Electromagnetic dosimetry in a sitting rhesus model at 225 MHz

Dosimetric measurements in a 9.5-kg tissue-equivalent rhesus model were conducted at 225 MHz using a nonperturbing temperature probe and a gradient-layer calorimeter. Temperature probe measurements showed deep penetration of electromagnetic energy, and calorimeter experiments showed an average SAR (0.285 W/kg per mW/cm²) that was nearly three times greater than that observed for the same model at 1.29 GHz.     

Predicting the required thickness of custom shielding materials in kilovoltage radiotherapy beams

The planning and delivery of kilovoltage (kV) radiotherapy treatments involves the use of custom shielding designed and fabricated for each patient. This study investigated methods by which the required thickness of custom shielding could be predicted for non-standard shielding materials fabricated using 3D printing techniques. Seven kV radiation beams from a WOmed T-300 X-ray therapy unit were modelled using SpekPy software, and AAPM TG-61 data were used to account for backscatter and spectral effects, for incrementally increasing thicknesses of Pb, W-PLA composite and Cu-PLA composite materials. The same beams were used to perform physical transmission measurements, and the thickness of each material required to achieve 5% beam transmission was determined. While the measured transmission factors for Pb, W-PLA and Cu-PLA shielding generally exceeded the calculated transmission factors, these differences had minimal effect on the derived thicknesses of shielding required to achieve 5% transmission, where calculations agreed with measurements within 0.5 mm for Pb at all available energies (70–300 kVp), within 1.4 mm for W-PLA at all available energies, and within 2.1 mm for Cu-PLA at superficial treatment energies (70–100 kVp). The incremental transmission factor calculation method described and validated in this study could be used, in combination with the conservative addition of 1–2 mm of additional material, to estimate shielding requirements for novel materials in therapeutic kilovoltage beams. However, if calculated shielding thicknesses equate to 10 mm or more, then additional verification measurements should be performed and the clinical suitability of the novel shielding material should be re-evaluated.     

TLD characteristic of glass,feldspathic and lithium disilicate ceramics

Thermoluminescence (TL) emission of dental ceramics could be potentially used for retrospective dosimetry purposes as this allows a quick and reliable dose assessment in case of nuclear accident or bad use of a nuclear attack. This paper reports on the chemical and luminescence characterization of glass, feldspathic and lithium disilicate glass ceramic (LS₂). Swedish and Turkish dental ceramics supplied by Vivadent Ivoclar considering: (i) the dose response in the range 10 Gy to 6.9 kGy which displays a linear dose?response at low dose values up to 36 Gy (glass and feldspathic ceramics) and shows sublinear behavior from 12 Gy to 6 kGy (lithium disilicate glass ceramics), (ii) a reproducibility of the TL signal in which the area under the glow curve increased about 25% after 10 cycles for glass and lithium disilicate ceramics and increased about 30% after seven cycles for feldspathic ceramics, (iii) stability of the luminescence emission with the elapsed time and (iv) effect of the heating rate. Glass, lithium disilicate and feldspathic ceramics display a complex UV‐blue glow emission that can be respectively fitted to five and four groups of components assuming first‐order kinetics behavior.     

Specific absorption rate in models of man and monkey at 225 and 2,000 MHz

Full-size models of a man and a rhesus monkey were exposed to radiofrequency (RF) radiation at 225 MHz. The model of man was also exposed to 2,000 MHz. Specific absorption rates (SARs) were measured in partial-body sections, such as the arms, legs, etc., using gradient-layer calorimeters. Also, front-surface thermographic images were obtained to qualitatively show the heating patterns. For all of the configurations used, the SAR in the limbs was much higher than in the torso. Agreement (whole-body SARs) with spheroidal models was better for both models at 225 MHz than at 2,000 MHz. These results indicate that in the frequency range two orders of magnitude above whole-body resonance, SAR in the limbs significantly contributes to the whole-body average SAR.     

Patient dose monitoring systems: A new way of managing patient dose and quality in the radiology department

PurposeDue to the upcoming European Directive (2013/59/EURATOM) and the increased focus on patient safety in international guidelines and regulations, Patient Dose Monitoring Systems, also called Dose Management Systems (DMS), are introduced in medical imaging departments. This article focusses on the requirements for a DMS, its benefits and the necessary implementation steps.MethodThe implementation of a DMS can be perceived as a lengthy, yet worthy, procedure: users have to select the appropriate system for their applications, prepare data collection, validate, perform configuration, and start using the results in quality improvement projects.ResultsA state of the art DMS improves the quality of service, ensures patient safety and optimizes the efficiency of the department. The gain is multifaceted: the initial goal is compliance monitoring against diagnostic reference levels. At a higher level, the user gets an overview of the performance of the devices or centers that are under his supervision. Error identification, generation of alerts and workflow analysis are additional benefits. It can also enable a more patient-centric approach with personalized dosimetry. Skin dose, size-specific dose estimates and organ doses can be calculated and evaluated per patient.ConclusionA DMS is a powerful tool and essential for improved quality and patient care in a radiology department. It can be configured to the needs of medical physicists, radiologists, technologists, even for the management of the hospital. Collaboration between all health professionals and stakeholders, input-output validation and communication of findings are key points in the process of a DMS implementation.     

Empirical validation of SAR values predicted by FDTD modeling.

Rapid increase in the use of numerical techniques to predict current density or specific absorption rate (SAR) in sophisticated three dimensional anatomical computer models of man and animals has resulted in the need to understand how numerical solutions of the complex electrodynamics equations match with empirical measurements. This aspect is particularly important because different numerical codes and computer models are used in research settings as a guide in designing clinical devices, telecommunication systems, and safety standards. To ensure compliance with safety guidelines during equipment design, manufacturing and maintenance, realistic and accurate models could be used as a bridge between empirical data and actual exposure conditions. Before these tools are transitioned into the hands of health safety officers and system designers, their accuracy and limitations must be verified under a variety of exposure conditions using available analytical and empirical dosimetry techniques. In this paper, empirical validation of SAR values predicted by finite difference time domain (FDTD) numerical code on sphere and rat is presented. The results of this study show a good agreement between empirical and theoretical methods and, thus, offer a relatively high confidence in SAR predictions obtained from digital anatomical models based on the FDTD numerical code.     

Monte Carlo-based patient internal dosimetry in fluoroscopy-guided interventional procedures: A review

PurposeThis systematic review aims to understand the dose estimation approaches and their major challenges. Specifically, we focused on state-of-the-art Monte Carlo (MC) methods in fluoroscopy-guided interventional procedures.MethodsAll relevant studies were identified through keyword searches in electronic databases from inception until September 2020. The searched publications were reviewed, categorised and analysed based on their respective methodology.ResultsHundred and one publications were identified which utilised existing MC-based applications/programs or customised MC simulations. Two outstanding challenges were identified that contribute to uncertainties in the virtual simulation reconstruction. The first challenge involves the use of anatomical models to represent individuals. Currently, phantom libraries best balance the needs of clinical practicality with those of specificity. However, mismatches of anatomical variations including body size and organ shape can create significant discrepancies in dose estimations. The second challenge is that the exact positioning of the patient relative to the beam is generally unknown. Most dose prediction models assume the patient is located centrally on the examination couch, which can lead to significant errors.ConclusionThe continuing rise of computing power suggests a near future where MC methods become practical for routine clinical dosimetry. Dynamic, deformable phantoms help to improve patient specificity, but at present are only limited to adjustment of gross body volume. Dynamic internal organ displacement or reshaping is likely the next logical frontier. Image-based alignment is probably the most promising solution to enable this, but it must be automated to be clinically practical.     

Development of patient-specific 3D models from histopathological samples for applications in radiation therapy

The biological effects of ionizing radiation depend on the tissue, tumor type, radiation quality, and patient-specific factors. Inter-patient variation in cell/nucleus size may influence patient-specific dose response. However, this variability in dose response is not well investigated due to lack of available cell/nucleus size data. The aim of this study was to develop methods to derive cell/nucleus size distributions from digital images of 2D histopathological samples and use them to build digital 3D models for use in cellular dosimetry.Nineteen of sixty hematoxylin and eosin stained lung adenocarcinoma samples investigated passed exclusion criterion to be analyzed in the study. A difference of gaussians blob detection algorithm was used to identify nucleus centers and quantify cell spacing. Hematoxylin content was measured to determine nucleus radius. Pouring simulations were conducted to generate one-hundred 3D models containing volumes of equivalent cell spacing and nuclei radius to those in histopathological samples.The nuclei radius distributions of non-tumoral and cancerous regions appearing in the same slide were significantly different (p < 0.01) in all samples analyzed. The median nuclear-cytoplasmic ratio was 0.36 for non-tumoral cells and 0.50 for cancerous cells. The average cellular and nucleus packing densities in the 3D models generated were 65.9% (SD: 1.5%) and 13.3% (SD: 0.3%) respectively.Software to determine cell spacing and nuclei radius from histopathological samples was developed. 3D digital tissue models containing volumes with equivalent cell spacing, nucleus radius, and packing density to cancerous tissues were generated.     

In phantom assessment of superficial doses under TomoTherapy irradiation

PurposeAim of this work is the assessment of build-up and superficial doses of different clinical Head&Neck plans delivered with Helical TomoTherapy (HT) (Accuray, Sunnyvale, CA). Depth dose profiles and superficial dose points were measured in order to evaluate the Treatment Planning System (TPS) capability of an accurate dose modeling in regions of disequilibrium. Geometries and scattering conditions were investigated, similar to the ones generally encountered in clinical treatments.MethodsMeasurements were performed with two dosimeters: Gafchromic® EBT3 films (Ashland Inc., Wayne, NJ) and a synthetic single crystal diamond detector (PTW-Frieburg microDiamond, MD). A modified version of the Alderson RANDO phantom was employed to house the detectors. A comparison with TPS data was carried out in terms of dose difference (DD) and distance-to-agreement (DTA).ResultsDD between calculated data and MD measurements are within 4% even in points with high spatial dose variation. For depth profiles, EBT3 data show a DDmax of 3.3% and DTAmax of 2.2 mm, in low and high gradient regions, respectively, and compare well with MD data. EBT3 superficial points always results in measured doses lower than TPS evaluated ones, with a maximum DTA value of 1.5 mm.ConclusionsDoses measured with the two devices are in good agreement and compare well with calculated data. The deviations found in the present work are within the reference tolerance level, suggesting that the HT TPS is capable of a precise dose estimation both in superficial regions and in correspondence with interfaces between air and PMMA.     

Clinical application of MOSkin dosimeters to rectal wall in vivo dosimetry in gynecological HDR brachytherapy

PurposeThree MOSkins dosimeters were assembled over a rectal probe and used to perform in vivo dosimetry during HDR brachytherapy treatments of vaginal cancer. The purpose of this study was to verify the applicability of the developed tool to evaluate discrepancies between planned and measured doses to the rectal wall.Materials and methodsMOSkin dosimeters from the Centre for Medical Radiation Physics are particularly suitable for brachytherapy procedures for their ability to be easily incorporated into treatment instrumentation. In this study, 26 treatment sessions of HDR vaginal brachytherapy were monitored using three MOSkin mounted on a rectal probe. A total of 78 measurements were collected and compared to doses determined by the treatment planning system.ResultsMean dose discrepancy was determined as 2.2 ± 6.9%, with 44.6% of the measurements within ±5%, 89.2% within ±10% and 10.8% higher than ±10%. When dose discrepancies were grouped according to the time elapsed between imaging and treatment (i.e., group 1: ≤90 min; group 2: >90 min), mean discrepancies resulted in 4.7 ± 3.6% and 7.1 ± 5.0% for groups 1 and 2, respectively. Furthermore, the position of the dosimeter on the rectal catheter was found to affect uncertainty, where highest uncertainties were observed for the dosimeter furthest inside the rectum.ConclusionsThis study has verified MOSkin applicability to in-patient dose monitoring in gynecological brachytherapy procedures, demonstrating the dosimetric rectal probe setup as an accurate and convenient IVD instrument for rectal wall dose verification. Furthermore, the study demonstrates that the delivered dose discrepancy may be affected by the duration of treatment planning.