Experimental and Monte Carlo-simulated spectra of standard mammography-quality beams

Simulations of X-ray spectra, half value layer, and mean energy from mammography using EGSnrc Monte Carlo and SpekPy

INTRODUCTION

This study presents the simulation results of X-ray spectra, half value layers (HVLs), and mean energies (E_mean) of two mammography units using EGSnrc Monte Carlo (MC) and SpekPy computer codes.

METHODS

The spectra caused by different combinations of targets/filters at various tube voltages (kVps) of two mammography units were simulated using two different computer codes. The EGSnrc MC simulated data of spectra and E_mean were compared with those obtained from SpekPy. The simulated values of two units' HVLs obtained from two computer codes were compared with those from physical measurements from mammography machines used in clinical practice.

RESULTS

The maximum discrepancies in E_mean simulated from two codes were less than 4.1% and 1.5% for the target/filter combination of W/Rh and Mo/Mo, respectively. The HVLs of the SpekPy were well matched to the physical measurements. The percentage differences were within 3.3% and 6.8% for two units, respectively. The EGSnrc MC simulated values of HVLs show the percentage differences within 8.9% and 7.0% with those from physical measurements for two units, respectively. All methods of HVLs determination comply with the requirements of IAEA Human Health Series No.17.

CONCLUSIONS

The HVLs, E_mean, spectra varied depending on the target/filter combinations and composition of the mammography tubes. The simulation results verify that the HVLs evaluation using the EGSnrc MC and SpekPy can be validated for mammography standard beam qualities and provide prediction almost immediately compared with physical experiments.

IMPLICATIONS FOR PRACTICE

EGSnrc MC and SpekPy have been considered powerful toolkits to simulate typical X-ray tubes used in mammography due to the good agreement between the calculation of E_mean, physical measurements and simulated HVLs.

Comparison of spectra and mean glandular dose (MGD) with tube voltage used in digital mammography for simulated, metrological and clinical cases

Mammography has a crucial role in breast cancer detection. The National Cancer Institute (INCA) estimates that 29.7% of the cancer cases in Brazil are related specifically to the breast. It is necessary to evaluate the mean glandular dose with a new solid-state detector in a digital radiography system, utilizing PMMA phantoms and spacers for different thicknesses. The Selenia Dimensions (Hologic, Bedford) direct radiography (DR) system can perform full-field digital mammographies through digital detectors. This system uses new technologies, such as the digital breast tomosynthesis system (DBT), and employs a sequence of projections acquired over the breast, resulting in images with low contrast. The estimation of breast dose is an important part of mammographic quality control for x-ray mammography. Nevertheless, there are currently no standard protocols for the dosimetry of breast imaging in 3D. Additionally, a x-ray spectra function is crucial to measure a considerable output in x-ray spectrometry. The purpose of this work was to assess the mean glandular dose (MGD) and the spectra in slabs of polymethyl methacrylate (PMMA) and breast equivalent thickness through digital mammography using four experiments: a Hologic Selenia Dimensions mammograph with a solid-state detector; a spectrometer (only for the spectra, in this case); a clinical COMET x-ray tube with a solid-state detector; and the MCNPX code. References recommend that the real environments that work well with digital mammography are in the following tube voltages: 25 kVp; 26 kVp; 28 kVp; 31 kVp and 33 kVp. Taking into account several thicknesses of PMMA, the results of both the MGD in metrological, clinical and simulated cases were in accordance with the references, from 30 mm of PMMA. All the spectra for all cases have indicated good agreement with the references.

COMPARISON OF SPECTRA AND MEAN GLANDULAR DOSE WITH TUBE VOLTAGES USED IN DIGITAL BREAST TOMOSYNTHESIS FROM SIMULATED, METROLOGICAL AND CLINICAL CASES

Digital breast tomosynthesis (DBT) is a screening and diagnostic modality that acquires images of a breast at multiple angles during a short scan. The Selenia Dimensions (Hologic, Bedford, Mass) DBT system can perform both full-field digital mammography and DBT. The system acquires 25 projections over a 15° angular range (from -7.5° to +7.5°). X-ray spectroscopy is generally linked with a high-resolution semiconductor detector through a correction to its energy response function. The energy spectrum describes the radiation field, in which several quality parameters can be extracted, such as the effective energy, half-value layer and exposure. X-ray spectroscopy is usually performed with solid-state semiconductor detectors. Radiation dose is a concern in mammography, as the current protocols recommend that medical physicians evaluate mean glandular dose (MGD) as a part of service quality control. Studies are needed for radiation dose optimization from tomosynthesis patients. The COMET metrological X-ray tube, considered as with a constant potential and cooled, has proved to be a crucial tool in order to obtain the high energy resolution for low-energy radiographs in mammography. The Monte Carlo method, through Monte Carlo N-Particle eXtended (MCNPX), was proven to be an essential tool for image formation and posterior analysis of the deposited dose from breast simulators and radiographic contrast evaluation, for several anode/filter combinations. The purpose of this work was to assess the MGD and spectra in slabs of polymethyl methacrylate (PMMA) and breast equivalent thicknesses using four experiments with a Hologic Selenia Dimensions mammography X-ray tube with multimeter, a spectrometer (only for spectra, in this case), a metrological X-ray tube with a multimeter, and the MCNPX code. References indicate that the real conditions for a mammography X-ray tube that conducts tomosynthesis include tube voltages of 26, 29, 30 and 33 kVp. Taking into account several thicknesses of PMMA, both the MGD and spectral results were in accordance with the references. Most of the spectra were in accordance with the references, showing that the resources used in the experiments can evaluate the energy level received by a patient. The MGD values were lower than those in the references from 30 to 50 mm PMMA, and the data can be used for improvements in the detectors used in the Laboratory of Metrology in the State of Rio de Janeiro University, Brazil. Additionally, in the future, optimization of image quality can be performed for both semiconductors and mammography X-ray equipment.

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.