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 imagingin 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 kVUniversity 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 geometryHomogeneous mixture adipose+glandular

4

Energy range: 4.25 – 82.25 keV

Skin thickness influence on MGD

5

- Cylindrical breast

- Breast height = 1, 1.5, 2*breast radius

- Homogeneous adipose/glandular mix

- Skin thickness = 1.45 mm

Photon and electron interactions

6

20 40 60 80

0.99

1.00

1.01

Dg

Nw

/o b

rem

sstr

ah

lun

g/D

gN

w/b

rem

sstr

ah

lun

g

Incident photon energy (keV)

Breast diameter = 14 cm

Glandular fraction = 20%Simulated processes:

- Compton Scatter

- Rayleigh Scatter

- Photoelectric effect

- Bremsstrahlung

Monoenergetic and Polyenergetic DgNCTHomogeneous mixture adipose+glandular

7

10 20 30 40 50 600.0

0.2

0.4

0.6

0.8

1.0

Dg

N (

mG

y/m

Gy)

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

(m

Gy/m

Gy)

Breast diameter (cm)

Glandular fraction:

0%

25%

50%

75%

100%

49 kVp

HVL = 1.29 mm Al

Polyenergetic DgN

pDgNCT = 𝐸𝑚𝑖𝑛𝐸𝑚𝑎𝑥𝛷(𝐸) × ϑ(𝐸) × DgN(𝐸) × ∆𝐸

𝐸𝑚𝑖𝑛

𝐸𝑚𝑎𝑥𝛷(𝐸) × ϑ(𝐸) × ∆𝐸

Interpolated data for monoenergetic DgNCTHomogeneous mixture adipose+glandular

8

10 20 30 40 50 60 70 800.0

0.2

0.4

0.6

0.8

1.0

Calculated values

8th-order polynomlial fit-curve

Dg

NC

T

Primary photon energy (keV)

Breast diameter = 120 mm

Breast glandularity = 25%

Breast height=

Radius x 2

Radius x 1.5

Radius x 1

Fitting R2 > 0.9999

Patient specific breast phantoms

9

Simple model vs patient specific breast phantoms

10

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

11

*by courtesy of Prof. Ioannis Sechopoulos - Radboud UMC – Nijmegen (NL)

20 segmented 3D images*of 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

12

Glandular dose distribution within a patient specific heterogeneous breast (left) and within ahomogeneous 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./R

eal bre

ast m

odel

DgN

90th percentile

10th percentile

75th percentile

25th percentile

MedianMean

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

14

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