Dosimetry in breast computed tomography Paolo Russo Universit di - - PowerPoint PPT Presentation

Dosimetry in breast computed tomography

Paolo Russo

Università di Napoli Federico II, Dipartimento di Fisica ‘E. Pancini’ INFN Sezione di Napoli Napoli, Italy

russo@na.infn.it

2 University of Rochester (Prof. R. Ning) Koning Corp. commercial scanner (FDA/EU approved for diagnostic imaging in combination with mammography, non contrast) 49 kVp University of Erlangen & CT Imaging (Prof. W.

• A. Kalender) Photon-counting detector 60 kVp

Phase-contrast breast microCT w/ SPECT (University of Naples «Federico II») 60-120 kV University of California Davis (Prof. J. M. Boone) 80 kVp University of Massachusetts (Prof. A. Karellas) 70 kVp

Dosimetry in dedicated breast CT

3

MGD = DgNCT × K

MAMMOGRAPHY: Mean Glandular Dose MGD = DgN x ESAK

MGD = Mean Glandular Dose (mGy) DgNCT = Normalized glandular dose coefficient in CT (mGy/mGy) K = Air kerma at scanner isocenter (mGy) in a full rotation

Breast model and irradiation geometry

Homogeneous mixture adipose+glandular

4 Energy range: 4.25 – 82.25 keV

Skin thickness influence on MGD

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• Cylindrical breast
• Breast height = 1, 1.5, 2*breast radius
• Homogeneous adipose/glandular mix
• Skin thickness = 1.45 mm

Photon and electron interactions

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20 40 60 80 0.99 1.00 1.01 DgNw/o bremsstrahlung/DgNw/bremsstrahlung Incident photon energy (keV)

Breast diameter = 14 cm Glandular fraction = 20%

Simulated processes:

• Compton Scatter
• Rayleigh Scatter
• Photoelectric effect
• Bremsstrahlung

Monoenergetic and Polyenergetic DgNCT

Homogeneous mixture adipose+glandular

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10 20 30 40 50 60 0.0 0.2 0.4 0.6 0.8 1.0 DgN (mGy/mGy) Incident photon energy (keV)

Monochromatic DgN Breast diameter = 12 cm Glandular fraction = 25%

6 8 10 12 14 16 18 0.25 0.30 0.35 0.40 0.45 0.50 0.55 DgN (mGy/mGy) Breast diameter (cm)

Glandular fraction: 0% 25% 50% 75% 100%

49 kVp HVL = 1.29 mm Al Polyenergetic DgN

pDgNCT = 𝐹𝑛𝑗𝑜

𝐹𝑛𝑏𝑦 𝛸(𝐹) × ϑ(𝐹) × DgN(𝐹) × ∆𝐹

𝐹𝑛𝑗𝑜

𝐹𝑛𝑏𝑦 𝛸(𝐹) × ϑ(𝐹) × ∆𝐹

Interpolated data for monoenergetic DgNCT

Homogeneous mixture adipose+glandular

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10 20 30 40 50 60 70 80 0.0 0.2 0.4 0.6 0.8 1.0

Calculated values 8th-order polynomlial fit-curve

DgNCT Primary photon energy (keV)

Breast diameter = 120 mm Breast glandularity = 25% Breast height= Radius x 2 Radius x 1.5 Radius x 1 Fitting R

2 > 0.9999

Patient specific breast phantoms

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Simple model vs patient specific breast phantoms

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A case study

In this specific case the MGD calculated with the homogeneous cylindrical model is 21% lower than the one calculated with the patient specific phantom (49 kVp; W/Al)

DgNCT coefficients assessment

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*by courtesy of Prof. Ioannis Sechopoulos - Radboud UMC – Nijmegen (NL) 20 segmented 3D images*

• f 20 different breasts

Mean Std Min Max Glandular fraction (%) 28.0 22.6 4.9 76.0 Diameter (cm) 11.2 2.1 6.4 14.6

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Glandular dose distribution within a patient specific heterogeneous breast (left) and within a homogeneous cylindrical breast model (right). The grey lines indicate the skin boundary. The coronal slice was selected at 2.1mm from the chest wall and the sagittal slice is at the mid-

• breast. Simulated beam quality: 49 kVp, HVL=1.40mm Al.

DgNCT coefficients assessment

13 0.8 1.0 1.2 1.4

Min Max

• Hom. cont./Real breast model

DgN

90

th percentile

10

th percentile

75

th percentile

25

th percentile

Median Mean

49 kVp HVL = 1.29 mm Al Mean

= 1.04

• Std. Dev. = 0.13

Min = 0.80 Max = 1.36 Mean = 1.08

• Std. Dev. = 0.10

Min = 0.85 Max = 1.27

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Conclusions

• Homogeneous and heterogeneous breast models for MGD

evaluation in breast CT have been presented, with 1.45 mm skin thickness

• Monoenergetic and polyenergetic DgN coefficients have been

provided up to 80 keV with Geant4 MC simulations

• MGD estimates differ by less than 8%, on average, with respect to a

patient specific breast model (min -15%, max +27% difference) (49 kVp, HVL 1.4 mmAl)

russo@na.infn.it