DOSIS: a Monte Carlo simulation program for dose related studies in mammography

Virtual dosimetry study with three cone-beam breast computed tomography scanners using a fast GPU-based Monte Carlo code

Objective. To compare the dosimetric performance of three cone-beam breast computed tomography (BCT) scanners, using real-time Monte Carlo-based dose estimates obtained with the virtual clinical trials (VCT)-BREAST graphical processing unit (GPU)-accelerated platform dedicated to VCT in breast imaging. Approach. A GPU-based Monte Carlo (MC) code was developed for replicatingin silicothe geometric, x-ray spectra and detector setups adopted, respectively, in two research scanners and one commercial BCT scanner, adopting 80 kV, 60 kV and 49 kV tube voltage, respectively. Our cohort of virtual breasts included 16 anthropomorphic voxelized breast phantoms from a publicly available dataset. For each virtual patient, we simulated exams on the three scanners, up to a nominal simulated mean glandular dose of 5 mGy (primary photons launched, in the order of 1011-1012per scan). Simulated 3D dose maps (recorded for skin, adipose and glandular tissues) were compared for the same phantom, on the three scanners. MC simulations were implemented on a single NVIDIA GeForce RTX 3090 graphics card.Main results.Using the spread of the dose distribution as a figure of merit, we showed that, in the investigated phantoms, the glandular dose is more uniform within less dense breasts, and it is more uniformly distributed for scans at 80 kV and 60 kV, than at 49 kV. A realistic virtual study of each breast phantom was completed in about 3.0 h with less than 1% statistical uncertainty, with 109primary photons processed in 3.6 s computing time.Significance. We reported the first dosimetric study of the VCT-BREAST platform, a fast MC simulation tool for real-time virtual dosimetry and imaging trials in BCT, investigating the dose delivery performance of three clinical BCT scanners. This tool can be adopted to investigate also the effects on the 3D dose distribution produced by changes in the geometrical and spectrum characteristics of a cone-beam BCT scanner.

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

Simple Summary

Virtual 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.

Abstract

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.

Evaluation of glandular dose in mammography in presence of breast cysts using Monte Carlo simulation

Background: Normalized glandular dose (DgN) is an important dosimetric quantity in mammography.

Aim: In this study, the effect of the presence of breast cysts and their size, number and location on DgN is evaluated.

Materials and methods: The effect of the presence of cysts in breast was examined using MCNPX code. This was performed by taking homogeneous breast phantoms containing spheroid breast cysts into account. The radius of the cysts, numbers of the cysts, and depth of the cysts, and their location were variable. Various electron energies were also considered. Finally, these results were compared with the results of a cyst-less breast phantom.

Results: The results show that the effect of the presence of cysts in the breast depends on the size, number and location of cysts. The presence of cysts at lower depths leads to a decrease in the DgN values, compared to the breast phantom without cysts. The presence of cysts in the breast phantom has an effect of -7 to +14 percent on the DgN values under the conditions considered in this modeling. This effect is independent of the X-ray tube voltage, the breast phantom thickness, and glandular ratio, and depends only on the number and size and location of the cysts. The bigger radius and number of cysts, the greater effect on DgN value.

Virtual clinical trials in medical imaging: a review

The accelerating complexity and variety of medical imaging devices and methods have outpaced the ability to evaluate and optimize their design and clinical use. This is a significant and increasing challenge for both scientific investigations and clinical applications. Evaluations would ideally be done using clinical imaging trials. These experiments, however, are often not practical due to ethical limitations, expense, time requirements, or lack of ground truth. Virtual clinical trials (VCTs) (also known as in silico imaging trials or virtual imaging trials) offer an alternative means to efficiently evaluate medical imaging technologies virtually. They do so by simulating the patients, imaging systems, and interpreters. The field of VCTs has been constantly advanced over the past decades in multiple areas. We summarize the major developments and current status of the field of VCTs in medical imaging. We review the core components of a VCT: computational phantoms, simulators of different imaging modalities, and interpretation models. We also highlight some of the applications of VCTs across various imaging modalities.

Validation of mammographic x-ray spectra generated using Particle and Heavy Ion Transport code System

A Monte Carlo (MC) code is a robust method to generate a mammographic x-ray spectrum because the geometry of a mammography system can be flexible and directly modeled in MC simulation. However, simulations from MC code need to be validated before it can be reliably used for specific applications. This study aimed to generate and validate the x-ray spectra of relevant anodes used in mammography and breast tomosynthesis using Particle and Heavy Ion Transport code System (PHITS). PHITS version 3.08 was used to generate the x-ray spectra of molybdenum (Mo), rhodium (Rh), and tungsten (W) anodes. The Mo anode spectrum derived using PHITS was compared with those obtained using other MC codes. The generated spectra of all anodes were compared with the literature. Parameters including spectral shape, K characteristic x-ray yield, heel effect, and half-value layer (HVL) were used for a comparative assessment. The differences in these assessment parameters conducted by PHITS and PHITSEGS5 simulations were studied. Regarding the comparative parameters, PHITSEGS5 simulation improved the accuracy of mammographic x-ray generation compared to PHITS simulation; K x-ray and bremsstrahlung yields of the Mo anode spectrum generated by PHITSEGS5 simulation were a better agreement with those generated by other MC code simulations. The PHITSEGS5 spectra overestimated K x-ray and low-energy bremsstrahlung photons in comparison with measured spectra. Subsequently, HVLs calculated from PHITSEGS5 spectra were 1.0% (Mo/Mo) and 7.0% (W/Al) lower than those derived from measured spectra. For Mo and Rh anodes, relative difference of HVLs calculated from PHITSEGS5 spectra and those obtained from literature and measurement were within the TRS 457 acceptance criteria (±0.02 mm Al). The observed difference exceeded the acceptance criteria for W anode. Regarding existed consistency in HVL between simulation and measurement, PHITSEGS5 simulation can be reliably used to generate x-ray spectra of Mo and Rh anodes. However, its accuracy should be improved for generating W anode spectrum.

A comparison of mean glandular dose diagnostic reference levels within the all-digital Irish National Breast Screening Programme and the Irish Symptomatic Breast Services

Data on image quality, compression and radiation dose were collected from symptomatic breast units within the Republic of Ireland. Quantitative and qualitative data were analysed using SPSS. Recommendations of mean glandular dose (MGD) diagnostic reference levels were made at various levels for film-screen and full field digital mammography units to match levels published worldwide. MGDs received by symptomatic breast patients within Ireland are higher than those received in the all-digital Irish Breast Screening service; 55-65 mm breast: 1.75 mGy (screening) vs. 2.4 mGy (symptomatic) at the 95th percentile; various reasons are proposed for the differences. MGDs achieved in the screening service may be lower because of the exacting requirements for radiographer training, characteristics of the patients and equipment quality assurance levels. More precise imaging guidelines, standards and training of symptomatic radiographers performing mammography are suggested to remediate MGDs delivered to the breasts of Irish women attending the symptomatic breast services.

Radiation doses from low energy X-ray beams measured with synthetic diamond compared with calculated values obtained from the PENELOPE Monte Carlo code

The utilization of a probe with synthetic diamond as the sensing material developed to measure radiation doses from mammography X-ray beams is described. The computer code system PENELOPE was used, with a geometry model simulating the experimental conditions, to compute the doses from the mammography X-ray beams to the diamond sensing material. The orientation of the diamond sensor to provide maximum absorption of the incident X-ray beam during exposure was also investigated using the PENELOPE code. The results from the theoretical model and experimental measurements are compared.

Contrast-to-noise ratio in magnification mammography: a Monte Carlo study

Magnification views are a common way to perform a secondary examination when suspicious abnormalities are found in a screening mammogram. The visibility of microcalcifications and breast lesions is restricted by the compromise between the image quality and the absorbed dose. In this study, image quality characteristics in magnification mammography were evaluated based on Monte Carlo techniques. A breast phantom was utilized, simulating a homogeneous mixture of adipose and glandular tissue in various percentages of glandularity, containing inhomogeneities of various sizes and compositions. The effect of the magnification degree, breast glandularity, tube voltage and anode/filter material combination on image quality characteristics was investigated in terms of a contrast-to-noise ratio (CNR). A performance index PI(nu) was introduced in order to study the overall performance of various anode/filter combinations under different exposure parameters. Results demonstrate that CNR is improved with the degree of magnification and degraded as the breast glandularity is increased. Degree of magnification 1.3 offers the best overall performance for most of the anode/filter combinations utilized. Under magnification conditions, the role of dose is demoted against the image quality, as magnification views are secondary, diagnostic examinations and not screening procedures oriented to non-symptomatic women. For decreased image quality weighting, some anode/filter combinations different from Mo/0.030 mmMo can be utilized as they offer a similar performance index. However, if the desired weighting for the image quality is high, the Mo/0.030 mmMo combination has the best overall performance.

Suitability of new anode materials in mammography: Dose and subject contrast considerations using Monte Carlo simulation

Mammography is the technique with the highest sensitivity and specificity, for the early detection of nonpalpable lesions associated with breast cancer. As screening mammography refers to asymptomatic women, the task of optimization between the image quality and the radiation dose is critical. A way toward optimization could be the introduction of new anode materials. A method for producing the x-ray spectra of different anode/filter combinations is proposed. The performance of several mammographic spectra, produced by both existing and theoretical anode materials, is evaluated, with respect to their dose and subject contrast characteristics, using a Monte Carlo simulation. The mammographic performance is evaluated utilizing a properly designed mathematical phantom with embedded inhomogeneities, irradiated with different spectra, based on combinations of conventional and new (Ru, Ag) anode materials, with several filters (Mo, Rh, Ru, Ag, Nb, Al). An earlier developed and validated Monte Carlo model, for deriving both image and dose characteristics in mammography, was utilized and overall performance results were derived in terms of subject contrast to dose ratio and squared subject contrast to dose ratio. Results demonstrate that soft spectra, mainly produced from Mo, Rh, and Ru anodes and filtered with k-edge filters, provide increased subject contrast for inhomogeneities of both small size, simulating microcalcifications and low density, simulating masses. The harder spectra (W and Ag anode) come short in the discrimination task but demonstrate improved performance when considering the dose delivered to the breast tissue. As far as the overall performance is concerned, new theoretical spectra demonstrate a noticeable good performance that is similar, and in some cases better compared to commonly used systems, stressing the possibility of introducing new materials in mammographic practice as a possible contribution to its optimization task. In the overall optimization task in terms of subject contrast to dose ratio, tube voltage was found to have a minor effect, while with respect to the filter material, a lesion specific performance was noticed, with Al filtered spectra showing improved characteristics in case of the inhomogeneities simulating microcalcifications, while softer k-edge filtered spectra are more suitable for the discrimination of inhomogeneities simulating masses.

Monte Carlo generated conversion factors for the estimation of average glandular dose in contact and magnification mammography

Magnification mammography is a special technique used in the cases where breast complaints are noted by a woman or when an abnormality is found in a screening mammogram. The carcinogenic risk in mammography is related to the dose deposited in the glandular tissue of the breast rather than the adipose, and average glandular dose (AGD) is the quantity taken into consideration during a mammographic examination. Direct measurement of the AGD is not feasible during clinical practice and thus, the incident air KERMA on the breast surface is used to estimate the glandular dose, with the help of proper conversion factors. Additional conversion factors adapted for magnification and tube voltage are calculated, using Monte Carlo simulation. The effect of magnification degree, tube voltage, various anode/filter material combinations and glandularity on AGD is also studied, considering partial breast irradiation. Results demonstrate that the estimation of AGD utilizing conversion factors depends on these parameters, while the omission of correction factors for magnification and tube voltage can lead to significant underestimation or overestimation of AGD. AGD was found to increase with filter material's k-absorption edge, anode material's k-emission edge, tube voltage and magnification. Decrease of the glandularity of the breast leads to higher AGD due to the increased penetrating ability of the photon beam in thick breasts with low glandularity.

Monte Carlo simulation of primary electron production inside an a-selenium detector for x-ray mammography: physics

Selenium is among the materials under investigation that may form effective detectors and provide a major contribution to digital mammography. Till the final image formation, there is an intervention of the x-ray photons transformation to primary electrons and their subsequent ionizing drift towards the electrodes that collect them. The characteristics of the generated primary electrons inside a-Se material such as their angular, spatial and energy distribution affect the characteristics of the final image. A Monte Carlo based model has been developed that simulates the x-ray irradiation of an a-Se detector plate, including primary photon interactions (photoelectric absorption, coherent and incoherent scattering), as well as secondary ones, such as fluorescence (Kalpha, Kbeta) and emission of Auger electrons. The angular, spatial and energy distributions for the generated primary electrons inside a-Se have been produced for various mammographic x-ray spectra and their usefulness in designing and optimizing a detector made of a-Se for digital mammography is discussed.