE® dosimeter can be useful for 3D verification of radiotherapy treatment delivery when careful attention is given to the dosimeter measurement technique. Ideally, a low noise and fast optical CT scanning system is required for imaging the dosimeter for verification of advanced radiotherapy treatment delivery. We have developed a complementary metal-oxide-semiconductor (CMOS)-based optical computed tomography (OCT) system (CMOS-OCT), implementing the recent advances in CMOS active pixel sensor (APS) technology that offer on-chip image processing and high-speed readout. In this study, we present the dosimetric response of irradiated PRESAGE® dosimeter measured with the CMOS-OCT system and describe a feasibility study of using the calibrated system to verify stereotactic radiosurgery (SRS) treatment. PRESAGE® dosimeters were irradiated using a 6 MV Elekta Synergy linear accelerator to investigate dosimetric and geometric properties of PRESAGE® dosimeter with CMOS-OCT imaging system, in order to verify a clinical SRS treatment plan of the brain. The tomographic projection images of the PRESAGE® dosimeter were captured by the image sensor within 100 s for a 360-degree rotation. The CMOS-OCT imaging system showed good linearity in dose-response measured by PRESAGE® dosimeter. The correlation of the optical density (OD) changes measured from the CMOS-OCT with attenuation value measured using a calibrated UV/Vis spectrophotometer was found to be better than 0.996. The tomographic reconstruction of the dose distribution from the PRESAGE® showed good agreement when compared with the calculated dose distributions from the treatment planning software. The in-house CMOS-OCT imaging system is capable of visualising and verifying the dose distribution of stereotactic radiosurgery treatment. The CMOS-OCT imaging of the PRESAGE® dosimeter provides accurate dosimetric and geometric information on radiotherapy delivery in a three-dimensional volume, which is useful in advanced radiotherapy. The introduction of CMOS technology in the OCT improves the imaging speed of the dosimeters dose readout.

OD57 - Dual wavelength reading method of Fricke-Xylenol orange-Gelatin gel dosimeters with cone-beam optical CT scanner for applications in stereotactic radiotherapy

OD57 - Dual wavelength reading method of Fricke-Xylenol orange-Gelatin gel dosimeters with cone-beam optical CT scanner for applications in stereotactic radiotherapy

PH-0601 3D dosimetry with a novel fast optical CT scanner utilizing fiber optic taper for collimated images

PH-0601 3D dosimetry with a novel fast optical CT scanner utilizing fiber optic taper for collimated images

Fiducial detection and registration for 3D IMRT QA with organ-specific dose information.

PurposeTwo‐dimensional (2D) IMRT QA has been widely performed in Radiation Oncology clinic. However, concerns regarding its sensitivity in detecting delivery errors and its clinical meaning have been raised in publications. In this study, a robust methodology of three‐dimensional (3D) IMRT QA using fiducial registration and structure‐mapping was proposed to acquire organ‐specific dose information.MethodsComputed tomography (CT) markers were placed on the PRESAGE dosimeter as fiducials before CT simulation. Subsequently, the images were transferred to the treatment planning system to create a verification plan for the examined treatment plan. Patient’s CT images were registered to the CT images of the dosimeter for structure mapping according to the positions of the fiducials. After irradiation, the 3D dose distribution was read‐out by an optical‐CT (OCT) scanner with fiducials shown on the OCT dose images. An automatic localization algorithm was developed in MATLAB to register the markers in the OCT images to those in the CT images of the dosimeter. SlicerRT was used to show and analyze the results. Fiducial registration error was acquired by measuring the discrepancies in 20 fiducial registrations, and thus the fiducial localization error and target registration error (TRE) was estimated.ResultsDosimetry comparison between the calculated and measured dose distribution in various forms were presented, including 2D isodose lines comparison, 3D isodose surfaces with patient’s anatomical structures, 2D and 3D gamma index, dose volume histogram and 3D view of gamma failing points. From the analysis of 20 fiducial registrations, fiducial registration error was measured to be 0.62 mm and fiducial localization error was calculated to be 0.44 mm. Target registration uncertainty of the proposed methodology was estimated to be within 0.3 mm in the area of dose measurement.ConclusionsThis study proposed a robust methodology of 3D measurement‐based IMRT QA for organ‐specific dose comparison and demonstrated its clinical feasibility.

19 Personalized pretreatment QA for intracranial stereotactic treatments using gel dosimetry and 3D printing of phantom: A feasibility study

Introduction.In stereotactic radiotherapy, pretreatment patient-specific quality controls (PSQA) are performed on generic phantoms to check the agreement between the dose calculated by the treatment planning system (TPS) and the measured dose by a point or 2D detector. For those controls, gel dosimetry seems a promising technique as it presents several advantages compared with conventional dosimeters such as 3D absolute dose determination with high spatial resolution [1] Babic S. et al. Three-dimensional dose verification for IMRT in the Radiological Physics Centre head-and-neck phantom using optical CT scans of Ferrous Xylenol-Orange gel dosimeters. Int J Radiat Oncol Biol Phys. 2008; 70: 1281-1291 Abstract Full Text Full Text PDF PubMed Scopus (59) Google Scholar . Thus, we investigated the feasibility of gel dosimetry in combination with a patient-based 3D printed phantom for personalized PSQA.

Preliminary characterization of the Duke Integrated-Lens Optical-CT scanner (DIOS)

The present study investigates a cost effective and practical non-telecentric optical-CT scanner developed for 3D dosimetry, the Duke Integrated-Lens Optical-CT scanner (DIOS). The DIOS system includes an upgraded light-collimating tank (the LC-tank) made of solid polyurethane with precision curved ends (with lensing functionality) and a precision cylindrical central hollow for the dosimeter. The LC-tank thus collimates light from a small area light source (~2cm diameter) into parallel rays through the dosimeter, with refocusing of emergent light onto a CCD camera with a focusing lens with an aperture. The solid nature of the LC-Tank dramatically reduces the amount of required refractive matching fluid compared to earlier scanning systems. The aim of this work was to perform preliminary characterization studies of DIOS in comparison to earlier systems, particularly telecentric systems. The preliminary results indicate promising performance for the DIOS approach.

Assessing CBCT-based patient positioning accuracy on the Gamma Knife IconTM via Presage® 3D absolute dosimetry

Cone beam computed tomography (CBCT) imaging has been implemented on the Leksell Gamma Knife IconTM for repeated patient positioning in mask-based Gamma Knife radiosurgery. The purpose of this study was to evaluate the accuracy of the CBCT-based patient positioning on the Gamma Knife IconTM. Two Presage phantoms of 15 cm diameter and 10 cm height were irradiated with identical shot placements on an Acoustic Neuroma target with the same prescription dose following standard mask-based treatment workflow according to two different fraction schedules: a single fraction of 7.5 Gy and 5 fractions with 1.5 Gy fraction dose. On the top and the bottom portions of each phantom, 8 single 16 mm collimator shots were delivered with maximum doses from 2 Gy to 20 Gy for dose sensitivity calibration. The irradiated Presage phantoms were scanned and analyzed using an OCTOPOUS optical CT scanner. Both the absolute dose distributions and the relative dose distributions for the Acoustic target on each phantom were compared with those from the treatment planning system. The relative dose distribution from the single fraction irradiation agrees better with the planning system than the 5 fraction irradiation, indicating noticeable change in the dose distribution caused by the phantom positioning/repositioning process. No difference between the absolute dose distributions from the two phantoms could be identified because of the large uncertainty in the experiment data.

Feasibility study of a dry optical CT scanner using aspherical lenses

Dry scanners have been proposed as alternatives to scanners in which a refractive index matching fluid is used. Previous dry scanners either have a small field-of-view or employ a thick cast with a higher refractive index than the phantom. This may not always be feasible. A new design is proposed in this study where two aspherical meniscus lenses are employed to direct the laser beams in parallel lines through the phantom.

Pre-clinical and small field dosimetry

Preclinical in vivo studies have drastically improved over the past decade with the development of cone beam computed tomography (CBCT) image-guided small animal irradiation systems. Such systems produce 220 or 225 kV x-rays with square and circular field sizes ranging from 0.5 to 10 mm. The dosimetry of such equipment involving kilovoltage small-field dosimetry has not received as much attention as the megavoltage small-field dosimetry. The dosimetry of megavoltage small fields can be challenging due to lateral charged particle disequilibrium, detector volume averaging effect, and high dose gradients. Clinically there has been a rapid increase in the use of small fields in modern radiotherapy techniques such as stereotactic radiosurgery (SRS) and stereotactic radiotherapy (SBRT). This study presents dosimetric properties of image-guided small animal irradiation systems. Both EBT Gafchromic films and 3-D PRESAGE measured beam data were presented and compared with the calculated dose distribution from a commissioned planning system. For megavoltage small-field dosimetry, EBT3 films and PRESAGE dosimeters were used to measure the dose distributions for MLC-delimited 6 x 6 to 20 x 20 mm2 square fields, and for selected IMRT and VMAT plans with small field sizes or segments of 1-4 cm. A single-beam optical CT scanner was used as the readout mechanism of the radiation-induced 3-D information in the PRESAGE phantoms. Measured data sets were compared with calculated results from Eclipse Acuros XB. The results for kilovoltage small animal irradiator showed that PDD data measured from EBT films and PRESAGE dosimeters are in agreement within 3%; profiles and 2-D dose distributions measured from PRESAGE present a larger penumbra compared with those from EBT films. Discrepancies between measured beam data and treatment planning data were identified. For megavoltage small fields, the measured data percent depth dose, beam profiles, and dose distributions were found to agree within experimental uncertainties for EBT films and PRESAGE dosimeters. Beam profiles for MLC-delimited field sizes less than 10 mm reveal some discrepancies in the penumbra region between measured data and calculated results. Dose distributions from EBT film and PRESAGE measurements demonstrate that 3-D dosimetry measurement for small-field IMRT and VMAT QA may be necessary to ensure a complete verification of dose delivery and accuracy.

Dose verification of dynamic MLC-tracked radiotherapy using small PRESAGE® 3D dosimeters and a motion phantom

With the increasing complexity of radiotherapy treatments typical 1D and 2D quality assurance (QA) detectors may fail to detect out-of-plane dose discrepancies, in particular in the presence of motion. In this work, small samples of the PRESAGE® 3D radiochromic dosimeter were used in combination with a motion phantom to measure real-time multileaf collimator (MLC)-tracked radiotherapy treatments. A different sample of PRESAGE® was irradiated for each of three different irradiation scenarios: (1) static: static sample, without tracking (2) motion: moving sample, without tracking and (3) tracking: moving sample, with tracking. Our in-house software DynaTrack dynamically moves the linac’s MLC leafs based on the target position. The doses delivered to the samples were reconstructed based on the recorded positions of the MLC and phantom during the beam delivery. PRESAGE® samples were imaged with an in-house optical-CT scanner. Comparison between simulated and measured 3D dose showed good agreement for all three irradiation scenarios (static: 99.2%; motion: 99.7%; tracking: 99.3% with a 3%, 2 mm and a 10% threshold local gamma criterion), failing only at the edges of the PRESAGE® samples (~ 6 mm). Given that the dose distributions deposited using the DynaTrack system have been independently verified, this experiment demonstrates the ability of PRESAGE to measure 3D doses correctly in a tracking context. We conclude that this methodology could be used in the future to validate the delivery of dynamic MLC-tracked radiotherapy.