Accurate 3D Dosimetry Research Articles

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.

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.

A deep-learning-based prediction model for the biodistribution of 90 Y microspheres in liver radioembolization.

Radioembolization with 90 Y microspheres is a treatment approach for liver cancer. Currently, employed dosimetric calculations exhibit low accuracy, lacking consideration of individual patient, and tissue characteristics. The purpose of the present study was to employ deep learning (DL) algorithms to differentiate patterns of pretreatment distribution of 99m Tc-macroaggregated albumin on SPECT/CT and post-treatment distribution of 90 Y microspheres on PET/CT and to accurately predict how the 90 Y-microspheres will be distributed in the liver tissue by radioembolization therapy. Data for 19 patients with liver cancer (10 with hepatocellular carcinoma, 5 with intrahepatic cholangiocarcinoma, 4 with liver metastases) who underwent radioembolization with 90 Y microspheres were used for the DL training. We developed a 3D voxel-based variation of the Pix2Pix model, which is a special type of conditional GANs designed to perform image-to-image translation. SPECT and CT scans along with the clinical target volume for each patient were used as inputs, as were their corresponding post-treatment PET scans. The real and predicted absorbed PET doses for the tumor and the whole liver area were compared. Our model was evaluated using the leave-one-out method, and the dose calculations were measured using a tissue-specific dose voxel kernel. The comparison of the real and predicted PET/CT scans showed an average absorbed dose difference of 5.42% ± 19.31% and 0.44% ± 1.64% for the tumor and the liver area, respectively. The average absorbed dose differences were 7.98 ± 31.39Gy and 0.03 ± 0.25Gy for the tumor and the non-tumor liver parenchyma, respectively. Our model had a general tendency to underpredict the dosimetric results; the largest differences were noticed in one case, where the model underestimated the dose to the tumor area by 56.75% or 72.82Gy. The proposed deep-learning-based pretreatment planning method for liver radioembolization accurately predicted 90 Y microsphere biodistribution. Its combination with a rapid and accurate 3D dosimetry method will render it clinically suitable and could improve patient-specific pretreatment planning.

Linear energy transfer characterization of five gel dosimeter formulations for electron and proton therapeutic beams

Gel dosimeters, including radiochromic types like Fricke, as well as polymer formulations, are considered to be the only reliable option for accurate 3D dosimetry. Nevertheless, their implementation in daily clinical quality assurance still remains strongly limited for a few high specialized radiotherapy centres. Although gel dosimeters present very good water-equivalence due to their inherent chemical and isotopic compositions, addressing the corresponding dosimetry outputs is highly challenging, needing careful assessment in terms of the different radiation qualities involved in the mixed field. Accurate estimations of the linear energy transfer for each gel dosimeter formulation stands as a baseline for further accurate dose deconvolution in mixed radiation fields. The present study reports on the linear energy transfer characterization of five different gel dosimeter formulations, Fricke, Itabis, Magic, Nipam, and Pagat, for electron and proton therapeutic beams as obtained by Monte Carlo approaches, along with experimental results for validation purposes. The linear energy transfer, as a function of beam quality and penetration depth, is obtained for electron and proton therapeutic beams remarking the presence of non-negligible variations, which need to be accounted for a further accurate implementation of gel dosimetry as well as for precise dose deconvolution in mixed radiation fields.

Preparation of W1/O/W2 emulsion to limit the diffusion of Fe3+ in the Fricke gel 3D dosimeter

The irradiation of tumors in radiotherapy requires accurate 3D dosimetry. The Fricke 3D dosimeters, which were considered to be high potential of application in 3D dosimetry, suffer from a reduced temporal integrity of dose distribution caused by Fe3+ ions diffusion. To overcome the drawback, we firstly synthesized a kind of amphiphilic molecules with critical micelle concentration of 0.45 g/L and hydrophile‐lipophile balance value of 10, then prepared multiple emulsions by self‐assembling those molecules in Fricke solution under liquid paraffin, and finally obtained Fricke hydrogel embedded with the multiple emulsions. The diffusion coefficient of Fe3+ ions in the embedded Fricke hydrogel was measured to be 0.17 mm2/h. The hydrogel dosimeter exhibits considerable potential for use in dose verification applications.

OA169] Advanced personalised 3D dosimetry for peptide receptor radionuclide therapy based on monte carlo method

Purpose Peptide Receptor Radionuclide Therapy (PPRT) is nowadays the most effective therapy for treating patients who suffered of neuroendocrine tumours [1] . In clinical practice, absorbed dose calculations are computed based on the Medical Internal Radiation Dose (MIRD) scheme (not planned or optimised for patients’ specific characteristics). To increase the quality and the efficacy of the clinical trials, a more accurate 3D dosimetry is required. Monte Carlo (MC) techniques showed to provide the most accurate approach for the MRT dose calculations. In this work, Raydose MC based pipeline [2] was used to compute the patient specific dose calculation for a clinical trial in PRRT. The dose obtained for lesions and organs at risk (OARs) were then compared to the ones based on the MIRD scheme. Methods One hundred patients were enrolled as part of a clinical trial in PRRT with a prescribed activity between 3.7 and 5.5 GBq of 177Lu-DOTA-Tyr3-octreotide. Patients were scanned five times with a SPECT/CT scanner and the sequential scans were co-registered using a non-rigid registration algorithm [3] . Volumes of interest (VOIs) including lesions and OARs were manually outlined on fused images. 3D dose maps calculated with Raydose were compared to the corresponding ones obtained with OLINDA/EXM software. Differences between calculations were evaluated using a paired samples Wilcoxon-Mann-Witney test (two-sided, p Results Mean doses calculated with OLINDA/EXM were higher than the corresponding mean doses calculated with Raydose. In particular, for the structures studied, the following median percentage differences were obtained: kidneys, −11.70% (Internal Quartile Range (IQR) = 7.14); liver, −14.06% (IQR = 5.62); spleen, −12.50% (IQR = 6.38); lesions, −21.01% (IQR = 18.52). Furthermore, significant differences were observed for kidneys and lesions. Conclusions Dose values obtained with the MC method suggested an underestimation of the dose calculated for OARs and lesions in PRRT treatment. A more accurate patient specific 3D dose calculation could be used to increase the injected activity and to rise the efficiency of the treatment but maintaining a safe level of toxicity.

Dose–response of EBT3 radiochromic films to proton and carbon ion clinical beams

We investigated the dose–response of the external beam therapy 3 (EBT3) films for proton and carbon ion clinical beams, in comparison with conventional radiotherapy beams; we also measured the film response along the energy deposition-curve in water. We performed measurements at three hadrontherapy centres by delivering monoenergetic pencil beams (protons: 63–230 MeV; carbon ions: 115–400 MeV/u), at 0.4–20 Gy dose to water, in the plateau of the depth-dose curve. We also irradiated the films to clinical MV-photon and electron beams. We placed the EBT3 films in water along the whole depth-dose curve for 148.8 MeV protons and 398.9 MeV/u carbon ions, in comparison with measurements provided by a plane-parallel ionization chamber. For protons, the response of EBT3 in the plateau of the depth-dose curve is not different from that of photons, within experimental uncertainties. For carbon ions, we observed an energy dependent under-response of EBT3 film, from 16% to 29% with respect to photon beams. Moreover, we observed an under-response in the Bragg peak region of about 10% for 148.8 MeV protons and of about 42% for 398.9 MeV/u carbon ions. For proton and carbon ion clinical beams, an under-response occurs at the Bragg peak. For carbon ions, we also observed an under-response of the EBT3 in the plateau of the depth-dose curve. This effect is the highest at the lowest initial energy of the clinical beams, a phenomenon related to the corresponding higher LET in the film sensitive layer. This behavior should be properly modeled when using EBT3 films for accurate 3D dosimetry.

Stray light in cone beam optical computed tomography: II. Reduction using a convergent light source

Optical cone beam computed tomography (CBCT) using a broad beam and CCD camera is a fast method for densitometry of 3D optical gel dosimeters. However, diffuse light sources introduce considerable stray light into the imaging system, leading to underestimation of attenuation coefficients and non-uniformities in CT images unless corrections are applied to each projection image. In this study, the light source of a commercial optical CT scanner is replaced with a convergent cone beam source consisting of almost exclusively image forming primary rays. The convergent source is achieved using a small isotropic source and a Fresnel lens. To characterize stray light effects, full-field cone beam CT imaging is compared to fan beam CT (FBCT) using a 1 cm high fan beam aperture centered on the optic axis of the system. Attenuating liquids are scanned within a large 96 mm diameter uniform phantom and in a small 13.5 mm diameter finger phantom. For the uniform phantom, cone and fan beam CT attenuation coefficients agree within a maximum deviation of (1 ± 2)% between mean values over a wide range from 0.036 to 0.43 cm−1. For the finger phantom, agreement is found with a maximum deviation of (4 ± 2)% between mean values over a range of 0.1–0.47 cm−1. With the convergent source, artifacts associated with refractive index mismatch and vessel optical features are more pronounced. Further optimization of the source size to achieve a balance between quantitative accuracy and artifact reduction should enable practical, accurate 3D dosimetry, avoiding time consuming 3D scatter measurements.

An investigation of PRESAGE® 3D dosimetry for IMRT and VMAT radiation therapy treatment verification

The purpose of this work was to characterize three formulations of PRESAGE® dosimeters (DEA-1, DEA-2, and DX) and to identify optimal readout timing and procedures for accurate in-house 3D dosimetry. The optimal formulation and procedure was then applied for the verification of an intensity modulated radiation therapy (IMRT) and a volumetric modulated arc therapy (VMAT) treatment technique.PRESAGE® formulations were studied for their temporal stability post-irradiation, sensitivity, and linearity of dose response. Dosimeters were read out using a high-resolution optical-CT scanner. Small volumes of PRESAGE® were irradiated to investigate possible differences in sensitivity for large and small volumes (‘volume effect’). The optimal formulation and read-out technique was applied to the verification of two patient treatments: an IMRT plan and a VMAT plan.A gradual decrease in post-irradiation optical-density was observed in all formulations with DEA-1 exhibiting the best temporal stability with less than 4% variation between 2–22 h post-irradiation. A linear dose response at the 4 h time point was observed for all formulations with an R2 value >0.99. A large volume effect was observed for DEA-1 with sensitivity of the large dosimeter being ~63% less than the sensitivity of the cuvettes. For the IMRT and VMAT treatments, the 3D gamma passing rates for 3%/3 mm criteria using absolute measured dose were 99.6 and 94.5% for the IMRT and VMAT treatments, respectively.In summary, this work shows that accurate 3D dosimetry is possible with all three PRESAGE® formulations. The optimal imaging windows post-irradiation were 3–24 h, 2–6 h, and immediately for the DEA-1, DEA-2, and DX formulations, respectively. Because of the large volume effect, small volume cuvettes are not yet a reliable method for calibration of larger dosimeters to absolute dose. Finally, PRESAGE® is observed to be a useful method of 3D verification when careful consideration is given to the temporal stability and imaging protocols for the specific formulation used.

Development and evaluation of convergent and accelerated penalized SPECT image reconstruction methods for improved dose-volume histogram estimation in radiopharmaceutical therapy.

Three-dimensional (3D) dosimetry has the potential to provide better prediction of response of normal tissues and tumors and is based on 3D estimates of the activity distribution in the patient obtained from emission tomography. Dose-volume histograms (DVHs) are an important summary measure of 3D dosimetry and a widely used tool for treatment planning in radiation therapy. Accurate estimates of the radioactivity distribution in space and time are desirable for accurate 3D dosimetry. The purpose of this work was to develop and demonstrate the potential of penalized SPECT image reconstruction methods to improve DVHs estimates obtained from 3D dosimetry methods. The authors developed penalized image reconstruction methods, using maximum a posteriori (MAP) formalism, which intrinsically incorporate regularization in order to control noise and, unlike linear filters, are designed to retain sharp edges. Two priors were studied: one is a 3D hyperbolic prior, termed single-time MAP (STMAP), and the second is a 4D hyperbolic prior, termed cross-time MAP (CTMAP), using both the spatial and temporal information to control noise. The CTMAP method assumed perfect registration between the estimated activity distributions and projection datasets from the different time points. Accelerated and convergent algorithms were derived and implemented. A modified NURBS-based cardiac-torso phantom with a multicompartment kidney model and organ activities and parameters derived from clinical studies were used in a Monte Carlo simulation study to evaluate the methods. Cumulative dose-rate volume histograms (CDRVHs) and cumulative DVHs (CDVHs) obtained from the phantom and from SPECT images reconstructed with both the penalized algorithms and OS-EM were calculated and compared both qualitatively and quantitatively. The STMAP method was applied to patient data and CDRVHs obtained with STMAP and OS-EM were compared qualitatively. The results showed that the penalized algorithms substantially improved the CDRVH and CDVH estimates for large organs such as the liver compared to optimally postfiltered OS-EM. For example, the mean squared errors (MSEs) of the CDRVHs for the liver at 5 h postinjection obtained with CTMAP and STMAP were about 15% and 17%, respectively, of the MSEs obtained with optimally filtered OS-EM. For the CDVH estimates, the MSEs obtained with CTMAP and STMAP were about 16% and 19%, respectively, of the MSEs from OS-EM. For the kidneys and renal cortices, larger residual errors were observed for all algorithms, likely due to partial volume effects. The STMAP method showed promising qualitative results when applied to patient data. Penalized image reconstruction methods were developed and evaluated through a simulation study. The study showed that the MAP algorithms substantially improved CDVH estimates for large organs such as the liver compared to optimally postfiltered OS-EM reconstructions. For small organs with fine structural detail such as the kidneys, a large residual error was observed for both MAP algorithms and OS-EM. While CTMAP provided marginally better MSEs than STMAP, given the extra effort needed to handle misregistration of images at different time points in the algorithm and the potential impact of residual misregistration, 3D regularization methods, such as that used in STMAP, appear to be a more practical choice.

3D tomodosimetry using long scintillating fibers: A feasibility study

3D dosimetry is recognized as an ideal for patient-specific quality assurance (QA) of highly conformal radiotherapy treatments. However, existing 3D dosimeters are not straightforward to implement in the clinic, as their read-out procedure is often tedious and their accuracy, precision, and∕or sample size exhibit limitations. The purpose of this work is to develop a 3D dosimeter based on the concept of tomodosimetry inside concentric cylindrical planes using long scintillating fibers for the QA of modern radiotherapy techniques such as intensity-modulated radiation therapy (IMRT) or intensity-modulated arc therapy (IMAT). Using a model-based simulation, scintillating fibers were modeled on three concentric cylindrical planes of radii 2.5, 5.0, and 7.5 cm, inside a 10 cm radius water-equivalent cylinder phantom. The phantom was set to rotate around its central axis, made parallel to the linac gantry axis of rotation. Light acquisitions were simulated using the calculated dose from the treatment planning software and reconstructed in each cylindrical plane at a resolution of 1 mm(2) using a total-variation minimization iterative reconstruction algorithm. The 3D dose was then interpolated from the reconstructed cylindrical plane doses at a resolution of 1 mm(3). Different scintillating fiber patterns were compared by varying the angle of each fiber in its cylindrical plane and introducing a light-tight cut in each fiber. The precision of the reconstructed cylindrical dose distribution was evaluated using a Poisson modeling of the acquired light signals and the accuracy of the interpolated 3D dose was evaluated using an IMRT clinical plan for a prostate case. Straight scintillating fiber patterns with light-tight cuts were the most accurate in cylindrical dose reconstruction, showing less than 0.5 mm distance-to-agreement in dose gradients and a mean local dose difference of less than 0.2% in the high dose region for a 10 × 10 cm(2) field. The precision attained with this fiber configuration was less than 0.9% in the high dose, low gradient region of an IMRT segment for light acquisitions of 0.1 MU over a 360 degree rotation of the cylinder phantom. 3D dose interpolation for the IMRT clinical plan yielded an overall dose difference with the reference input of less than 1%, except in high dose gradients. Using long scintillating fibers inside rotating, concentric cylindrical planes, the authors demonstrate that their tomodosimetry method has the potential for high resolution, precise, and accurate 3D dosimetry. Moreover, because of its water-equivalence and rotational symmetry, this design should find interesting application for both treatment QA and machine commissioning.

SU-E-T-67: Evaluation of the 3D Dosimetry of the Nucletron MHDR-V2 Source Using PRESAGETM Dosimeters

Purpose: To characterize PRESAGE™ dosimeters for interstitial sources through comparison of direct measurements of the 3D dose parameters of Nucletron mHDR-v2 sources to published dose parameters in AAPM Report No. 229. This project aims to establish PRESAGE™ as an accurate brachytherapy dosimeter and investigates an effective technique in 3D dosimetry. Methods: PRESAGE™ dosimeters are solid, polyurethane-based dosimeters doped with radiochromic leuco-dyes that produce a linear optical-density response to dose. Dosimeters were scanned prior to irradiation to measure background signal. To eliminate potential imaging artifacts due to oversaturated responses in the immediate range of the source, channels 0.4cm and 1.4cm in diameter and 6cm depth were cast in the center of PRESAGE™ dosimeters 9.5cm in height and 11.6cm in diameter. Polyurethane plugs of the same dimensions as the channels were inserted into the dosimeters during irradiation and the HDR catheter positioned in the center of the plug. The irradiation times of 10 and 35 seconds for the 0.4 cm-channel and 1.4 cm-channel dosimeters resulted in doses of 3 Gy at y=0.5cm and y=1cm respectively. After irradiation, the plugs were removed and the dosimeters were scanned with an optical-CT scanner. The reconstructed pre-and post-irradiation images were subtracted and exported to CERR for dose measurements. The calculated dose rates were normalized to the dose rate values at y=1cm and y=1.5cm for the 0.4 cm-channel and 1.4 cm-channel dosimeters. Results: Comparing measured to published dose rates, the average absolute percent differences were approximately 25.8% (stdev. 6.4%) and 12.8% (stdev. 2.8%) for the 0.4cm and 1.4 cm-channel dosimeters respectively. Conclusion: Initial results suggest PRESAGE™ with optical-CT has high potential in measuring accurate 3D dosimetry of Nucletron mHDR-v2 sources. The 1.4 cm-channel dosimeters measured more accurate results, perhaps due to the increased dwell time and dose delivered. Characterization of PRESAGE™ with a low-dose-rate source will follow.

SU‐E‐T‐149: 3D Proton Gel Dosimetry

Purpose: To evaluate the water equivalency of PRESAGE and genipin gel for 3D proton dosimetry. Methods: The GEANT4 Hadrontherapy example was modified to calculate depth doses in four different formulations of PRESAGE dosimeters and in water for a proton beam with a mean energy of 62 MeV. To compare the depth doses in each phantom, the water equivalent thickness (WET) was calculated for each PRESAGE dosimeter according to IAEA technical report 398. Results: Our GEANT4 results show that the maximum energy deposition occurs at 2.9 cm depth in water, at 2.7 cm in the two new chemical formulations of PRESAGE®, at 2.6 cm in the original formulation of PRESAGE®, at 2.55 cm in PRESAGETM and at 2.85 cm in genipin gel. The deposited energy in PRESAGETM is 15% higher than that in water, while it is approximately 5% higher than water in the new formulations of PRESAGE®. For the calculated relative dose versus WET, the new PRESAGE® dosimeters are in better agreement with the depth doses in water. At Bragg peak the maximum relative dose difference with that of water is up to 1% for the PRESAGE® formulation A and more than 2% for the PRESAGE® formulation B and C and the PRESAGETM dosimeter. For the genipin gel the maximum relative dose difference with water is less than 0.5%. Conclusions: The PRESAGE® dosimeters with the new chemical formulations are more water equivalent than the existing PRESAGE® and PRESAGETM dosimeters and are thus more suitable for accurate 3D dosimetry of proton beams. The depth dose in the genipin gel dosimeter shows better agreement with water than the PRESAGE dosimeters. Further experimental studies are planned to investigate possible saturation in the new PRESAGE® formulations and in genipin gel for high LET radiation fields.

Optical CT scanning of cross-linked radiochromic gel without cylinder wall

Genipin cross-linked gelatin hydrogels with 0.2 M sulphuric acid are radiochromic and have sufficient sensitivity for investigating doses less than 50 Gray. Because of the gelatine cross-links, these gels have sufficient strength to allow removal from the vessel container in which they were cast. Placing the gels in the same liquid that was used for preparing the gel allows the radiochemistry to the same throughout the gels and provides physical support. In this buoyancy neutral environment the gel has the same shape and the preparation vessel. This allows optical CT scanning without wall artefacts due to reflection, refraction and optical activity. A gel was irradiated to dose of 25 Gray with a 10 MV photon beam of 20 × 20 mm cross section. Full 3D optical CT scanning was performed with a Vista 10 optical cone-beam CT scanner. Central beam axis profiles and depth dose agree with diode and parallel plate ion chamber measurements. These results demonstrate that genipin cross-linked gel can be used for accurate 3D dosimetry, including surface dose measurements.

WE‐E‐AUD‐03: Characterization of a Novel, Fast, In‐House, CCD‐Based Optical‐CT Scanner for 3D Dosimetry in Radiochromic Dosimeters

Purpose: Practical tools for comprehensive three‐dimensional (3D) dosimetry are urgently needed. Here we evaluate a novel, in‐house, second‐generation optical‐computed tomography (optical‐CT) scanner for 3D‐dosimetry in PRESAGE™ dosimeters. Primary advantages over laser‐based, first‐generation scanners include much faster scanning times and potential for high resolution. Methods: The optical‐CT scanner performance was evaluated by comparing dose readout with that from the ‘gold‐standard’ first‐generation (MGS) scanner for three irradiation schemes of progressive complexity (a single rectangular‐beam, a 3D‐conformal and an IMRT treatment). The scanner incorporates a red‐filtered (633 nm) uniform area‐backlight, but is distinguished by incorporation of a customized tertiary telecentric lens‐system, which enables image formation with parallel light‐ray geometry. 2D projection images were acquired on a CCD camera of an irradiated PRESAGE™ cylinder (5 cm diameter). Multiple 2D projections over a 360°‐scan were used to reconstruct a 3D map of attenuation coefficients. Results: A complete set of high‐resolution (1392×1040 pixels, 50μm) projection images was acquired in ∼5 minutes with the new (CCD) scanner. This represents an improvement in speed of &gt;2 orders of magnitude and potential for high‐resolution dosimetry compared to the first‐generation MGS scanner. The speed advantage for CCD‐scanner arises because, unlike the MGS scanner, an entire 2D‐projection is acquired in a single acquisition. Line profiles showed that 3D dose readout from the CCD scanner was in good agreement with independent readout from the MGS scanner as well as the calculated treatment plan for all three irradiation schemes. Gamma comparison indicated agreement with MGS readout within 4% dose‐difference and 4mm distance‐to‐agreement. While the signal‐to‐noise ratio for MGS scanner is better by a factor of 3, the attributes of faster speed and higher resolution are highly desirable for practical 3D‐dosimetry. Conclusion: This work demonstrates the feasibility of accurate 3D dosimetry with the new optical‐CT scanner used in combination with radiochromic (non‐scattering) dosimeters.

TU‐C‐224C‐02: A Practical and Accurate 3D Dosimetry System for Radiation Therapy

Purpose: At present, clinical dosimeters are limited to point or planar measurement, and hence do not provide the comprehensive 3D information ideal for verification of advanced delivery techniques. In this work we present a clinically viable 3D dosimetry system comprising a PRESAGE™ dosimeter with read out by an optical‐CT scanner. Method and Materials: A novel solid dosimeter called PRESAGE™ has been developed which is composed of polyurethane polymer and radiochromic leuco dyes. PRESAGE™ exhibits a stable color change and hence optical density (OD) change when exposed to ionizing radiation. A PRESAGE™ cylinder of 16cm diameter × 11cm height was taken through the treatment planning process and a 5‐field 6MV conformal radiation treatment was delivered by a Varian® linear accelerator. The radiation induced OD change was imaged in 3D by an optical‐CT scanner and this measured distribution was then compared with the corresponding dose distribution calculated by the treatment planning system, as well as to independent measurement by GAFCHROMIC® film. Intercomparisons between the three dose distributions were made by superimposing isodose lines and calculating gamma maps (with criteria 4% dose difference and 4mm distance to agreement). Results: Given stable temperature and protection from exposure to incandescent light, the dose response of PRESAGE™ was observed to be robust to all aspects of the lab. The 3D dose distribution measured in PRESAGE™ showed good agreement with the calculated treatment plan (Eclipse) as well as the independent film measurement at all percent doses &gt;30% (i.e. in regions further than 1cm from the wall). Gamma comparison shows that the PRESAGE™ measurement agrees with both the calculation in treatment plan and the film measurement within 4% dose difference and 4mm distance to agreement. Conclusion: This work presents the PRESAGE™/optical‐CT combination as a practical 3D dosimetry system which can provide comprehensive quality assurance of advanced treatment techniques.

Focusing optics of a parallel beam CCD optical tomography apparatus for 3D radiation gel dosimetry

Optical tomography of gel dosimeters is a promising and cost-effective avenue for quality control of radiotherapy treatments such as intensity-modulated radiotherapy (IMRT). Systems based on a laser coupled to a photodiode have so far shown the best results within the context of optical scanning of radiosensitive gels, but are very slow (∼9 min per slice) and poorly suited to measurements that require many slices. Here, we describe a fast, three-dimensional (3D) optical computed tomography (optical-CT) apparatus, based on a broad, collimated beam, obtained from a high power LED and detected by a charged coupled detector (CCD). The main advantages of such a system are (i) an acquisition speed approximately two orders of magnitude higher than a laser-based system when 3D data are required, and (ii) a greater simplicity of design. This paper advances our previous work by introducing a new design of focusing optics, which take information from a suitably positioned focal plane and project an image onto the CCD. An analysis of the ray optics is presented, which explains the roles of telecentricity, focusing, acceptance angle and depth-of-field (DOF) in the formation of projections. A discussion of the approximation involved in measuring the line integrals required for filtered backprojection reconstruction is given. Experimental results demonstrate (i) the effect on projections of changing the position of the focal plane of the apparatus, (ii) how to measure the acceptance angle of the optics, and (iii) the ability of the new scanner to image both absorbing and scattering gel phantoms. The quality of reconstructed images is very promising and suggests that the new apparatus may be useful in a clinical setting for fast and accurate 3D dosimetry.