Abstract
PurposeTo validate Monte Carlo (MC)‐based breast dosimetry estimations using both a homogeneous and a 3D anthropomorphic breast phantom under polyenergetic irradiation for internal breast dosimetry purposes.MethodsExperimental measurements were performed with a clinical digital mammography system (Mammomat Inspiration, Siemens Healthcare), using the x‐ray spectrum selected by the automatic exposure control and a tube current‐exposure time product of 360 mAs. A homogeneous 50% glandular breast phantom and a 3D anthropomorphic breast phantom were used to investigate the dose at different depths (range 0–4 cm with 1 cm steps) for the homogeneous case and at a depth of 2.25 cm for the anthropomorphic case. Local dose deposition was measured using thermoluminescent dosimeters (TLD), metal oxide semiconductor field‐effect transistor dosimeters (MOSFET), and GafChromic™ films. A Geant4‐based MC simulation was modified to match the clinical experimental setup. Thirty sensitive volumes (3.2 × 3.2 × 0.38 mm3) on the axial‐phantom plane were included at each depth in the simulation to characterize the internal dose variation and compare it to the experimental TLD and MOSFET measurements. The experimental 2D dose maps obtained with the GafChromic™ films were compared to the simulated 2D dose distributions.ResultsDue to the energy dependence of the dosimeters and due to x‐ray beam hardening, dosimeters based on these three technologies have to be calibrated at each depth of the phantom. As expected, the dose was found to decrease with increasing phantom depth, with the reduction being ~93% after 4 cm for the homogeneous breast phantom. The 2D dose map showed nonuniformities in the dose distribution in the axial plane of the phantom. The mean combined standard uncertainty increased with phantom depth by up to 5.3% for TLD, 6.3% for MOSFET, and 9.6% for GafChromic™ film. In the case of a heterogeneous phantom, the dosimeters are able to detect local dose gradient variations. In particular, GafChromic™ film showed local dose variations of about 46% at the boundaries of two materials.ConclusionsResults showed a good agreement between experimental measurements (with TLD and MOSFET) and MC data for both homogeneous and anthropomorphic breast phantoms. Larger discrepancies are found when comparing the GafChromic™ dose values to the MC results due to the inherent less precise nature of the former.MC validations in a heterogeneous background at the level of local dose deposition and in absolute terms play a fundamental role in the development of an accurate method to estimate radiation dose. The potential introduction of a breast dosimetry model involving a nonhomogeneous glandular/adipose tissue composition makes the validation of internal dose distributions estimates crucial.
Highlights
Digital mammography is used as the primary diagto the exposure to x rays, there is a risk of carcinogenesis in all mammography examinations
Concerning the breast shape, current models only represent the breast using a semi-elliptical approximation of the cranio-caudal (CC) view,[8,9] while only a subjective model was proposed for the medio-lateral oblique (MLO) view.[10]
In our previous work,[18] we proposed experimental methodologies to validate Monte Carlo (MC) simulations using three different dosimeters [GafChromicTM films, thermoluminescent dosimeters (TLDs), and metal oxide semiconductor fieldeffect transistor (MOSFET) dosimeters] and we validated a MC code for internal breast mammography dosimetry using a homogeneous phantom irradiated by a monoenergetic x-ray beam
Summary
Digital mammography is used as the primary diagto the exposure to x rays, there is a risk of carcinogenesis in all mammography examinations This risk, albeit small,[2] has nostic technology for early detection of breast cancer. Models with varying glandular percentage between pure adipose (i.e., 0% glandular tissue) and pure glandular (i.e., 100% glandular tissue) can be defined This homogeneous tissue approximation introduces an overestimation in the dose evaluations as shown by the works of Dance et al.,[14] Sechopoulos et al.,[15] and Hernandez et al.[16]. These studies pointed out the importance in investigating the effect of assuming a simple homogeneous distribution against a heterogeneous distribution of the glandular tissue in the breast. The acquisition of 3D glandular tissue distribution information from breast tomosynthesis, albeit limited, could allow for patient-specific dose estimates
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