Effect of nonhomogeneous breast 3ssue on mean glandular dose assessment in digital breast tomosynthesis M. Bap3sta, S. Di Maria, C. Figueira, L. Orvalho, A. Silva, P. Vaz, M. Zankl
o Mo3va3on Mammography is the standard technique for breast cancer screening. Limited Sensi3vity 1530% of detectable cancers are not detected by screenfilm mammography. Poor contrast of tumors compared with the surrounding 3ssue. Dense breast 3ssue can mask the tumors by lying directly above and below the tumor. Digital Breast Tomosynthesis (DBT) Overlapping breast /ssue is par/ally solved by 3D procedure. Preliminary clinical studies assert the poten/al to reduce the recall rate.
o Digital Breast Tomosynthesis DBT could replace mammography technique in clinical screening exams. Defini3on of standard dosimetric protocols for Mean Glandular Dose (MGD) es3ma3on. Mean Glandular Dose (MGD): average dose absorbed during image acquisi/on in the breast glandular /ssue; use of Monte Carlo (MC) methods to quan/fy the energy deposited in the glandular part. Aim of this work: study the effect of nonhomogeneous breast 3ssue on the breast dose assessment Homogeneous breast computa3onal phantom Vs Real segmented voxel phantom Fig.1 Example of a reconstructed image of DBT.
o Part 1 Valida3on of the DBT system Dose Measurements 24 +24 Entrance Surface Dose (ESD) measured by placing 7 thermoluminescent dosimeters (TLD100) above the phantom: Xray tube Compression padlle Digital detector Fig.3 Tissue equivalent mammography phantom (average breast with 50% adipose and 50% glandular /ssue, realis/cally shaped and enveloped inside an adipose equivalent /ssue case of 0.5 cm thickness). Acquisi3on parameters Fig. 2 Clinical system for breast tomosynthesis (Siemens MAMMOMAT Inspira/on). Voltage (kvp) 28 mas 172.5 Anode/Filter WRh
o Part 1 Valida3on of the DBT system MCNPX Simula4ons Monte Carlo (MC) code MCNPX v2.7.0. Xray point source collimated into a cone. SID of 65cm for the 0 tomosynthesis projec/on. Tally F6 energy deposited (MeV/g per par/cle). 10 7 histories sta/s/cal error less than 1%. Fig. 4 Geometrical setup simplified of the DBT system developed for the MCNPX simula/on. adipose 3ssue 50% glandular + 50% adipose 3ssue Mylar Holder TLD chip Fig.5 Geometry of the breast and of the TLD, in the transverse plane, developed for de MCNPX simula/on.
o Part 1 Valida3on of the DBT system MCNPX Simula4ons Structure Material Density (g/cm3) Breast (semicircular cylinder, 4 cm thick) Adipose /ssue (0.5 cm layer ) Glandular/Adipose (50%50%) 0.985 Adipose (100%) 0.93 Holder Mylar 1.380 TLD chip LiF:Mg,Ti 2.64 (composi/ons according to: hfp://physics.nist.gov/cgibin/star/compos.pl) Different contribu3ons from the several projec3ons for ESD. Conversion Factor (CF) between measurements and simula3ons. CF 28kVp = Dose TLD free in air / Tally F6 free in air ESD simualted ( mgy.100mas 1 )=Tally F6 CF 28kVp
Valida4on Results TLD ESD measured (mgy) ESD simulated (mgy) Difference (%) A B C D A B C D E F G 10.19 8.92 12.51 10.33 9.15 11.45 10.37 8.92 14.02 10.78 9.01 16.40 10.86 9.21 15.20 8.95 9.18 2.55 10.59 8.73 17.55 E F G A B C D E F G Uncertain3es: Placement of the TLDs; TLD measurements uncertain/es about 1015%; Intensity of the photon fluence about 1520%. Fig.6 A DBT slice of the breast phantom and TLDs. B Geometry of the experimental setup implemented with MCNPX.
o Part 2 Effect of nonhomogeneous breast 3ssue on MGD Breast Dosimetry for DBT (Dance et al.) (1) D(θ)=K.g.c.s.t(θ) Dose for a single projec/on at angle θ K incident air kerma measured in 0º but for the mas used at projec/on angle θ g, c, s conversion factor for breast glandularity and xray spectra t(θ) tomo factor at angle θ (2) t(θ)= D(θ)/D(0) MGD for both angles is calculated using MC simula/ons for the same value of mas Backscacer factor (BF) It could be important BF= ESD phantom / ESD free for in the air calcula/on of MGD in vivo with TLDs
o Part 2 Effect of nonhomogeneous breast 3ssue on MGD MCNPX Simula4ons Dose calcula/ons (tally f6) homogeneous breast computa/onal phantom and a real segmented breast voxel phantom. For BF calcula3on: For tfactors calcula3on: Energy: 28kVp Thicknesses: 4cm, 6cm, 5cm and 7cm. One TLD above the phantoms with the detector chip aligned with the center of the xray source (0⁰projec/on). Energy: 24kVp, 28kVp and 32kVp. Thicknesses: 4cm and 7cm. Voxel resolu3on: 4cm 0,471x0,267x1mm 3 5cm 0,471x0,333x1 mm 3 6cm 0,471x0,400x1 mm 3 7cm 0,471x0,467x1 mm 3 skin 2,6% glandular 3ssue adipose 3ssue 10 7 10 8 histories sta/s/cal uncertainty less than 5%. Fig.7 Breast voxel phantom with 2,6% of glandular /ssue.
o Part 2 Effect of nonhomogeneous breast 3ssue on MGD 1.00 1.00 0.98 0.98 0.96 0.96 0.94 0.92 t-factor 0.90 4cm homogeneous 0.88 4cm voxel 0.86-24-20-16-12-8 -4 0 4 8 12 16 20 24 1.00 0.98 0.96 Angle 0.94 0.92 t-factor 0.90 0.88 5cm homogeneous 5cm voxel 0.86-24 -20-16 -12-8 -4 0 4 8 12 16 20 24 1.00 0.98 0.96 Angle 0.94 0.92 0.90 0.88 t-factor 6cm homogeneous 6cm voxel 0.86-24-20-16-12-8 -4 0 4 8 12 16 20 24 Angle t-factor 0.94 0.92 0.90 7cm homogeneous 0.88 7cm voxel 0.86-24 -20-16 -12-8 -4 0 4 8 12 16 20 24 Angle
o Part 2 Effect of nonhomogeneous breast 3ssue on MGD Thickness (cm) [(Homogeneous Voxel)/ Homogeneous]x100 (%) Homogeneous phantom: 4 25.07 5 10.09 6 2.41 7 23.54 BackscaAer factor Thin breast Overs/mate. Thicker breast Unders/mate. Thickness (cm) kvp Homogeneous Voxel Difference (%) 4 24 1,095 1,078 1.553 28 1,109 1,155 4.148 32 1,124 1,030 8.363 7 24 1,096 1,021 6.843 28 1,108 1,095 1.173 32 1,121 1,686 50.401
o Conclusions DBT system was sucessefully validated taking into account the good agreement achieved between the MCNPX simula/on values and ESD measurements performed with TLDs. The t(θ) factors introduced in the dosimetric tomosynthesis formalism provided by Dance et al. (2011) should be carefully handled taking into account their variability between homogeneous and real segmented breast. Considering an homogeneous breast phantom there is a MGD underes/ma/on for thicker breasts (up to 23%) and a MGD overes/ma/on (up to 25%) for thin breasts with respect to a real segmented one. Also the BF in real segmented phantom can be different from the homogeneous one, taking to a final dose overs/ma/on of about 50% to thicker breast and high kv spectra. These results highlight the importance of an exact breast density evalua/on in order to improve the accuracy of the screening exams from the MGD assessment point of view.
References D.R. Dance et al.,es/ma/on of mean glandular dose for breast tomosynthesis: factors for use with the UK, European and IAEA breast dosimetry protocols, Phys.Med.Biol. 56 (2011) 453471 M. Zankl, U. Fill, C. Hoeschen, W. Panzer, D. Regulla; AVERAGE GLANDULAR DOSE CONVERSION COEFFICIENTS FOR SEGMENTED BREAST VOXEL MODELS; Radia/on Protec/on Dosimetry (2005), Vol. 114, Nos 13, pp. 410 414 Na/onal Ins/tute of Standards and, T., s.d. Physical Reference Data. [Online] Available at: hfp://physics.nist.gov/cgibin/star/compos.pl?matno=001 Pelowwitz, D. B., 2011. MCNPX User's Manual Version 2.7.0. Los Alamos NaFonal Laboratory. Siemens, 2009. MAMMOMAT Inspira/on Digital Mammography Plauorm for Screening, Diagnos/cs, Biopsy and Tomosynthesis. Medical SoluFons.
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