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Skin Model and its impact on Digital Mammography
1
Rodrigo T. Massera; Alessandra Tomal
Institute of Physics "Gleb Wataghin”
University of Campinas
Campinas, Brazil
Outline
2
Motivation
• Mammography
• Dosimetry – Mean Glandular Dose
Methodology
• Implemented Models
• How?
Results
• MGD vs Skin Model
• Differences
Conclusions• Summary
Introduction
3
Population-based screening programs
Use Ionizing Radiation
Quality Control and Optimization
Why is dosimetry important in
mammography?
Mean Glandular Dose (MGD)
Adipose
Tissue
Glandular
Tissue
Skin
Real Breast
Incident Photons
Adipose
Tissue
Glandular
Tissue
Skin
Real Breast
Energy Deposited: Glandular Tissue Directly measured?
Monte Carlo simulation
Mean Glandular Dose (MGD)
6
Parameters to consider...
• X-ray spectrum;
• Geometric Model;
• Breast Thickness;
• Breast Composition (𝑓𝑔);
• Skin Model?• 5 mm Adipose Tissue ( Dance 1990)
• 4 mm Skin Tissue (Wu 1991/Boone 1999)
Using breast-CT: ≈1.44 mm (Vedantham et al 2012)
≈1.45 mm (Huang et al 2008);
+ adipose layer
Previous Estimations:
Current Measures:
64% thinnerCredits: Boone & Hernandez 2016, AAPM. “Changing
Perceptions and Updated Methods for Mammography
Dosimetry”
Mean Glandular Dose (MGD)
Objectives
7
Study the impact of skin models on Mean
Glandular Dose in Digital Mammography
• Geometry
• MGD calculus
Adapt MC Code
• MGD X Skin ModelsAnalysis
Outline
8
Motivation
• Mammography
• Dosimetry – Mean Glandular Dose
Methodology
• Implemented Models
• How?
Results
• MGD vs Skin Model
• Differences
Conclusions• Summary
9
Monte Carlo code:
• PENELOPE (2014) + penEasy (2015)
Beam Parameters:
• Monoenergetic (8 – 60 keV)
• Polyenergetic (22 – 35 kV):
• Mo (Mo-Rh)
• Rh (Rh)
• W (Rh-Al-Ag)
*X-ray spectra from Hernandez et al (2014)
Methodology
Methodology
10*Compositions from Hammerstein et al 1979
Breast Model*
• t = 2 cm – 8 cm
• Glandularity (𝑓𝑔) = 1%-100%
11
Breast Model*
• t = 2 cm – 8 cm
• Glandularity (𝑓𝑔) = 1%-100%
Skin shielding Models
I. 5 mm adipose;
II. 4 mm skin;
III. 1,45 mm skin;
IV. 1,45 mm skin + 2 mm adipose;
V. 1,45 mm skin + 3,55 mm adipose;
*Compositions from Hammerstein et al 1979
Methodology
12
Adipose
Tissue
Homogeneous Glandular-
Adipose Tissue Mixture
Skin
Monte Carlo Simulations
How do we separate the deposited
energy between tissues?
penEasy: 𝐸𝑎𝑣𝑔
Mean Glandular Dose (MGD)
13
MGD Weighing method (Dance 1990)
Simulation starts:
𝐸𝑔𝑙𝑎𝑛𝑑=0
InteractionType
IncoherentPhotoelectric
(I)
(II)
(III)
Simulation Ends:
Return𝐸𝑔𝑙𝑎𝑛𝑑
MGD = 𝐸𝑔𝑙𝑎𝑛𝑑
𝑀𝑎𝑠𝑠 × 𝑓𝑔
nMGD = 𝑀𝐺𝐷
𝐾𝑎𝑖𝑟
Mean Glandular Dose (MGD)
14
Dosimetry
Air Kerma from Primary
PhotonsMGD
Code modifications...
nMGD = 𝑀𝐺𝐷
𝐾𝑎𝑖𝑟
~40.000 simulations
15
User Input
• Mono/Poly;
• Energy Range/Anode Filter;
• Breast Thickness range;
• Breast Composition;
• Skin Model;
• Detector Type;
• Antiscatter Grid Model;
• etc
Python
Script
List of Simulations
Parallel Simulations
Windows/Linux full compatibility
PENELOPE
Data Collection
and savingPython
Return Data
Automatization with Python™
~40.000 simulations
16
Automatization with Python™
3-10 min/simulation – Uncertainty (1 - 0.25%)
Processor i7 7700 3.6 Ghz
Outline
17
Motivation
• Mammography
• Dosimetry – Mean Glandular Dose
Methodology
• Implemented Models
• How?
Results
• MGD vs Skin Model
• Differences
Conclusions• Summary
Results - Code Validation
18
AAPM – Report 195 (2015)
19
• 5 cm thick;
• 20% 𝑓𝑔;
• 1.45 mm skin;
Results: Skin shielding models
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Monoenergetic Beam
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1% 𝑓𝑔 100% 𝑓𝑔
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Monoenergetic Beam – Depth Dose 18 keV20% 𝑓𝑔
2 cm breast 8 cm breast
Results: Skin shielding models
22
Polyenergetic Beam
2 cm 8 cm
20% 𝑓𝑔
36%
15%
34%
16%
Results: Skin shielding models
23
Polyenergetic Beam Mo/Mo 28 kV
2 cm breast
21%
23%
Results: Skin shielding models
24
Polyenergetic Beam Mo/Mo 28 kV
21%
17%
Results: Skin shielding models
1% 𝑓𝑔
Results: Summary
25
Polyenergetic Beam – Skin Models
Outline
26
Motivation
• Mammography
• Dosimetry – Mean Glandular Dose
Methodology
• Implemented Models
• How?
Results
• MGD vs Skin Model
• Differences
Conclusions• Summary
Conclusions
27
𝑓𝑔 (≈50%) BreastThickness
(≈350%)
SkinModel
(≈40%)
MGD
Variation
Tube Potential(≈130%)
Depth Dose : skin attenuation and Homogeneous Mixture Volume
Anode/Filter(≈90%)
• Skin model affects the MGD up to 37%;
• Larger variations: low energies; high glandularity, thin breasts
• The Skin Model has a significant impact on MGD estimates;
• Reduce the uncertanties;
• Patient-specific dosimetry;
• Heterogeneous breast
Acknowledgement28
• Process 2016/15366-9
• Process 2015/21873-8 • Process 483170/2015-3
Rodrigo T. Massera Bruno L. Rodrigues
José Maria
Fernandez-Varea
Lab Members and AlumniCollaborators
Our Institution
29
University of Campinas (UNICAMP): 1st in Latin America
Credits: Lucas Rodolfo de Castro Moura -
http://www.lrdronecampinas.com.br/
Funded in 1966
30
Thank You!
Rodrigo T. Massera
MSc Student
IFGW - UNICAMP