Optical Cone Beam CT Research Articles

A cone-beam optical CT based on a convergent light source – Characterization and optimization

PurposeEmploying a Fresnel lens and a point-like light source to create a convergent light beam for the camera effectively minimizes stray light and enhances image quality in optical computed tomography (OCT), benefiting 3D dosimetry applications. This study outlines the development of an economical cone-beam optical computed scanner for 3D dosimetry. MethodsOptical performance was assessed by calculating modulation transfer function (MTF) with pattern charts. Stray light was evaluated by imaging a cylinder flask and a square grid with 5 mm diameter holes to determine the stray-to-primary ratio. Reconstruction quality was determined using SIRT-TV and compared with spectrophotometry attenuation coefficients, with the best regularization parameter (λ = 0.01) chosen based on contrast-to-noise ratio (CNR). Dosimetry performance was assessed by determining percentage dose depth (PDD) for a 6MV beam with a 5 × 5 cm2 field using FXO-f gel dosimeter, compared with ionization chamber data. ResultsMTF evaluation yielded ≥ 50 % agreement with pattern charts. Stray-to-primary ratio was less than 0.1 or 10 % of the total signal. Reconstruction showed low noise and artifacts, with optimal CNR at λ = 0.01. Attenuation coefficients from optical CT aligned with spectrometer measurements within 1.2 %. PDD calculated with FXO-f gel dosimeter closely matched ionization chamber data (<1.2 % difference), achieving a dose resolution of 0.1 Gy. ConclusionThe built and optimization the de optical-CT based on a convergent beam is read to perform the 3D quality assurance in clinical applications.

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.

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.

Evaluation of the accuracy of a six-degree-of-freedom robotic couch using optical surface and cone beam CT images of an SRS QA phantom

Abstract Purpose: To assess the accuracy of the Varian PerfectPitch six-degree-of-freedom (6DOF) robotic couch by using a Varian SRS QA phantom. Methods: The stereotactic radiosurgery (SRS) phantom has five tungsten carbide BBs each with 7·5 mm in diameter arranged with the known geometry. Optical surface images and cone beam CT (CBCT) images of the phantom were taken at different pitch, roll and rotation angles. The pitch, roll, and rotation angles were varied from −3 to 3 degrees by inputs from the linac console. A total of 39 Vision RT images with different rotation angle combinations were collected, and the Vision RT software was used to determine the rotation angles and translational shifts from those images. Eight CBCT images at most allowed rotational angles were analysed by in-house software. The software took the coordinates of the voxel of the maximum CT number inside a 7·5-mm sphere surrounding one BB to be the measured position of this BB. Expected BB positions at different rotation angles were determined by multiplying measured BB positions at zero pitch and roll values by a rotation matrix. Applying the rotation matrix to 5 BB positions yielded 15 equations. A linear least square method was used for regression analysis to approximate the solutions of those equations. Results: Of the eight calculations from CBCT images, the maximum rotation angle differences (degree) were 0·10 for pitch, 0·15 for roll and 0·09 for yaw. The maximum translation differences were 0·3 mm in the left–right direction, 0·5 mm in the anterior–posterior direction and 0·4 mm in the superior–inferior direction. Conclusions: The uncertainties of the 6-DOF couch were examined with the methods of optical surface imaging and CBCT imaging of the SRS QA phantom. The rotational errors were less than 0·2 degree, and the isocentre shifts were less than 0·8 mm.

Development and Evaluation of Cone-Beam Optical Ct Based on the Convergent Light Source for Larger Volumes

Development and Evaluation of Cone-Beam Optical Ct Based on the Convergent Light Source for Larger Volumes

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

Sensitivity enhancement of methacrylic acid gel dosimeters by incorporating iodine for computed tomography scans

PurposePolymer gel dosimeters provide three-dimensional absorbed dose information and have gradually become a popular tool for quality assurance in radiotherapy. This study aims to incorporate iodine into the MAGAT-based gel as radiation sensitizer and investigate whether it can be used to measure the radiation dose and slice thickness for CT scans. MethodsThe nMAGAT(I) gel was doped with 0.03, 0.05, and 0.07-M iodine. The absorbed dose was delivered using a CT scanner (Alexion 16, Toshiba Medical Systems, Japan) with tube voltages of 80, 100, 120, and 135 kVp. The irradiated nMAGAT(I) gel was read using a cone beam optical CT scanner to produce dose-response curves. The nMAGAT(I) gel was used to obtain the slice sensitivity profile (SSP) and the CT dose index (CTDI) for quality assurance of CT scans. ResultsThe 0.07-M iodine-doped nMAGAT(I) gel exhibited maximum sensitivity with the dose enhancement ratio of 2.12. The gel was chemically stable 24 h after its preparation, and the polymerization process was completed 24–48 h after the irradiation. For CT quality assurance, the full width at half maximum measured by the nMAGAT(I) gel matched the nominal slice thickness of CT. The CTDI at center, CTDI at peripheral, and weighted CTDI obtained by the nMAGAT(I) gel differed from those obtained by the ionization chamber by −4.2%, 3.1%, and 0.7%, respectively. ConclusionsThe nMAGAT(I) gel can be used to assess radiation doses and slice thickness in CT scans, thus rendering it a potential quality assurance tool for CT and other radiological diagnostic applications.

A preliminary study on the use of FX-Glycine gel and an in-house optical cone beam CT readout for IMRT and RapidArc verification

The radiochromic FX gel with Optical CT readout has been investigated by several authors and has shown promising results for 3D dosimetry. One of the applications of the gel dosimeters is their use in 3D dose verification for IMRT and RapidArc quality assurance. Though polymer gel has been used successfully for clinical dose verification, the use of FX gel for clinical dose verification with optical cone beam CT needs further validation. In this work, we have used FX gel and an in- house optical readout system for gamma analysis between the dose matrices of measured dose distribution and a treatment planning system (TPS) calculated dose distribution for a few test cases.

Quantifying refractive index mismatch effects on cone beam optical CT scanner measurements

A series of experiments are conducted to evaluate the effects of refractive index (RI) mismatch in readout with a Vista cone beam optical CT scanner. The changing optical attenuation in cuvettes of different colours and RI are quantified using a spectrophotometer, which shows decreasing attenuation as RI increases. A small jar of a colourless solution inside a large jar of another colourless solution with different combinations of fluids of varying RI is imaged in the Vista scanner. Large RI solutions surrounding small RI solutions create a converging lens, while small RI solutions surrounding large RI solutions create a diverging lens. The same dual-jar system is used with solutions of varying RI and colour (attenuation) to assess the effect of RI on attenuation coefficient determination.

Small-Field Measurements of 3D Polymer Gel Dosimeters through Optical Computed Tomography.

With advances in therapeutic instruments and techniques, three-dimensional dose delivery has been widely used in radiotherapy. The verification of dose distribution in a small field becomes critical because of the obvious dose gradient within the field. The study investigates the dose distributions of various field sizes by using NIPAM polymer gel dosimeter. The dosimeter consists of 5% gelatin, 5% monomers, 3% cross linkers, and 5 mM THPC. After irradiation, a 24 to 96 hour delay was applied, and the gel dosimeters were read by a cone beam optical computed tomography (optical CT) scanner. The dose distributions measured by the NIPAM gel dosimeter were compared to the outputs of the treatment planning system using gamma evaluation. For the criteria of 3%/3 mm, the pass rates for 5 × 5, 3 × 3, 2 × 2, 1 × 1, and 0.5 × 0.5 cm2 were as high as 91.7%, 90.7%, 88.2%, 74.8%, and 37.3%, respectively. For the criteria of 5%/5 mm, the gamma pass rates of the 5 × 5, 3 × 3, and 2 × 2 cm2 fields were over 99%. The NIPAM gel dosimeter provides high chemical stability. With cone-beam optical CT readouts, the NIPAM polymer gel dosimeter has potential for clinical dose verification of small-field irradiation.

Evaluation of the OSC‐TV iterative reconstruction algorithm for cone‐beam optical CT

The present work evaluates an iterative reconstruction approach, namely, the ordered subsets convex (OSC) algorithm with regularization via total variation (TV) minimization in the field of cone-beam optical computed tomography (optical CT). One of the uses of optical CT is gel-based 3D dosimetry for radiation therapy, where it is employed to map dose distributions in radiosensitive gels. Model-based iterative reconstruction may improve optical CT image quality and contribute to a wider use of optical CT in clinical gel dosimetry. This algorithm was evaluated using experimental data acquired by a cone-beam optical CT system, as well as complementary numerical simulations. A fast GPU implementation of OSC-TV was used to achieve reconstruction times comparable to those of conventional filtered backprojection. Images obtained via OSC-TV were compared with the corresponding filtered backprojections. Spatial resolution and uniformity phantoms were scanned and respective reconstructions were subject to evaluation of the modulation transfer function, image uniformity, and accuracy. The artifacts due to refraction and total signal loss from opaque objects were also studied. The cone-beam optical CT data reconstructions showed that OSC-TV outperforms filtered backprojection in terms of image quality, thanks to a model-based simulation of the photon attenuation process. It was shown to significantly improve the image spatial resolution and reduce image noise. The accuracy of the estimation of linear attenuation coefficients remained similar to that obtained via filtered backprojection. Certain image artifacts due to opaque objects were reduced. Nevertheless, the common artifact due to the gel container walls could not be eliminated. The use of iterative reconstruction improves cone-beam optical CT image quality in many ways. The comparisons between OSC-TV and filtered backprojection presented in this paper demonstrate that OSC-TV can potentially improve the rendering of spatial features and reduce cone-beam optical CT artifacts.

SU-F-BRA-11: An Experimental Commissioning Test of Brachytherapy MBDCA Dosimetry, Based On a Commercial Radiochromic Gel/optical CT System

Purpose: To implement a 3D dose verification procedure of Model-Based Dose Calculation Algorithms (MBDCAs) for 1 9 2Ir HDR brachytherapy, based on a novel Ferrous Xylenol-orange gel (FXG) and optical CT read-out. Methods: The TruView gel was employed for absolute dosimetry in conjunction with cone-beam optical CT read-out with the VISTA scanner (both from Modus Medical Inc, London, ON, Canada). A multi-catheter skin flap was attached to a cylindrical PETE jar (d=9.6cm, h=16cm) filled with FXG, which served as both the dosimeter and the water equivalent phantom of bounded dimensions. X- ray CT image series of the jar with flap attached was imported to Oncentra Brachy v.4.5. A treatment plan consisting of 8 catheters and 56 dwell positions was generated, and Oncentra-ACE MBDCA as well as TG43 dose results were exported for further evaluation. The irradiation was carried out with a microSelecton v2 source. The FXG dose-response, measured via an electron irradiation of a second dosimeter from the same batch, was linear (R2>0.999) at least up to 12Gy. A MCNP6 input file was prepared from the DICOM-RT plan data using BrachyGuide to facilitate Monte Carlo (MC) simulation dosimetry in the actual experimental geometry. Agreement between experimental (reference) and calculated dose distributions was evaluated using the 3D gamma index (GI) method with criteria (5%-2mm applied locally) determined from uncertainty analysis. Results: The TG-43 GI failed, as expected, in the majority of voxels away from the flap (pass rate 59% for D>0.8Gy, corresponding to 10% of prescribed dose). ACE performed significantly better (corresponding pass rate 92%). The GI evaluation for the MC data (corresponding pass rate 97%) failed mainly at low dose points of increased uncertainty. Conclusion: FXG gel/optical CT is an efficient method for level-2 commissioning of brachytherapy MBDCAs. Target dosimetry is not affected from uncertainty introduced by TG43 assumptions in 192Ir skin brachytherapy. Research co-financed by the ESF and Greek funds through the Operational Program Education and Lifelong Learning Investing in Knowledge Society of the NSRF. Research Funding Program: Aristeia. Modus Medical Devices Inc. provided a TruView dosimeter batch and Nucletron, and Elekta company, provided access to Oncentra Brachy v4.5, for research purposes.

Stray light reduction in optical computed tomography using a convergent cone-beam source

The planar diffuser light source for a cone-beam optical CT scanner was replaced with a filtered LED and large Fresnel lens. The source was focused on a camera and convergent cone-beam images were acquired. Images are sensitive to mismatches in the refractive index of vessels, samples and matching liquid. For PETE jars and water solutions, a loss of approximately 30% of the projections was demonstrated. Teflon PFA cylinders provided better refractive index matching and more accurate reconstructions. The convergent cone-beam source dramatically increased imaging efficiency by a factor of 1000 and reduced stray light levels by confining illumination to image forming rays.

The Accuracy of Tooth Image Reconstruction from Spiral CT, Micro-CT, and Cone-Beam CT Scans

Objectives The aim of this study was to compare 3D accuracy of tooth image reconstruction from three kinds of CT scans using 3D superimpositional method. Methods 18 sound extracted human teeth were scanned by 3D optical system, spiral CT, micro-CT, and cone-beam CT scanner. The digital teeth images reconstructed from three kinds of CT scans were superimposed onto the standard image from optical scans respectively. Distribution patterns of shape discrepancy were presented using histogram, as well as showed in different colors on the superimposed imagines. The ratio of voluminal discrepancy versus the volume of the standard image (RVD/VS) was calculated and analyzed, using the matched-pair t-test and rank sum test. Results Compared with the standard tooth image, the average RVD/VS of digital teeth images by the micro-CT, cone-beam CT, spiral CT scans were 5.11%, 20.73%, 24.60% respectively, and there were statistically significant difference among the three kinds of CT scans (P&lt;0.01). Significant difference were also found among the anterior teeth, bicuspids, and molars (P&lt;0.01). Histogram gave the description about the counts and magnitude of the discrepancies. Marked by difference colors, the superimposed images could give visualized information about the magnitude and distribution patterns of discrepancies. Conclusions The digital teeth models reconstructed from the spiral CT, micro-CT, cone-beam CT images are inhomogeneous enlarged, compared with the original models. As the only realizable way to individualized FEM analysis, tooth modeling by CT scans needs more efforts and refinements to improve its accuracy.

Characterization of a cone beam optical scanner

The use of radiochromic FX gel for mapping 3D dose distribution is hampered by the diffusion of gel and the slow scanning techniques. The development of fast optical cone beam scanning has improved the chances of using radiochromic gel as a feasible dosimeter for radiotherapy applications. In this work an optical cone beam scanner has been developed in-house and its performance characteristics have been studied. The reconstructed image of the optical scanner was analyzed by studying the resolution, signal-to-noise ratio and contrast to noise ratio (CNR). The resolution of the optical cone beam CT scanner was studied by scanning a catheter of 1 mm outer diameter and the scanner was able to detect the catheter. The geometrical accuracy of the reconstruction was studied by placing catheters in spiral geometry in the gel phantom and measuring the distances. It has been observed that the in-house Optical Cone beam scanner is suitable for scanning radiochromic gels for radiotherapy applications.

3-D dose verification by cone-beam optical CT scanning of PRESAGE dosimeter

There is a clear need for an accurate and practical dosimeter that is able to verify 3-D dose distributions from complex radiation treatments. The purpose of this study is to evaluate the dosimetric performance of PRESAGE radiochromic plastic dosimeter in conjunction with a cone-beam optical CT scanner, VistaTM, for 3-D dosimetry. The cone-beam optical CT scanner presented in this study can perform optical readout in less than 30 min, which makes same day dose verification for treatment possible. For dosimetric accuracy, a complete 3-D dose verification procedures were performed, including dose response calibration with a 12 MeV electron beam, a 6 MV photon square field irradiation, as well as a 5-field IMRT irradiation. This study shows that 3-D dose verification with fast optical CT scan of PRESAGE dosimeter is feasible. First-generation optical CT scanner can be used as a gold standard for optical readout of dosimeters.

The performance of an optical cone-beam CT scanner adapted for radiochromic film dosimetry

The primary purpose of this study was to evaluate commercial optical cone-beam computed tomography (CBCT) scanners as devices for reading EBT2 radiochromic film. A secondary objective was to implement a spatial correction for stray light present within optical CBCT systems. Square (12.7 × 12.7 cm2) EBT2 films were positioned vertically in the middle of a small water-filled tank, co-linear with the central beam axis of a 12 MeV electron beam. A total dose of 4.0 Gy was delivered at depth of 3.0 cm. Films were imaged prior to irradiation and 24 hours post-irradiation. Two different models of scanners, Vista15™ and Vista10™, were used to read out the irradiated films. In the Vista15™ scanner, residual light scatter was corrected for using: 1) a single vertical slot array and 2) a slot pair array that produced a vertical fan beam of light. Vista10™ was modified to have a smaller acceptance angle of scattered light and further corrections for residual scatter were made using a multiple slot array. With these different geometries, composite ‘open field’ and ‘shadow field’ images were generated and processed to create ‘glare-free’ pre and post-irradiation film images respectively, from which the net optical density (OD) was calculated. Results were compared against the open light field measurement in which no correction for stray light was made. Using the above scanners, EBT2 films were additionally read out to obtain 12 MeV electron and 6 MV photon percentage depth doses. By correcting for stray light it was found that the central-axis change in the net OD increased particularly in the 12 MeV electron build-up region and at the depth of maximum dose (dmax = 3.0 cm) where light transmission is lowest. In the open light field measurement acquired with the Vista15™ scanner the net OD was 0.87 +/−0.02. Using single vertical slot array geometry to correct for stray light, the net OD was 0.94 +/−0.02, while with the slot pair array the net OD was 0.99 +/−0.02. For the same film read out with the ‘modified’ Vista10™ scanner and corrected for scattered light using the multiple slot array technique, a maximum net OD of 1.30 +/−0.02 was obtained. This latter value is comparable to published data obtained with EBT2 film read out with a spectrophotometer. For 12 MeV electron and 6 MV photon percentage depth doses measured with EBT2 film, agreement with thimble ion chamber measurements is shown. The results demonstrate that accurate radiochromic film densitometry can be achieved with optical CBCT scanners and slot array masks that correct for spatially varying stray light within these devices.

SU-E-T-156: Fast Cone-Beam Optical CT Scanning of a Radiochromic Solid Dosimeter for Clinical 3-D Dose Verification

Purpose: There is an urgent need for fast 3-D dose verification for advanced radiotherapy techniques such as IMAT. This study is to evaluate the accuracy and clinical relevance utilizing a fast cone-beam optical CT scanner with a radiochromic solid dosimeter, PRESAGE®. Methods: A first-generation laser optical CT scanner, in conjunction with gels and radiochromic solid dosimeters, has been successfully used for 3-D dose verification of complex treatments such as IMRT and radiosurgery. However, a major limitation with this type scanner is the long scanning time (8-hours), which makes same-day dose verification for treatment impossible. In this study three radiochromic dosimeters (PRESAGE®) were irradiated with 12Mev electron beam, 6MV photon beam, and 5-field IMRT, respectively. Dose readout was performed using a Vista™ cone beam optical scanner (Modus Medical Devices Inc.). This scanner utilizes a LED diffuse light panel and a lens based CCD camera to capture a series of 2D optical projections through the region of interest while the object is being rotated 360°. Feldkamp filtered back projection was used to reconstruct the 3D dose map, with a spatial resolution of 1×1×1mm. Each dosimeter was scanned both before and after irradiation in order to eliminate artifacts caused by light scattering and dosimeter un-uniformity. Dose distributions were compared with results from a first-generation laser optical CT scanner and Eclipse planning system. Results: The PDDs measured with PRESAGE® and the beam data from Eclipse agree within 3% and 1mm. Comparison of the IMRT dose distributions between PRESAGE® measurements and Eclipse calculation shows 93% Gamma index passing rate (3%, 3mm criteria). The scanning time for a 3-D dose map is less then 30 minutes. Conclusions: The Vista™ cone-beam optical CT scanner and radiochromic dosimeter, PRESAGE®, has shown the potential for fast 3-D dose verification.

SU‐C‐BRA‐01: Efficient Generation of Beamlet Arrays with Hybrid Multileaf Collimator for Grid Therapy

Purpose: To develop a simple method of generating megavoltage beamlet arrays for efficient grid therapy. Methods: Multileaf collimators (MLC) were constructed by mounting equally‐spaced, rectangular, lead sheets on Varian linac block trays, in a divergent geometry. The lead leaves were positioned orthogonal to the same alternating leaf pattern beneath a Varian MLC. Lead sheets with nominal thicknesses of 6.4, 3.2 and 1.6 mm resulted in projected beamlets at linac isocenter of 10×10, 5×5 and 5×2.5 mm respectively. Relative to an open beam, the symmetric grid beam area is 25% open, 50% shadow of single MLC leaf and 25% shadow of overlapping MLC leaves. Kodak EDR‐2 film, ISP Gafchromic EBT film and leuco crystal violet micelle gels with optical cone beam CT readout were used to characterize dose distributions. The impact of the x‐ray beam flattening filter was measured by operating the Varian 2100 series linac in 6 MV mode with and without the filter. Modulation ratios were recorded at water depths of 15 to 200 mm. Orthogonal 5×5 mm grid beams offset by 5 mm were delivered to a 95 mm diameter cylinder of gel in a cubic water phantom at 100 mm depths. Results: Dose rates from beamlet arrays and single beamlets, defined by linac jaws, were similar. Removing the flattening filter doubled the dose rate but had a minor effect on dose modulation. Optical density modulation ratio (5×5 mm beamlet / single leaf shadow) decreased from 7 at 15 mm to 4 at 200 mm depth in water. The gel 3D distribution demonstrated the anticipated features of intersecting diverging beamlet arrays with central dose modulations of 3.5. Conclusions: A hybrid system of orthogonal linac and block tray MLC leaves provides a simple method of generating arrays of megavoltage photon beamlets for efficient delivery of grid therapy.W Francis was partially supported by a London Regional Cancer Program Small Grants for grid therapy development.

Cone Beam Optical CT Investigation on Tissue Equivalent Normoxic Polymer Gel Dosimeter

A potential method has emerged in the form of water-equivalent "3D gel dosimetry" using optical computed-tomography (optical-CT) which enables accurate, high resolution, 3D measurement of dose distributions associated with modern radiation treatments.. Optical Cone Beam CT (CBCT) scanner plays a major role for Gel dosimeter readout and clinical radiation therapy as 3-Dimensional Radiation Dosimetry. The normoxic PAGAT (Polyacrylamide Gelatin and Tetrakis) gel is used as a dosimeter for this cone beam CT analysis due to its tissue equivalent behaviour. Applying a uniform background subtraction of open field intensity resulted in cone beam CT reconstructed attenuation coefficient for a PAGAT Gel Dosimeter.