The Role of Monte Carlo and other
Advanced Dose Calculation
Algorithms for Breast APBI
Luc Beaulieu, Ph.D.
Associate Prof, Département de physique, Université Laval
Researcher, Centre de recherche en cancérologie de l’Université Laval
Medical physicist and Head of medical physics research, CHUQ
CE – Therapy SAMS Session, Tuesday July 20
Disclosure
• CHUQ is a Nucletron center of
excellence
– I am part of a standing research
agreement with Nucletron
Learning Objectives
• Understanding the current limitation related to
breast brachytherapy dosimetry.
• Identify situations where Monte Carlo or other
advanced dose calculation algorithms could be
useful.
• Identify the underlying dosimetric impact of the
choice of treatment isotope or source energy
(eBx) in breast brachytherapy.
Topics
• What’s wrong with TG43?
–Scatter conditions
–Heterogeneities
–Effect of source energy
• Alternatives and Challenges
What’s wrong with TG43?
What’s wrong with TG43?
Source position
within a 15 (40)
cm radius water
sphere
The Reality of Breast Brachytherapy!
Interstitial
Mammo
Contura
SAVI
Limitation of TG43 in relation to
clinical/physical parameters
Rivard, Venselaar and Beaulieu, Med Phys 36 (2009)
Limitation of TG43 for Breast
Brachytherapy
Energy Range
192Ir
Effect
Scatter condition
Shielding (applicator related)
103Pd/eBx
Absorbed dose (μen/ρ)
Attenuation (μ/ρ)
Shielding (applicator, source)
Tissue Heterogeneity
Tissue Heterogeneity
From NIST website
Tissue Heterogeneity: Adipose vs Water
G Landry et al., submitted to Med Phys
Tissue Heterogeneities not a problem at
192Ir energy
103Pd
Breast Brachytherapy
Tissue effects can be large
http://physmed.fsg.ulaval.ca/
14
H Afsharpour et al., PMB (in press)
Spectral measurements AXXENT
electronic source
WL
YK
AgK CdTe K-edge
[Liu et al. Spectroscopic characterization of a novel electronic brachytherapy
system. PMB 53, 61, 2008]
Tissue Heterogeneities
• Energy dependant
– Less than 1% for 192Ir over
useful distances
• 10-40% effect for 103Pd
– Similar for eBx
– Difference increase with
• Decreasing energy.
• Increasing distance from ref point.
Scatter Condition
Scatter Condition
Interstitial
CONTURA
Dashed line = TG43 ; Full line = MC
Scatter Condition
Scatter Condition
• Finite dimension of the breast for 192Ir
– No large effect on the highest isodoses
– Reduces skin doses by about 5%
• Pentalis et al., IJROBP 2005
• For lower energy such as 169Yb
– Skin and lung doses overestimated by 15-30%
• Lymperopoulou et al., Med Phys 2006
• For even lower energy
– Not the most important effect anymore…
Shielding: Contrast Agent
…Contrast and Air
Interstitial
Mammo
Contura
SAVI
Article Agents de contrastMed Phys 31
Med Phys 37 (2010)
0-4%
0-6%
0-9%
Contrast (15%) with an eBx source?
192Ir
eBx
Contrast with an eBx source (15%)
Effect of contrast agent
Contrast + tissues
The ballon for Xoft source does not use contrast
Alternatives…
If not TG43, what?
– Model-Based Dose Calculation
Algorithms
• Stochastic methods: Monte Carlo
• Deterministic approaches: GBBS (Acuros)
• Primary/Scatter separation: CCC
– Analytical Methods
• Ray tracing with scaling
– Hybrid approaches
Alternatives to TG43
Rivard, Beaulieu and Mourtada, Vision 20/20, Med Phys 2010
…and Challenges
TPS – Dose Calc
Dose parameters,
DVHs, isodoses
Going Beyond TG43
• Must maintain inter-institution consistency
• Must high QC/QA standards
• Must be able to
• Define for each voxel: material and density
• Define the dose scoring medium
• Might have to address retrospectively and prospectively
dose-toxicity and dose-outcome relationships
In the meantime…
• TG186 is working toward providing guidelines
• Look for it at a conference near you… in 2011
• For the time being, we recommend that
• All clinical plans be generated with TG43
• If using a MBDCA for dose recalculation
• Be sure to read Rivard et al., Vision 20/20 (2010)
• MBDCA should fall back to TG43 in well controlled
conditions (15 or 40 cm radius water geometry)
• Transport should be carried out in the medium (CT #) if
no artifacts
• Full documentation of tissue (Zeff) assignments
TG-186
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Luc Beaulieu, CHUQ (Chair)
Äsa Carlsson-Tedgren, Linköping University
Jean-François Carrier, CHUM
Stave Davis, UW Maddison
Firas Mourtada, MD Anderson
Mark Rivard, Tuft University
Rowan Thomson, Carleton University
Frank Verhaegen, Maastro
Todd Waering, Transpire inc (consultant)
Jeff Williamson, VCU
Quiz Time
For 192Ir breast brachytherapy, what
is the most important factor
influencing the dosimetry relative to
TG43?
33%
33%
33%
1. Tissue heterogeneities
2. None
3. Scatter condition
10
For 192Ir breast brachytherapy, what is
the most important factor influencing
the dosimetry relative to TG43?
1. Tissue heterogeneities
2. None
3. Scatter condition
In your clinical practice, what role do
you see for advanced dose
calculation algorithms?
33%
1. I do not want to use these yet.
33%
2. For dose recalculation after the fact.
33%
3. In parallel to TG43, allowing plan
modification.
10
What do you think will be the biggest
impact of moving away from TG43?
33%
33%
33%
1. Affect dose prescription levels and
dose-outcome studies.
2. Will provide better tx (in term of
outcome).
3. High risk of losing center-to-center
uniformity that we have with TG43.
10
Discussion
• The limitation of TG43 are modulated by
– Calculation geometry
– Applicator composition
– Source energy
Energy
Range
192Ir
Effect
Scatter condition
Shielding (applicator related)
103Pd/eBx
Absorbed dose (μen/ρ)
Attenuation (μ/ρ)
Shielding (applicator, source)
Conclusions
• TG43 has numerous useful qualities but is
limited
• MBDCA (MC, Acuros, …) promises more
accurate dose calculations
– Higher chance of GIGO
• AAPM recommendations are in the work