3D Dosimetry Research Articles

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.

CyberKnife Xsight versus fiducial-based target-tracking: a novel 3D dosimetric comparison in a dynamic phantom

BackgroundThe CyberKnife Xsight lung-tracking system (XLTS) provides an alternative to fiducial-based target-tracking systems (FTTS) for non-small-cell lung cancer (NSCLC) patients without invasive fiducial insertion procedures. This study provides a method for 3D independent dosimetric verification of the accuracy of the FTTS compared to the XLTS without relying on log-files generated by the CyberKnife system.MethodsA respiratory motion trace was taken from a 4D-CT of a real lung cancer patient and applied to a modified QUASAR™ respiratory motion phantom. A novel approach to 3D dosimetry was developed using Gafchromic EBT3 film, allowing the 3D dose distribution delivered to the moving phantom to be reconstructed. Treatments were planned using the recommended margins for one and three fiducial markers and XLTS 2-view, 1-view and 0-view target-tracking modalities. The dose delivery accuracy was analysed by comparing the reconstructed dose distributions to the planned dose distributions using gamma index analysis.ResultsFor the 3%/2 mm gamma criterion, gamma passing rates up to 99.37% were observed for the static deliveries. The 3-fiducial and 1-fiducial-based deliveries exhibited passing rates of 93.74% and 97.82%, respectively, in the absence of target rotation. When target rotation was considered, the passing rate for 1-fiducial tracking degraded to 91.24%. The passing rates observed for XLTS 2-view, 1-view and 0-view target-tracking were 92.78%, 96.22% and 76.08%, respectively.ConclusionsExcept for the XLTS 0-view, the dosimetric accuracy of the XLTS was comparable to the FTTS under equivalent treatment conditions. This study gives us further confidence in the CyberKnife XLTS and FTTS systems.

EFFECT OF DIFFERENT SOLVENTS ON THE OPTICALLY STIMULATED LUMINESCENCE SIGNAL FROM MGB4O7:CE,LI-LOADED POLYMER FILMS.

The detailed dose analysis at the extremities remains a challenge, without affecting operators' mobility and their tactile sense. Using films loaded with optically stimulated luminescence (OSL) crystals have been studied in order to overcome some of these challenges in 2D dosimetry. In this work, we investigated flexible polymeric films loaded with MgB4O7:Ce,Li to acquire a better understanding of the dependence of the dosimetric signal characteristics on the production process and the influence of using different powder grain sizes. In film production, five different solvents were used: acetone-benzene, dichloromethane, chloroform, tetrahydrofuran and formic acid. Our results indicate that acetone-benzene is the solvent mixture that less influences the signal emitted by treated crystals, in comparison with the signal emitted by the pristine crystal powder. Conversely, by using formic acid, the crystalline structure of the sample was most severely modified, leading to a drastic reduction of the emitted OSL signal. We found that the extent of the grain surface in contact with the solvent in the process is important and should be taken into consideration when choosing the proper grain size to be used. Highlights Polymeric films loaded with MgB4O7:Ce,Li crystals were produced using different solvents.Different effect on the OSL signal was found depending on the used solvent.Among the evaluated solvents, acetone-benzene was the one that less affected the OSL signal.

A Review of PRESAGE Radiochromic Polymer and the Compositions for Application in Radiotherapy Dosimetry.

Recent advances in radiotherapy technology and techniques have allowed a highly conformal radiation to be delivered to the tumour target inside the body for cancer treatment. A three-dimensional (3D) dosimetry system is required to verify the accuracy of the complex treatment delivery. A 3D dosimeter based on the radiochromic response of a polymer towards ionising radiation has been introduced as the PRESAGE dosimeter. The polyurethane dosimeter matrix is combined with a leuco-dye and a free radical initiator, whose colour changes in proportion to the radiation dose. In the previous decade, PRESAGE gained improvement and enhancement as a 3D dosimeter. Notably, PRESAGE overcomes the limitations of its predecessors, the Fricke gel and the polymer gel dosimeters, which are challenging to fabricate and read out, sensitive to oxygen, and sensitive to diffusion. This article aims to review the characteristics of the radiochromic dosimeter and its clinical applications. The formulation of PRESAGE shows a delicate balance between the number of radical initiators, metal compounds, and catalysts to achieve stability, optimal sensitivity, and water equivalency. The applications of PRESAGE in advanced radiotherapy treatment verifications are also discussed.

Assessment of Statistical Process Control Based DVH Action Levels for Systematic Multi-Leaf Collimator Errors in Cervical Cancer RapidArc Plans.

BackgroundIn the patient-specific quality assurance (QA), DVH is a critical clinically relevant parameter that is finally used to determine the safety and effectiveness of radiotherapy. However, a consensus on DVH-based action levels has not been reached yet. The aim of this study is to explore reasonable DVH-based action levels and optimal DVH metrics in detecting systematic MLC errors for cervical cancer RapidArc plans.MethodsIn this study, a total of 148 cervical cancer RapidArc plans were selected and measured with COMPASS 3D dosimetry system. Firstly, the patient-specific QA results of 110 RapidArc plans were retrospectively reviewed. Then, DVH-based action limits (AL) and tolerance limits (TL) were obtained by statistical process control. Secondly, systematic MLC errors were introduced in 20 RapidArc plans, generating 380 modified plans. Then, the dose difference (%DE) in DVH metrics between modified plans and original plans was extracted from measurement results. After that, the linear regression model was used to investigate the detection limits of DVH-based action levels between %DE and systematic MLC errors. Finally, a total of 180 test plans (including 162 error-introduced plans and 18 original plans) were prepared for validation. The error detection rate of DVH-based action levels was compared in different DVH metrics of 180 test plans.ResultsA linear correlation was found between systematic MLC errors and %DE in all DVH metrics. Based on linear regression model, the systematic MLC errors between -0.94 mm and 0.88 mm could be caught by the TL of PTV95 ([-1.54%, 1.51%]), and the systematic MLC errors between -1.00 mm and 0.80 mm could also be caught by the TL of PTVmean ([-2.06%, 0.38%]). In the validation, for original plans, PTV95 showed the minimum error detection rate of 5.56%. For error-introduced plans with systematic MLC errors more than 1mm, PTVmean showed the maximum error detection rate of 88.89%, and then was followed by PTV95 (86.67%). All the TL of DVH metrics showed a poor error detection rate in identifying error-induced plans with systematic MLC errors less than 1mm.ConclusionIn 3D quality assurance of cervical cancer RapidArc plans, process-based tolerance limits showed greater advantages in distinguishing plans introduced with systematic MLC errors more than 1mm, and reasonable DVH-based action levels can be acquired through statistical process control. During DVH-based verification, main focus should be on the DVH metrics of target volume. OARs in low-dose regions were found to have a relatively higher dose sensitivity to smaller systematic MLC errors, but may be accompanied with higher false error detection rate.

Features of PABIGnx 3D Polymer Gel as an Ionising Radiation Dosimeter.

This work presents the features of the PABIGnx 3D polymer gel dosimeter. It consists of two cross-linkers: poly(ethylene glycol) diacrylate (PEGDA), as one biacrylic component, and N,N′-methylenebisacrylamide (MBA), which is another cross-linker often used in 3D dosimeters. Additionally, it contains oxygen scavenges of copper sulfate pentahydrate and ascorbic acid. All ingredients are embedded in a physical gel matrix of gelatine. Upon irradiation, the biacrylic cross-linking agents (PEGDA and MBA) undergo radical polymerisation and cross-linking, which is manifested by the appearance of the opacity of the intensity related to the absorbed dose. PABIGnx was irradiated with an oncological source of ionising radiation, and analysed by using a nuclear magnetic resonance (0.5 T). The following characteristics were obtained: (i) linear and dynamic dose-response of 0.5 to ~18 Gy and 40 Gy, respectively, (ii) dose sensitivity of 0.071 ± 0.001 Gy−1 s−1, (iii) integral 3D dose distribution for at least 24 days after irradiation, (iv) adequate batch-to-batch reproducibility, (v) dose-response independent of irradiation with 6 MV photons, 15 MV photons, 6 MV photons FFF of 0.0168–0.1094 Gy/s dose rates, and (vi) soft tissue equivalence. The study showed that the features of PABIGnx confirm its suitability for use in 3D radiotherapy dosimetry.

Virtual Clinical Trials in 2D and 3D X-ray Breast Imaging and Dosimetry: Comparison of CPU-Based and GPU-Based Monte Carlo Codes.

Simple SummaryVirtual clinical trials in X-ray breast imaging may permit substantial reduction of the costs, times, and exposure risk to patient of clinical trials. Monte Carlo simulation techniques are increasingly adopted for VCT in breast imaging and dosimetry studies. This work aims to compare three different platforms for breast VCT studies, to develop real-time virtual DM, DBT and BCT examinations, where the in-silico image acquisition process takes a computational time comparable to that typical of a corresponding real clinical examination.Computational reproductions of medical imaging tests, a form of virtual clinical trials (VCTs), are increasingly being used, particularly in breast imaging research. The accuracy of the computational platform that is used for the imaging and dosimetry simulation processes is a fundamental requirement. Moreover, for practical usage, the imaging simulation computation time should be compatible with the clinical workflow. We compared three different platforms for in-silico X-ray 3D breast imaging: the Agata (University & INFN Napoli) that was based on the Geant4 toolkit and running on a CPU-based server architecture; the XRMC Monte Carlo (University of Cagliari) that was based on the use of variance reduction techniques, running on a CPU hardware; and the Monte Carlo code gCTD (University of Texas Southwestern Medical Center) running on a single GPU platform with CUDA environment. The tests simulated the irradiation of cylindrical objects as well as anthropomorphic breast phantoms and produced 2D and 3D images and 3D maps of absorbed dose. All the codes showed compatible results in terms of simulated dose maps and imaging values within a maximum discrepancy of 3%. The GPU-based code produced a reduction of the computation time up to factor 104, and so permits real-time VCT studies for X-ray breast imaging.

A Novel Nanocomposite Material for Optically Stimulated Luminescence Dosimetry.

Radiotherapy is a well-established and important treatment for cancer tumors, and advanced technologies can deliver doses in complex three-dimensional geometries tailored to each patient's specific anatomy. A 3D dosimeter, based on optically stimulated luminescence (OSL), could provide a high accuracy and reusable tool for verifying such dose delivery. Nanoparticles of an OSL material embedded in a transparent matrix have previously been proposed as an inexpensive dosimeter, which can be read out using laser-based methods. Here, we show that Cu-doped LiF nanocubes (nano-LiF:Cu) are excellent candidates for 3D OSL dosimetry owing to their high sensitivity, dose linearity, and stability at ambient conditions. We demonstrate a scalable synthesis technique producing a material with the attractive properties of a single dosimetric trap and a single near-ultraviolet emission line well separated from visible-light stimulation sources. The observed transparency and light yield of silicone sheets with embedded nanocubes hold promise for future 3D OSL-based dosimetry.

Post-treatment three-dimensional voxel-based dosimetry after Yttrium-90 resin microsphere radioembolization in HCC

BackgroundPost-therapy [90Y] PET/CT-based dosimetry is currently recommended to validate treatment planning as [99mTc] MAA SPECT/CT is often a poor predictor of subsequent actual [90Y] absorbed dose. Treatment planning software became available allowing 3D voxel dosimetry offering tumour-absorbed dose distributions and dose-volume histograms (DVH). We aim to assess dose–response effects in post-therapy [90Y] PET/CT dosimetry in SIRT-treated HCC patients for predicting overall and progression-free survival (OS and PFS) and four-month follow-up tumour response (mRECIST). Tumour-absorbed dose and mean percentage of the tumour volume (V) receiving ≥ 100, 150, 200, or 250 Gy and mean minimum absorbed dose (D) delivered to 30%, 50%, 70%, and 90% of tumour volume were calculated from DVH’s. Depending on the mean tumour -absorbed dose, treated lesions were assigned to a < 120 Gy or ≥ 120 Gy group.ResultsThirty patients received 36 SIRT treatments, totalling 43 lesions. Median tumour-absorbed dose was significantly different between the ≥ 120 Gy (n = 28, 207 Gy, IQR 154–311 Gy) and < 120 Gy group (n = 15, 62 Gy, IQR 49–97 Gy, p <0 .01). Disease control (DC) was found more frequently in the ≥ 120 Gy group (79%) compared to < 120 Gy (53%). Mean tumour-absorbed dose optimal cut-off predicting DC was 131 Gy. Tumour control probability was 54% (95% CI 52–54%) for a mean tumour-absorbed dose of 120 Gy and 90% (95% CI 87–92%) for 284 Gy. Only D30 was significantly different between DC and progressive disease (p = 0.04). For the ≥ 120 Gy group, median OS and PFS were longer (median OS 33 months, [range 8–33 months] and median PFS 23 months [range 4–33 months]) than the < 120 Gy group (median OS 17 months, [range 5–33 months] and median PFS 13 months [range 1–33 months]) (p < 0.01 and p = 0.03, respectively).ConclusionsHigher 3D voxel-based tumour-absorbed dose in patients with HCC is associated with four-month DC and longer OS and PFS. DVHs in [90Y] SIRT could play a role in evaluative dosimetry.

Investigation of 2D radiophotoluminescence films in radiotherapy: Multileaf collimator Quality Assurance and small field dosimetry

In radiotherapy, accurate dose determination and precise dose delivery to the tumor are directly associated with better treatment outcomes, such as better tumor control and lower post treatment complications. Challenges in Quality Assurance (QA) are a.o. (1) the varying amounts of primary and secondary scattered photons, the varying gantry angles and the energy heterogeneity and (2) the small field sizes used in Intensity Modulated Radiotherapy (IMRT) and Volumetric Modulated Arc Therapy (VMAT).As alternative to the commonly used GafChromic™ EBT films, radiophotoluminescence (RPL) films gained attention for high resolution 2D dosimetry in radiotherapy thanks to their insensitivity to ambient light, reusability, absence of fading and the few image corrections needed.In this work we characterize the response of improved Al2O3:C,Mg RPL films in a clinical 6 MV beam. Film response was evaluated in terms of uniformity, dose response, minimum detectable dose (MDD), small field dosimetry and its performance was evaluated for multileaf collimator (MLC) QA tests.The new films showed linear dose response up to 8 Gy, followed by supralinear response for higher doses up to 24 % at 20 Gy. QA checks for the MLC, including the chair and the alternating leaves, were compared with calculations from the treatment planning system (TPS) and showed an agreement within 5% for alternating leaves and a deviation in Full Width Half Maximum (FWHM) below 1.5 % for the chair test. The maximum FWHM discrepancy between RPL film results and reference data (diode) for small field sizes (30 × 30, 20 × 20, 10 × 10, 8 × 8, 6 × 6 mm2) was 1.9 %. Notwithstanding the variations in dose response, the studied films exhibit overall good agreement with reference data and demonstrate potential for QA in IMRT and small field radiotherapy

In Vivo Comparison of Micro-Balloon Interventions (MBI) Advantage: A Retrospective Cohort Study of DEB-TACE Versus b-TACE and of SIRT Versus b-SIRT.

The purpose of this study was to evaluate in vivo the role of the micro-balloon by comparing trans-arterial chemoembolization (DEB-TACE) and selective internal radiotherapy (SIRT) procedures performed with and without balloon micro-catheter (b-DEB-TACE and DEB-TACE/SIRT and b-SIRT) for the treatment of hepatocellular carcinoma (HCC). The impact of a balloon micro-catheter on trans-arterial loco-regional treatment was analyzed using non-enhanced post-procedural cone-beam CT (Ne-CBCT) by comparing the attenuation values in the embolized area and the surrounding liver tissue before and after DEB-TACE versus b-DEB-TACE and by comparing 2D/3D dosimetry in single-photon emission computed tomography after SIRT versus b-SIRT, and by comparing the histological count of the beads following orthotopic liver transplantation in the DEB-TACE versus b-DEB-TACE subgroup. We treated 84 HCC patients using trans-arterial loco-regional therapy. Fifty-three patients (26 DEB-TACE and 27 b-DEB-TACE) were analyzed in the TACE group. Contrast, signal-to-noise ratio, and contrast-to-noise ratio were all significantly higher in b-DEB-TACE subgroup than DEB-TACE (182.33 HU [CI95% 160.3-273.5] vs. 124 HU [CI95% 80.6-163.6]; 8.3 [CI95% 5.7-10.1] vs. 4.5 [CI95% 3.7-6.0]; 6.9 [CI95% 4.3-7.8] vs. 3.1 [CI95% 2.2-5.0] p < 0.05). Thirty-one patients (24 SIRT and 7 b-SIRT) were analyzed in the SIRT group. 2D dosimetry profile evaluation showed an activity intensity peak significantly higher in the b-SIRT than in the SIRT subgroup (987.5 ± 393.8 vs. 567.7 ± 302.2, p = 0.005). Regarding 3D dose analysis, the mean dose administered to the treated lesions was significantly higher in the b-SIRT than in the SIRT group (151.6Gy ± 53.2 vs. 100.1Gy ± 43.4, p = 0.01). In histological explanted liver analysis, there was a trend for higher intra-tumoral localization of embolic microspheres for b-DEB-TACE in comparison with DEB-TACE. Due to the use of three different methods, the results of this study demonstrate in vivo, a better embolization profile of oncological intra-arterial interventions performed with balloon micro-catheter regardless of the embolic agent employed.

Effect of total body irradiation lung block parameters on lung doses using three-dimensional dosimetry.

PurposeTotal body irradiation (TBI) is an integral part of stem cell transplant. However, patients are at risk of treatment‐related toxicities, including radiation pneumonitis. While lung dose is one of the most crucial aspects of TBI dosimetry, currently available data are based on point doses. As volumetric dose distribution could be substantially altered by lung block parameters, we used 3D dosimetry in our treatment planning system to estimate volumetric lung dose and measure the impact of various lung block designs.Materials and methodsWe commissioned a TBI beam model in RayStation that matches the measured tissue‐phantom ratio under our clinical TBI setup. Cerrobend blocks were automatically generated in RayStation on thoracic Computed Tomography (CT) scans from three anonymized patients using the lung, clavicle, spine, and diaphragmatic contours. The margin for block edge was varied to 0, 1, or 2 cm from the superior, lateral, and inferior thoracic borders, with a uniform margin 2.5 cm lateral to the vertebral bodies. The lung dose was calculated and compared with a prescription dose of 1200 cGy in six fractions (three with blocks and three without).ResultThe point dose at midplane under the block and the average lung dose are at the range of 73%–76% and 80%–88% of prescription dose respectively regardless of the block margins. In contrast, the percent lung volume receiving 10 Gy increased by nearly two‐fold, from 31% to 60% over the margins from 0 to 2 cm.ConclusionsThe TPS‐derived 3D lung dose is substantially different from the nominal dose assumed with HVL lung blocks. Point doses under the block are insufficient to accurately gauge the relationship between dose and pneumonitis, and TBI dosimetry could be highly variable between patients and institutions as more descriptive parameters are not included in protocols. Much progress remains to be made to optimize and standardize technical aspects of TBI, and better dosimetry could provide more precise dosimetric predictors for pneumonitis risk.

Preface

IC3Ddose 2021 - 11th International Conference on 3D and Advanced Dosimetry The 11th edition of the International Conference on Three Dimensional (3D) and Advanced Dosimetry, IC3Ddose, was scheduled to meet in Quebec City, Quebec, Canada in 2020. The conference, initiated originally in Kentucky in 1999 and then continued roughly every two years thereafter in venues across the globe, was set for a Canadian venue.By late February 2020 the organizers had received over 60 submissions for proffered presentations to the conference and had recruited a strong cohort of 16 invited speakers. The submissions covered topics from film dosimetry, 3D dosimetry with chemical systems, EPID dosimetry, scintillation and Cherenkov based techniques, challenges of clinical dose delivery validation with dynamic techniques and small radiation fields, end-to-end QA, and more. All was ready for a very successful meeting in June 2020, and then Covid-19 stopped all life as usual in March of that year.In late 2020 it became clear that it would not be possible to just postpone the 2020 meeting to 2021, so it was decided to turn IC3Ddose021 into a virtual meeting. This enabled those who had submitted work for the conference to still present their research albeit remotely in a meeting from May 10th to 13th, 2021. Some of the original submissions were withdrawn because they had been already published in the interim or because authors could not attend the meeting. Thirty-eight papers were presented in 21 oral presentations and 17 posters over the four days of IC3Ddose21. The sessions were scheduled to accommodate 93 participants from 13 countries across multiple time-zones. The oral sessions over the meetings first three days were given over the Zoom remote platform that everyone had become proficient with in the last year. To replicate the relaxed personal interactions long fostered in IC3Ddose, interactive sessions after each day’s oral presentation with poster viewing, vendor exhibits and informal gathering rooms for open discussion were moved to an interactive virtual platform called Spatial. Chat.List of International Scientific Committee, Local Organizing Committee are available in this pdf.

Synthesis and structural characterization of AI2O3 nanoparticles: Towards 3D optically stimulated luminescence dosimetry

Advanced radiotherapy techniques, which plan and deliver a treatment in complicated 3D geometries with steep dose gradients, push 3D dosimetry with correspondingly high spatial resolution to the top of scientific and clinical agendas. This paper presents the first steps taken towards an inexpensive and reusable material for 3D dosimetry based on optically stimulated luminescence (OSL). Carbon-doped alumina (Al2O3:C) nanoparticles were synthesized using supercritical flow synthesis, in which product properties can be finely controlled. The particles were characterized using electron microscopy and powder x-ray diffraction. C-doping did not alter the crystallographic structure appreciably, and a high elemental signal from C could be measured. Nanoparticles of amorphous γ-Al2O3:C were achieved, however calcining these to produce the OSL-relevant α-phase yielded microparticles. Future work will aim at producing phase-pure α-Al2O3:C nanoparticles with a narrow size distribution below 10 nm, and controllable C-concentration and O-deficiency.

RSC: Gel dosimetry as a tool for clinical implementation of image-guided radiotherapy

The implementation of new image-guided radiotherapy (IGRT) treatment techniques requires the development of new quality assurance (QA) methods including geometric and dosimetric validation of the applied dose in 3D. Polymer gels (PG) provide a promising tool to perform such tests. However, to be used in a large variety of clinical applications, the PG must be flexibly applicable. In this work, we present a variety of phantoms used in clinical routine to perform both hardware and workflow tests in IGRT. This includes the validation of isocenter accuracy in magnetic resonance (MR)-guided RT (MRgRT) and end-to-end tests of online adaptive treatment techniques for inter- and intra-fraction motion management in IGRT. The phantoms are equipped with one or more PG containers of different materials including 3D printed containers to allow for 3D dosimetry in arbitrarily shaped structures. The proposed measurement techniques and phantoms provide a flexible application and show a clear benefit of PG for 3D dosimetry in combination with end-to-end tests in many clinical QA applications.

Investigation of target dose conformity using normoxic polymer gel dosimetry techniques: A clinical example of 12th thoracic vertebrae SBRT treatment with VMAT

This paper describes the 3D dosimetry verification of a 24Gy in 3 fractions VMAT SBRT treatment delivery to the 12th thoracic vertebrae (T12). With this fractionation regime, the critical isodose value is 17.5Gy to the outer edge of the spinal cord and must not be exceeded. This is a high-risk treatment that could result in severe injury to the patient unless administered precisely. With the use of 3D normoxic polymer gel dosimetry techniques, using MRI readout, a clinical example is presented which shows that the Mid North Coast Cancer Institute’s paraspinal program is capable of delivering complex distributions with high doses to within very tight tolerances. This also demonstrates the strength of normoxic polymer gel as a 3D dosimeter.

3D dose delivery QA using couch and gantry mounted cameras

While 2D planar radiation therapy quality assurance techniques suffice for linac verification, a full 3D dosimetry measurement is ideal for robust treatment plan verification. In this study, an optical 3D dose reconstruction technique based on radioluminescence imaging is presented, using two orthogonal views provided by gantry and couch mounted cameras. Both small field and multi-target beam apertures were tested, mimicking typical 3D radiosurgery plans. Pass rates >94% for 3D gamma with 2%/2mm were achieved in all cases. Slice images were used for gamma analysis in each of transverse, sagittal and coronal planes, through the point of maximum dose. This proposed system can enable real-time imaging of beam delivery dynamics in a cylindrical water-equivalent phantom with high spatial resolution 3D reconstruction of the dose distribution, and verification of each beamlet used.

P85 - A IOERT Geant4 Monte Carlo simulation for the computation of field output factors and 3D dosimetry

P85 - A IOERT Geant4 Monte Carlo simulation for the computation of field output factors and 3D dosimetry

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.