Normalized glandular dose coefficients for digital breast tomosynthesis systems with a homogeneous breast model

Abstract

This work aims at calculating and releasing tabulated values of dose conversion coefficients, DgNDBT, for mean glandular dose (MGD) estimates in digital breast tomosynthesis (DBT). The DgNDBT coefficients are proposed as unique conversion coefficients for MGD estimates, in place of dose conversion coefficients in mammography (DgNDM or c, g, s triad as proposed in worldwide quality assurance protocols) used together with the T correction factor. DgNDBT is the MGD per unit incident air kerma measured at the breast surface for a 0° projection and the entire tube load used for the scan. The dataset of polyenergetic DgNDBT coefficients was derived via a Monte Carlo software based on the Geant4 toolkit. Dose coefficients were calculated for a grid of values of breast characteristics (breast thickness in the range 20–90 mm and glandular fraction by mass of 1%, 25%, 50%, 75%, 100%) and the simulated geometries, scan protocols, irradiation geometries and typical spectral qualities replicated those of six commercial DBT systems (GE SenoClaire, Hologic Selenia Dimensions, GE Senographe Pristina, Fujifilm Amulet Innovality, Siemens Mammomat Inspiration and IMS Giotto Class). For given breast characteristics, target/filter combination, tube voltage and half value layer (HVL), two spectra with two HVL values have been simulated in order to permit MGD estimates from experimental HVL values via mathematical interpolation from tabulated values. The adopted breast model assumes homogenous composition of glandular and adipose tissues; it includes a 1.45 mm thick skin envelope in place of the 4–5 mm envelope commonly adopted in dosimetry protocols. The simulation code was validated versus AAPM Task group 195 Monte Carlo reference data sets (absolute differences not higher than 1.1%) and by comparison to relative dosimetry measurements with radiochromic film in a PMMA test object (differences within the maximum experimental uncertainty of 11%). The calculated coefficients show maximum relative deviations of −17.6% and +6.1% from those provided by the DBT dose coefficients adopted in the EUREF protocol and of 1.5%, on average, from data in the AAPM TG223 report. A spreadsheet is provided for interpolating the tabulated DgNDBT coefficients for arbitrary values of HVL, compressed breast thickness and glandular fraction, in the corresponding investigated ranges, for each DBT unit modeled in this work.

Comparisons of glandular breast dose between digital mammography, tomosynthesis and breast CT based on anthropomorphic patient-derived breast phantoms

“…The Monte Carlo code was based on the Geant4 simulation toolkit and used the Option4 physics list. It was developed in-house from previous code versions [5,31] and was validated both against literature data and measurements in previous works. The previous code version used in Ref.…”

“…The previous code version used in Ref. [5] computed the MGD for a homogeneous breast model and relied on the G-factor [2] for the calculation of the sole dose to the glandular…”

“…( 3)) evaluated as the air kerma for a source at 0 deg position and for the total tube load of the exam. Similarly, Sarno et al [5] proposed comprehensive conversion coefficients (DgN DBT ) for MGD DBT estimates:…”

“…These coefficients (DgN, DgN DBT , g, c, s and T) are evaluated via Monte Carlo (MC) simulations, and mainly depend on the x-ray beam quality and on the breast characteristics, as well as on the scanning protocols when tomographic breast images are acquired in DBT and BCT. In these MC simulations, the breast is commonly modelled as a homogeneous mixture of adipose and glandular tissue, enveloped in a layer mimicking the skin [1][2][3][4][5][9][10][11]. Such a homogeneous breast model is a simplification of the real condition of a 3D heterogeneous distribution of the glandular mass in the patient breast.…”

“…This work aims at comparing normalized glandular dose coefficients in DBT provided in Ref. [5] as DgN DBT coefficients, and dose coefficients (g, c, s, T) provided by Dance et al [4] -both evaluated by means of conventional homogeneous breast model -to DgN DBT values evaluated by using heterogeneous and anthropomorphic computational breast models [28], reflecting real breasts anatomies. This study completes previous studies in DM [10,21,23,26] and in BCT [8] which showed that homogeneous breast models lead to a bias in MGD estimates, when compared to MGD estimated with patient-derived computational phantoms.…”