Collimator Settings Research Articles

Characterisation of a hand-held CZT-based gamma camera for 177Lu imaging

BackgroundCurrently, hand-held gamma cameras are being developed for 99mTc imaging, mainly for sentinel lymph node detection. These cameras offer advantages, such as mobility and ease of access, and may be useful also for other applications such as biokinetic studies in animals or for imaging of small, superficial structures in patients. In this work, the suitability of a CZT-based hand-held camera for 177Lu imaging is investigated. The energy response of CZT-based detectors combined with the multiple photon emissions of 177Lu poses new challenges compared to 99mTc imaging, and a thorough camera characterisation is thus warranted.MethodsThree collimators (LEHR, LEHS, and MEGP) and three energy windows (55 keV, 113 keV, and 208 keV) are investigated. Characterised camera properties include the system spatial resolution, energy resolution, sensitivity, image uniformity, septal penetration, and temperature dependence. Characterisations are made starting from NEMA guidelines when applicable, with adjustments made when required. The applicability of the camera is demonstrated by imaging of a superficially located tumour in a patient undergoing [177 Lu]Lu-DOTA-TATE therapy.ResultsOverall, the results are encouraging. Compared to a conventional gamma camera, the hand-held camera generally has a higher sensitivity for a given collimator. For source-collimator distances below 3 cm, the spatial resolution FWHM is within 6 mm for the LEHR and MEGP collimators. Before uniformity correction, the central field-of-view integral uniformity shows best results for the 113-keV window, with values obtained between 11 and 14%. The corresponding values after uniformity correction are within 3%. Effects of septal penetration are observed but are manageable with a proper combination of collimator and energy window setting. Septal penetration and collimator scatter not only affect the 208-keV window but also contribute with counts in lower windows due to energy-tailing effects. The patient study revealed non-uniform uptake patterns in a region that appeared uniform in a conventional gamma camera image.ConclusionsThe results show that the hand-held camera can be used for 177Lu imaging. A 113-keV energy window combined with LEHR or MEGP collimators provides the best image system characteristics.

Tunable and precise two-bunch generation at FLASHForward

Beam-driven plasma-wakefield acceleration based on external injection has the potential to significantly reduce the size of future accelerators. Stability and quality of the acceleration process substantially depends on the incoming bunch parameters. Precise control of the current profile is essential for optimising energy-transfer efficiency and preserving energy spread. At the FLASHForward facility, driver-witness bunch pairs of adjustable bunch length and separation are generated by a set of collimators in a dispersive section, which enables fs-level control of the longitudinal bunch profile. The design of the collimator apparatus and its commissioning is presented.

The helically-acquired CTDIvol as an alternative to traditional methodology.

PurposeMost clinical computed tomography (CT) protocols use helical scanning; however, the traditional method for CTDIvol measurement replaces the helical protocol with an axial scan, which is not easily accomplished on many scanners and may lead to unmatched collimation settings and bowtie filters. This study assesses whether CTDIvol can be accurately measured with a helical scan and determines the impact of pitch, collimation width, and excess scan length.MethodsCTDIvol was measured for 95 helical protocols on 31 CT scanners from all major manufacturers. CTDIvol was measured axially, then again helically, with the scan range set to the active area of the pencil chamber seen on the localizer image. CTDIvol measurements using each method were compared to each other and to the scanner‐displayed CTDIvol. To test the impact of scan length, the study was repeated on four scanners, with the scan range set to the phantom borders seen on the localizer.ResultsIt was not possible to match the collimation width between the axial and helical modes for 12 of the 95 protocols tested. For helical and axial protocols with matched collimation, the difference between the two methods averaged below 1 mGy with a correlation of R2 = 0.99. The difference between the methods was not statistically significant (P = 0.81). The traditional method produced four measurements that differed from the displayed CTDIvol by >20%; no helical measurements did. The accuracy of the helical CTDIvol was independent of protocol pitch (R2 = 0.0) or collimation (R2 = 0.0). Extending the scan range to the phantom borders increased the measured CTDIvol by 2.1%–9.7%.ConclusionThere was excellent agreement between the two measurement methods and to the displayed CTDIvol, without protocol or vendor dependence. The helical CTDIvol measurement can be accomplished more easily than the axial method on many scanners and is reasonable to use for QC purposes.

Contrast-enhanced, conebeam CT-based, fractionated radiotherapy and follow-up monitoring of orthotopic mouse glioblastoma: a proof-of-concept study

BackgroundDespite aggressive treatment regimens comprising surgery and radiochemotherapy, glioblastoma (GBM) remains a cancer entity with very poor prognosis. The development of novel, combined modality approaches necessitates adequate preclinical model systems and therapy regimens that closely reflect the clinical situation. So far, image-guided, fractionated radiotherapy of orthotopic GBM models represents a major limitation in this regard.MethodsGL261 mouse GBM cells were inoculated into the right hemispheres of C57BL/6 mice. Tumor growth was monitored by contrast-enhanced conebeam CT (CBCT) scans. When reaching an average volume of approximately 7 mm3, GBM tumors were irradiated with daily fractions of 2 Gy up to a cumulative dose of 20 Gy in different beam collimation settings. For treatment planning and tumor volume follow-up, contrast-enhanced CBCT scans were performed twice per week. Daily repositioning of animals was achieved by alignment of bony structures in native CBCT scans. When showing neurological symptoms, mice were sacrificed by cardiac perfusion. Brains, livers, and kidneys were processed into histologic sections. Potential toxic effects of contrast agent administration were assessed by measurement of liver enzyme and creatinine serum levels and by histologic examination.ResultsTumors were successfully visualized by contrast-enhanced CBCT scans with a detection limit of approximately 2 mm3, and treatment planning could be performed. For daily repositioning of the animals, alignment of bony structures in native CT scans was well feasible. Fractionated irradiation caused a significant delay in tumor growth translating into significantly prolonged survival in clear dependence of the beam collimation setting and margin size. Brain sections revealed tumors of similar appearance and volume on the day of euthanasia. Importantly, the repeated contrast agent injections were well tolerated, as liver enzyme and creatinine serum levels were only subclinically elevated, and liver and kidney sections displayed normal histomorphology.ConclusionsContrast-enhanced, CT-based, fractionated radiation of orthotopic mouse GBM represents a versatile preclinical technique for the development and evaluation of multimodal radiotherapeutic approaches in combination with novel therapeutic agents in order to accelerate translation into clinical testing.

A Method of High-Resolution Radiotherapy Delivery Fluences with a Pair of Fields with Orthogonal Collimator Settings: A Study on Ten Head-and-Neck Cancer Patients.

Context:Introduction of dual-layer multileaf collimator (MLC) radiotherapy linear accelerators into clinical practice is an important development in advanced external beam radiotherapy. A method of delivering comparable high-resolution fluences with a single-layer MLC is presented.Aims:The aims of this study are to present new algorithms and approaches to define high-resolution hypermodulated fluences, obtain orthogonal decomposition of fluences, and deliver them on a linear accelerator with single MLC from two perpendicular collimator settings.Materials and Methods:High-resolution fluences were defined using Monte Carlo (MC) calculation. A novel use of a limited-memory, bounded, Broyden–Fletcher–Goldfarb–Shanno algorithm was used to decompose such fluences to ones deliverable with a pair of fields with mutually orthogonal collimator settings. Such a technique, here named cross motion leaf calculator (XMLC), is compared against single sliding window (SSW) technique typically used in intensity-modulated radiation therapy (IMRT). An electronic portal imaging device (EPID) is used, and the results were compared with gamma analysis. Furthermore, MC was used to determine dose distributions for computed tomography images of ten head-and-neck cancer patients.Results:Gamma analysis (3%, 3 mm) against ideal fluence is considerably more favorable to XMLC (94% ± 4%) versus SSW (76% ± 5%). Furthermore, the dose–volume histogram (DVH) analysis showed that XMLC enables delivery of fluences superior to that of IMRT and these results in clinically relevant enhancements in DVH results.Conclusions:At the time of writing of this study, there were more than 12,000 medical linear accelerators in clinical use, and XMLC can prove itself useful wherever linac is equipped with MLC but cannot delivery latest techniques, such as volumetric modulated arc therapy.

A method to verify sections of arc during intrafraction portal dosimetry for prostate VMAT

This study investigates the use of a running sum of images during segment-resolved intrafraction portal dosimetry for volumetric modulated arc therapy (VMAT), so as to alert the operator to an error before it becomes irremediable. At the time of treatment planning, predicted portal images were created for each segment of the VMAT arc, and at the time of delivery, intrafraction monitoring software polled the portal imager to read new images as they became available. The predicted and measured images were compared and displayed on a segment basis. In particular, a running sum of images from ten segments (a ‘section’) was investigated, with mean absolute difference between predicted and measured images being quantified. Images for 13 prostate patients were used to identify appropriate tolerance values for this statistic. Errors in monitor units of 2%–10%, field size of 2–10 mm, field position of 2–10 mm and path length of 10–50 mm were deliberately introduced into the treatment plans and delivered to a water-equivalent phantom and the sensitivity of the method to these errors was investigated. Gross errors were also considered for one case. The patient images show considerable variability from segment to segment, but when using a section of the arc the variability is reduced, so that the maximum value of mean absolute difference between predicted and measured images is reduced to below 12%, after excluding the first 10% of segments. This tolerance level is also found to be applicable for delivery of the plans to a water-equivalent phantom. Using this as a tolerance level for the error plans, a 10% increase in monitor units is detected, 4 mm increase or shift in multileaf collimator settings can be detected, and an air gap of dimensions 40 mm × 50 mm is detected. Gross errors can also be detected instantly after the first 10% of segments. The running difference between predicted and measured images over ten segments is able to identify errors at specific regions of the arc, as well as in the overall treatment.

Measurements of Qf СйП, fmsr by Using Different Detectors in Radiation Qclin, Qmse Therapy Systems

AbstractBackground: It worth to be mentioned that the use of small size photon beam is frequently used in modern radio-therapy to treat brain tumors and functional disorders.Aim of Study: To measure output factors (OFs) and cal-culate the field factor ( ƒclin, ƒmsr ) of small fields andV clin, Qmsestudy its suitability for small field relative dosimetry.Material and Methods: Numerous detectors were used for measuring the output factors for 6 MV photon beams by a CyberKnife®. To normalize different detector responses for the same field configuration, a correction factor was calculated for each detector by simulating the radiation delivery using Monte Carlo (MC) methods. Detectors used in the study were PTW60019 MicroDiaond, PTW 60018 Silicon diode, PTW31018 MicroLion, and Exradin W1 Scintillator. Field factors were calculated using Alfonso formula. Output factors for a CyberKnife were measured in circular fields with the diameters range from 5mm to 60mm. Measurements were made in a water tank at a 1.5cm in term of depth and at 80 cm for source-to-axis distance.Results: The results of the current study show that the output measured by the detectors Micro Diamond and Exradin W1 Scintillator (PSD) were within the uncertainties of the Monte Carlo simulations for all the beam cones. The silicon diode detector was over-responding, while the MicroLion was under-responding. We found that an accurate dosimeter could be the MicroDiamond and Exradin W1 Scintillators in small field dosimetry.Conclusion: We found that at small collimator settings only the synthetic microdiamond (PTW60019) and Exradin W1 scintillation detectors can be used as relative dosimeters without applying any correction factor.

Data-driven cross-talk modeling of beam losses in LHC collimators

The Large Hadron Collider at CERN is equipped with a collimation system to intercept potentially dangerous beam halo particles before they damage its sensitive equipment. The collimator settings are determined following a beam-based alignment procedure, in which the collimator jaws are moved towards the beam until losses appear in the beam loss monitors. When the collimator reaches the beam envelope, beam losses propagate mainly in the direction of the beam and are, therefore, also observed by other nearby beam loss monitors. This phenomenon is known as cross talk. Due to this, collimators are aligned sequentially to be able to identify which losses are generated by which collimator, such that any cross talk across beam loss monitors positioned close to each other is avoided. This paper seeks to quantify the levels of cross-talk observed by beam loss monitors when multiple collimators are moving, to be able to determine the actual beam loss signals generated by their corresponding collimators. The results obtained successfully predicted the amount of cross-talk observed for each of the cases tested in this study. This was then extended to predict loss map case studies and the proton impacts at each collimator by comparing them to simulations.

Imaging cardiac sympathetic innervation with MIBG: linear conversion of the heart-to-mediastinum ratio between different collimators

BackgroundThe heart-to-mediastinum (H/M) ratio is a commonly used parameter to measure cardiac I-123 metaiodobenzylguanidine (MIBG) uptake. Since the H/M ratio is substantially influenced by the collimator type, we investigated whether an empirical linear conversion of H/M ratios between camera systems with low-energy (LE) and medium-energy (ME) collimator is possible.MethodsWe included 18 patients with parkinsonism who were referred to one of the two participating molecular imaging facilities for the evaluation of cardiac sympathetic innervation by MIBG scintigraphy. Two consecutive planar image datasets were acquired with LE and ME collimators at 4 h after MIBG administration. Linear regression analyses were performed to describe the association between the H/M ratios gained with both collimator settings, and the accuracy of a linear transfer of the H/M ratio between collimators and across centers was assessed using a leave-one-out procedure.ResultsH/M ratios acquired with LE and ME collimators showed a strong linear relationship both within each imaging facility (R2 = 0.99, p < 0.001 and R2 = 0.90, p < 0.001) and across centers (H/M-LE = 0.41 × H/M-ME + 0.63, R2 = 0.97, p < 0.001). A linear conversion of H/M ratios between collimators and across centers was estimated to be very accurate (mean absolute error 0.05 ± 0.04; mean relative absolute error 3.2 ± 2.6%).ConclusionsThe present study demonstrates that a simple linear conversion of H/M ratios acquired with different collimators is possible with high accuracy. This should greatly facilitate the exchange of normative data between settings and pooling of data from different institutions.

Importance of Voxel Size in Defect Localization Using Gamma-Ray Scattering

The voxel, defined as the volume of the intersection between incident (primary) and scattered beams, plays an important role in the localization of defects in samples having several interests. In this work, the gamma rays emitted from a 137Cs radioactive source (having the strength of 222 GBq) are scattered from various regions of a wood sample. The scattered gamma flux is detected by an NaI(Tl) scintillation detector placed at 110 deg to the primary gamma-ray beam. Defect (decay) in the wood is simulated by drilling two collinear cylindrical flaws (having diameters of 0.8 and 1.2 cm) in the wood sample and then filling it with a mixture of sawdust and glue. Three sets of collimators with diameters of 6, 7, and 8 mm for the source and detector are used to vary the voxel size (volume). It has been found that better contrast (29.43% for a 1.2-cm defect and 16.37% for an 0.8-cm defect) is achieved for the smallest voxel (16.13 cm3) in comparison to the other two voxels (25.65 and 38.36 cm3). Further, better contrast for the smallest voxel is confirmed by comparing gray images obtained using MATLAB for all three voxel sizes at different scan positions. It has been concluded that for a given experimental setup, the accuracy of defect (decay) detection demands reduced voxel size.

Automatic spike detection in beam loss signals for LHC collimator alignment

Abstract A collimation system is installed in the Large Hadron Collider to protect its super-conducting magnets and sensitive equipment from potentially dangerous beam halo particles. The collimator settings are determined following an alignment procedure, whereby collimator jaws are moved towards the beam until a suitable spike pattern, consisting of a sharp rise followed by a slow decay, is observed in nearby beam loss monitors. This indicates the collimator jaw is aligned to the beam. The current method for aligning collimators is semi-automated whereby an operator must continuously observe the loss signals to determine whether the jaw has touched the beam, or if some other perturbation in the beam caused the losses. The human element in this procedure can result in errors and is a major bottleneck in automating and speeding up the alignment. This paper proposes to automate the human task of spike detection by using machine learning. A data set was formed from previous alignment campaigns, from which fourteen manually engineered features were extracted and six machine learning models were trained, analysed in-depth and thoroughly tested. The suitability of using machine learning in LHC operation was confirmed during collimator alignments performed in 2018, which significantly benefited from the models trained through machine learning in this study.

Collimation-induced experimental background studies at the CERN Large Hadron Collider

The data produced at the particle physics experiments at the Large Hadron Collider (LHC) contain not only the signals from the collisions, but also a background component from proton losses around the accelerator. Understanding, identifying and possibly mitigating this machine-induced background is essential for an efficient data taking, especially for some new physics searches. Among the sources of background are hadronic and electromagnetic showers from proton losses on nearby collimators due to beam-halo cleaning. In this article, the first dedicated LHC measurements of this type of background are presented. Controlled losses of a low-intensity beam on collimators were induced, while monitoring the backgrounds in the ATLAS detector. The results show a clear correlation between the experimental backgrounds and the setting of the tertiary collimators (TCTs). Furthermore, the results are used to show that during normal LHC physics operation the beam halo contributes to the total beam-induced background at the level of a percent or less. A second measurement, where the collimator positions are tightened during physics operation, confirms this finding by setting a limit of about 10% to the contribution from all losses on the TCTs, i.e. the sum of beam halo and elastic beam-gas scattering around the ring. Dedicated simulations of the halo-related background are presented and good agreement with data is demonstrated. These simulations provide information about features that are not experimentally accessible, like correlations between backgrounds and the distributions of proton impacts on the collimators. The results provide vital information about the dependence between background and collimator settings, which is of central importance when optimizing the LHC optics for maximum peak luminosity.

Conceptual design of a large pixelated CZT detector with four-hole collimator matched pixel detector for SPECT imaging: a Monte Carlo simulation study

Significant progress has been achieved in the design of pixelated CZT detectors dedicated to cardiac and breast SPECT imaging. However, their detector geometry and associated collimators' design have limited their clinical use. The aim of this work is to determine the best combination between a large pixelated CZT detector and parallel-hole collimator that can provide high spatial resolution at low injected activity with low-energy radiotracers. Our proposed combination resulted in the design of a novel four-hole matched pixel detector (4-HMPD) configuration. Our novel 4-HMPD design based on large pixelated CZT detector was firstly compared to the standard one-hole matched pixel detector (1-HMPD) configuration using Monte Carlo simulation. We have also predicted the influence of pixel size, interpixel gap and source-to-collimator distance on the basis of resulting spatial resolution, sensitivity and crosstalk events fraction for three collimator hole lengths for Tc-99m (140 keV) . Thereafter, we used the same detector and collimator settings of the 1-HMPD configuration as constructed with the D-SPECT camera module (Redlen Technologies, BC, Canada) for our 4-HMPD design to compare the performance of the two configurations. Our preliminary results showed that a large pixel size, a small interpixel gap and a small collimator hole length increased significantly the sensitivity at the detriment of spatial resolution. The performance comparison between the 4-HMPD and the 1-HMPD configurations demonstrated an improved reconstructed spatial resolution (by a factor two), higher contrast with the large sphere of the modified Jaszczak phantom (from 63.1% to 39.1%), clear appearance of cold spheres (>14 mm diameter) and the cold cylinders (> 11.1 mm diameter). The crosstalk events fraction varied from 8.5% to 12.8%. Our novel detector/collimator combination allows less electronic readout complexity, less crosstalk events between pixels and twofold increase in septal thickness resulting in low septal penetration compared to the classical 1-HMPD configuration. It also showed the highest enhancement in terms of spatial resolution even in cases of low sensitivity with less injected activity, and outperformed the performance of existing conventional NaI (TI) crystal-based systems.

Impact of beam configuration on VMAT plan quality for Pinnacle3Auto-Planning for head and neck cases

BackgroundThe purpose of this study was to compare automatically generated VMAT plans to find the superior beam configurations for Pinnacle3 Auto-Planning and share “best practices”.MethodsVMAT plans for 20 patients with head and neck cancer were generated using Pinnacle3 Auto-Planning Module (Pinnacle3 Version 9.10) with different beam setup parameters. VMAT plans for single (V1) or double arc (V2) and partial or full gantry rotation were optimized. Beam configurations with different collimator positions were defined. Target coverage and sparing of organs at risk were evaluated based on scoring of an evaluation parameter set. Furthermore, dosimetric evaluation was performed based on the composite objective value (COV) and a new cross comparison method was applied using the COVs.ResultsThe evaluation showed a superior plan quality for double arcs compared to one single arc or two single arcs for all cases. Plan quality was superior if a full gantry rotation was allowed during optimization for unilateral target volumes. A double arc technique with collimator setting of 15° was superior to a double arc with collimator 60° and a two single arcs with collimator setting of 15° and 345°.ConclusionThe evaluation showed that double and full arcs are superior to single and partial arcs in terms of organs at risk sparing even for unilateral target volumes. The collimator position was found as an additional setup parameter, which can further improve the target coverage and sparing of organs at risk.

A Collimator Setting Optimization Algorithm for Dual-Arc Volumetric Modulated Arc Therapy in Pancreas Stereotactic Body Radiation Therapy.

Purpose:To optimize collimator setting to improve dosimetric quality of pancreas volumetric modulated arc therapy plan for stereotactic body radiation therapy.Materials and Methods:Fifty-five volumetric modulated arc therapy cases in stereotactic body radiation therapy of pancreas were retrospectively included in this study with internal review board approval. Different from the routine practice of initializing collimator settings with a template, the proposed algorithm simultaneously optimizes the collimator angles and jaw positions that are customized to the patient geometry. Specifically, this algorithm includes 2 key steps: (1) an iterative optimization algorithm via simulated annealing that generates a set of potential collimator settings from 39 cases with pancreas stereotactic body radiation therapy, and (2) a multi-leaf collimator modulation scoring system that makes the final decision of the optimal collimator settings (collimator angles and jaw positions) based on organs at risk sparing criteria. For validation, the other 16 cases with pancreas stereotactic body radiation therapy were analyzed. Two plans were generated for each validation case, with one plan optimized using the proposed algorithm (Planopt) and the other plan with the template setting (Planconv). Each plan was optimized with 2 full arcs and the same set of constraints for the same case. Dosimetric results were analyzed and compared, including target dose coverage, conformity, organs at risk maximum dose, and modulation complexity score. All results were tested by Wilcoxon signed rank tests, and the statistical significance level was set to .05.Results:Both plan groups had comparable target dose coverage and mean doses of all organs at risk. However, organs at risk (stomach, duodenum, large/small bowel) maximum dose sparing (D0.1 cc and D0.03 cc) was improved in Planopt compared to Planconv. Planopt also showed lower modulation complexity score, which suggests better capability of handling complex shape and sparing organs at risk .Conclusions:The proposed collimator settings optimization algorithm successfully improved dosimetric performance for dual-arc pancreas volumetric modulated arc therapy plans in stereotactic body radiation therapy of pancreas. This algorithm has the capability of immediate clinical application.

Dual SPECT imaging of 111In and 67Ga to simultaneously determine in vivo the pharmacokinetics of different radiopharmaceuticals: a quantitative tool in pre-clinical research

Dual-isotope (DI) studies offer a number of advantages in pre-clinical imaging. These include: reducing study times when compared with sequential scans, reducing the number of animals required for any given study, and most importantly, producing images perfectly registered in space and time that provide simultaneous information about two distinct body functions. The ability of single photon emission computed tomography (SPECT) to measure and differentiate energies of the emitted photons makes it well suited for DI imaging. However, since scattered photons originating from one radioisotope may be detected in the energy window of the other and thus degrade image quality and quantitative accuracy, scatter and crosstalk corrections must be applied.The decay characteristics of 111In and 67Ga, which are suitable for quantitative DI imaging for up to 2 weeks post-injection, led us to investigate the performance of simultaneous 111In/67Ga SPECT imaging using a small-animal pre-clinical scanner. A series of phantom experiments were performed to investigate image quality and accuracy of activity quantification in 111In/67Ga images acquired with three different collimators and reconstructed from different photopeak combinations. The triple energy window (TEW) method was used to correct for scatter and crosstalk. Based on these phantom studies, the optimal selection of collimator and energy window settings was determined. When using these optimal settings, submillimeter-size structures were distinguishable in the reconstructed images and quantification errors below 20% were achieved for both isotopes. The optimal parameters were subsequently applied to an in vivo animal study. The determination of the distinct pharmacokinetic profiles of two polymer radiopharmaceuticals injected simultaneously, but by different administration routes (intravenously and intraperitoneally) into a single animal demonstrated the feasibility of simultaneous 111In/67Ga SPECT.

11 Quality analysis of cranio-spinal RT treatments (CSI) in helical Tomotherapy (HT) and protontherapy at the Institut Curie

Introduction Since 2010, about fifty patients have been treated with CSI in HT (Accuray) with a photon or proton boost depending on the indication. This population is the subject of an analysis of the quality of treatment plans. Since the installation of dynamic jaws (TomoEdge, 2014) and TPS updates (Volo), we have modified our clinical protocols to optimize treatments. The objectives and constraints of brain PTV and nevrax PTV volumes, ±boost PTV, and OARS are based on our clinical experience (SIOP group: brain tumors). Quality indices (IQ) such as the homogeneity index HI (optimal value zero) and the conformity index: Target Coverage TCO (optimal value 100%) are determined. Methods All patient data were collected and sorted according to their medical prescription in MS Excel. The QI are obtained by exporting the TPS treatment plans to the Artiview software (Aquilab, France) in DICOM format. The representation of dosimetric data on Artiview has been fully validated, notably on dosimetric indicators such as average dose, maximum dose, minimum dose, QI, D95%, computing grid, etc…The feasibility of this project was then performed on a sample of ten patients treated in 36 Gy (1.8 Gy/fraction) without boost. Results We observe an evolution in the clinical protocols implemented, particularly in the choice of collimator settings (2.5 cm to 5 cm), pitch and modulation factor, with a significant reduction in treatment time (35 min to 10 min), while maintaining PTV coverage and the constraints imposed on OAR. The relative differences between TPS and Artiview dosimetric indicators are less than 1.3%. For PTV: Brain PTV: Dmean = 36.3 Gy, D95% = 34.3 Gy, HI = 0.125 (0.056–0.502) and TCO = 97.3% (87.6%–99.8%). PTV nevrax: Dmean = 36.4 Gy, D95% = 34.8 Gy, HI = 0.095 (0.061–0.211) and TCO = 97.1% (82.1%–99.9%). For OAR: Eye: Dmean = 13.9 Gy. Crystallin: Dmax = 7.8 Gy. Oral cavity: Dmean = 9.6 Gy. Thyroid: Dmean = 12.9 Gy. Lungs: Dmean = 6.6 Gy, V20Gy = 6.4%. Heart: Dmean = 11.1 Gy. Liver: Dmean = 7.0 Gy. Kidneys: Dmean = 4.7 Gy, V12Gy = 5.8%. Ovaries: Dmax = 1 Gy. Testicles: Dmax = 0.2 Gy. Conclusions We validated the feasibility of the whole process. The complete study will cover the entire population according to therapeutic indications. The modification of the QI is under evaluation for a better representation of the data. Finally, a comparison with conformational and/or proton treatments can also be made.

Development of a Practical Calibration Procedure for a Clinical SPECT/MRI System Using a Single INSERT Prototype Detector and Multimini Slit-Slat Collimator

In the context of the INSERT project, we have been developing a clinical single photon emission computed tomography (SPECT) insert for a magnetic resonance imaging (MRI) system, in order to perform simultaneous SPECT/MRI of the human brain. This system will consist of 20 CsI:Tl scintillation detectors, 5-cm wide and 10-cm long, with a 72-channel silicon photomultiplier (SiPM) readout per detector, and a multimini slit-slat (MSS) collimator set up in a stationary partial ring. Additionally the system has a custom-built transmit/receive MR coil to ensure compatibility with the SPECT system. Due to the novel design of the system/collimator, existing geometric calibration methods are not suitable. Therefore we propose a novel and practical calibration procedure that consists of a set of specific independent measurements to determine the geometric parameters of the collimator. This procedure was developed utilizing a prototype system that consists of a reduced-size single detector with a 36-channel SiPM-based readout and a single MSS collimator module. Validation was performed by reconstructing different imaging phantoms, using a rotating stage to simulate a tomographic acquisition. Regarding uniformity, the coefficient of variation (COV) for the cylinder phantom reconstructed with correct calibration parameters is 6.7%, whereas the COV using incorrect parameters is 9.4%. The quality of the phantom reconstructions provide evidence of the applicability of the proposed method to the calibration of the prototype system. This procedure can be easily adapted for the final INSERT system.

Optimal collimator rotation based on the outline of multiple brain targets in VMAT

BackgroundThe aim of this study was to investigate the dosimetric quality in volumetric modulated arc therapy (VMAT) plans with optimal collimator angles that can represent the outline of multiple brain targets.MethodsTwenty patients with multiple target volumes in the brain cases were selected retrospectively. To better represent the outline of the multiple brain targets, four conformal arc plans were generated for each patient using one full arc with four collimator settings. The optimal collimator angles calculated from the integrated multi-leaf collimator (MLC) aperture that had the smallest aperture size for certain collimator settings of the conformal arc plan were selected. VMAT plans with the optimal collimator angles with angular sections of 40° and 60° (Colli-VMAT (40°), Colli-VMAT (60°)) were generated, followed by evaluation of field sizes, dose-volumetric parameters and total monitor units (MUs).ResultsPatient-averaged values of field sizes for Colli-VMAT (40°) (111.5 cm2) were lowest and 1.3 times smaller than those for Std-VMAT (143.6 cm2). Colli-VMAT plans improved sparing of most normal organs but for brain stem and left parotid gland. For the total MUs, the averaged values obtained with the Colli-VMAT (40°) (390 ± 148 MU) were smaller than those obtained with the Std-VMAT (472 ± 235 MU).ConclusionsThe Colli-VMAT plans with smaller angular sections could be suitable in the clinic for multiple brain targets as well as for irregularly shaped targets. Determination of the optimal collimator rotation generally showed good normal tissue sparing and MU reduction for multiple brain targets.

Electronic collimation of radiographic images: does it comprise an overexposure risk?

To investigate whether electronic collimation software, which is available in all digital X-ray systems, may comprise an overexposure risk. In the context of surveys on Diagnostic Reference Levels carried out in two radiographic facilities, along with data on exposure factors, the radiographic field sizes were also recorded. In one facility (Unit A), a wireless flat panel detector is used with a conventional X-ray unit, while in the other, a fully digital system is installed (Unit B). The electronically collimated image sizes were compared with the original radiation field sizes. The differences between these two systems concerning the field sizes and the mode of electronic collimation utilization were investigated. In Unit A, manual electronic collimation was extensively used and cases where the radiation field size was up to three times larger than that electronically collimated, were identified. On the contrary, in Unit B radiation fields were smaller and electronic collimation was automatic. When electronic collimation is used in manual mode instead of proper pre-exposure collimation, then it does comprise an overexposure risk. The risk is larger in radiographic units where the field size is not automatically selected according to the examination protocol and no interlocks against oversized collimation settings exist. Advances in knowledge: When radiologists review masked images to make the diagnosis, possible suboptimal X-ray field collimation practices may go unnoticed for long. Therefore, radiologists and medical physicists should periodically survey the original images to determine the actual radiation field sizes used for each radiographic examination type.