Model-Based Dose Calculation Algorithms in Brachytherapy
Luc Beaulieu Professor, Department of Physics, Université Laval
Medical Physicist and Head of Research, Department of Radia>on Oncology, CHU de Quebec
Contents • Introduction: TG43 and beyond…
• Advances in brachytherapy dose calculations
• Recommendations from AAPM/ESTRO/ABS/ABG Task Group 186
• Conclusion
Brachytherapy is state-of-the-art • Exquisite dose distribution and intensity modulation
• Dose deposition "kernel" better than proton
• Real-time image guidance and dose guidance
• Addition of robotic brachytherapy
• Possibility of shielding, directional source, multiple isotope/energie tx
…but dose calculation is not
≠
Interstitial Contura
Mammo SAVI
One size does not fit all!
Vision 20/20 Paper Medical Physics The evolution of brachytherapy treatment planning Mark Rivard,1 Jack L. M. Venselaar,2 and Luc Beaulieu3 1Department of Radiation Oncology, Tufts University School of Medicine, Boston, Massachusetts, USA 2Department of Medical Physics, Instituut Verbeeten, P.O. Box 90120, 5000 LA Tilburg, The Netherlands 3Département de Radio-Oncologie et Centre de Recherche en Cancérologie de l’Université Laval, Quebec
Brachytherapy is a mature treatment modality that has benefited from technological advances. Treatment planning has advanced from simple lookup tables to complex, computer-based dose calculation algorithms. The current approach is based on the AAPM TG-43 formalism with recent advances in acquiring single-source dose distributions. However, this formalism has clinically relevant limitations for calculating patient dose. Dose-calculation algorithms are being developed based on Monte Carlo methods, collapsed cone, and the linear Boltzmann transport equation. In addition to improved dose-calculation tools, planning systems and brachytherapy treatment planning will account for material heterogeneities, scatter conditions, radiobiology, and image guidance. The AAPM, ESTRO, and other professional societies are coordinating clinical integration of these advancements. This Vision 20/20 article provides insight on these endeavors. Med. Phys. 36, 2136-2153 (2009)
Sensitivity of Anatomic Sites to Dosimetric Limitations of Current Planning Systems
anatomic site
photon energy
absorbed dose attenuation shielding scattering beta/kerma
dose
prostate high low XXX XXX XXX
breast high XXX low XXX XXX XXX
GYN high XXX low XXX XXX
skin high XXX XXX low XXX XXX XXX
lung high XXX XXX low XXX XXX XXX
penis high XXX low XXX XXX
eye high XXX XXX XXX low XXX XXX XXX XXX
Rivard, Venselaar, Beaulieu, Vision 20/20, Med Phys 36, 2136-‐2153 (2009)
Importance of the Physics: Water vs Tissues
< 100 keV large differences
Impact of tissue composition: 192Ir
Importance of the Physics: Attenuation by Metals
10−3 10−2 10−1 100 101100
101
102
103
104
Photon Energy (MeV)
Atte
nuat
ion
coef
ficie
nt (r
atio
to w
ater
)
WCuAgAuTi
From NIST website
http://physmed.fsg.ulaval.ca/ 12
Poon et al., IJROBP (2008)
How important in the clinic?
Site / Application Importance
Shielded Applicators Huge
Eye plaque -10 to -30% (TG129)
Breast Brachy -5% to -40%
Prostate Brachy -2 to -15% on D90
GYN Depends on applicators
Accurate dose calculation should be a priority
• The dose-outcomes, tolerence doses, prescription doses will probably need to be revisited
• e.g. 192Ir breast skin tolerance dose • -16% Raffi et al, Med Phys 2010
Rule of tumb
Energy Range Effect
192Ir Scatter condition
Shielding (applicator related) 103Pd/125I/eBx Absorbed dose (µen/ρ)
Attenuation (µ/ρ)
Shielding (applicator, source)
Contents • Introduction: TG43 and beyond…
• Advances in brachytherapy dose calculations
• Recommendations from AAPM/ESTRO/ABS/ABG Task Group 186
• Conclusion
TG43 PSS CCC MC
Brachytherapy Dose Calculation Methods
GBBS Physics Content
Analytical / Factor-‐based Model-‐Based Dose Calculation : MBDCA
Rivard, Beaulieu and Mourtada, Vision 20/20, Med Phys 2010
TG43 PSS CCC MC
BT Dose Calc.
GBBS
Current STD: Full scatter water medium
No particle transport. No heterogeneity, shields. Primary can be used in more complex dose engine
Implicit particle transport: Heteregoneities. Accurate to 1st scatter. GPU friendly
Only commercial MDBCA. Solves numerically transport equtations. Full heteregoneities.
Explicit particle transport simulation. Gold STD for source characterization and other applications
TG43 PSS CCC MC
BT Dose Calc.
GBBS
First-scatter kernel
Outline of the brachy-CC MBDCA IV. Summation I. Raytrace source II. CC convolution III. CC convolution
S1sc S2sc
Dprim D1sc Drsc + + = Dtot
Scatter transport line Residual-scatter kernel
-4
-12
log(D/R) 192Ir msel-v2
Details in Carlsson and Ahnesjö (2000) Med Phys p 2320-2332
∝1sc prim
CPES D ∝2sc 1sc
CPES D
Scatter transport line
First scerma Second scerma
Primary source rays Material info
Material info Material info
I. TG43
Superposition of sngle-source water-dose Imaging in TG43: localise dose -anatomy
Dm,m Collapsed Cone Dw-TG 43
II. MBDCA
Information on tissue, etc composition from images or elsewhere
water
Brachy-CC MBDCA
From Åsa Carlsson-Tedgren
Monte Carlo simulations
• Mimics the discrete par>cle, sta>s>cal nature of ioniza>on radia>on
• "Golden standard" for dose calcula>ons • TG43 parameters • Primary ScaKer Separa>on
• Model complex geometries
• Derive informa>on not accessible in measurements
DWO Rogers, Review paper, PMB 51 (2006); TG43-U1 by Rivard et al., Med Phys 2004;
Monte Carlo Dose Calculations: Brachy
Williamson (1987) Med Phys p 567-576, Hedtjärn et al (2002) Phys Med Biol p 351-376
• General Purpose • EGSnrc • MCNP (5,X) • Penelope • Geant4
• Brachytherapy specific • MCPI – Seeds (Chibani and Williamson (2005) Med Phys
3688-3698) • BrachyDose - Seeds (Taylor et al (2007) Med Phys
445-457) • PTRAN CT (Williamson et al (1987) Med Phys p 567-576) • ALGEBRA (Afsharpour et al., (2012), PMB)
2D: Daskalov et al (2002), Med Phys 29, p.113-124 3D: Gifford et al (2006), Phys Med Biol vol 53, p 2253-2265
– Position: mesh position discretization (finite elements)
– Energy: E Energy bins (cross section) – Direction: Angular discretization
Ω̂ ⋅∇Ψ(r ,E,Ω̂) +σ t (
r ,E)Ψ(r ,E,Ω̂) =Qscat (r ,E,Ω̂) +Qex (r ,E,Ω̂)
« multi-‐group discrete ordinates grid-‐based …»
r = (x, y, z)
Ω̂ = (θ ,φ)
Grid-‐Based Boltzmann Solver (GBBS)
• Varian BV-Acuros® implementation: only commercial MBDCA solution at this time • CPE assumption : Primary dose analytical (ray-tracing
with scaling) • Dprim = Kcoll • First scatter from primary : Scerma = Dprim•((µ-µen)/uen) • Share this step with CCC
• 3D scatter integration through GBBS
• Source modeling done in Atilla® (Transpire Inc)
Grid-‐Based Boltzmann Solver (GBBS)
Example
Figure from : L. Petrokokkinos et. al. Med. Phys. 38, 1981-‐1992 (2011). . More references on the algorithm, see e.g.: K. A. Gifford et. al. Med. Phys. 35, 2279-‐2285 (2008)
• Speed: 40 sec to 12 min depending on complexity
Factor-based vs Model-based
Superposition of data from source characterization
Dw-TG43
Dm,m Dw,m
Source characterization
Tissue/applicator information
Source characterization
INPUT OUTPUT CALCULATION
TG43
MBDC
INPUT OUTPUT CALCULATION
From Åsa Carlsson-Tedgren
Model-Based Dose Calculation Algorithms
Contents • Introduction: TG43 and beyond…
• Advances in brachytherapy dose calculations
• Recommendations from AAPM/ESTRO/ABS/ABG Task Group 186
• Conclusion
TG-186 • For early adopters
• Report approved by • AAPM (BTSC, TPC) • ESTRO (BRAPHYQS, EIR) • ABS (Physics committee) • ABG
• Published in Medical Physics
1. Definition of the scoring medium
2. Cross section assignments
(segmentation)
3. Specific commissioning process
Three main areas identified as critical
Dx,y x: dose specification
medium
y: radiation transport medium
1. Definition of the scoring medium
� x,y: Local medium (m) or water (w)
!
FROM: G Landry, Med Phys 2011
Which dose to report? • Dm,m is inherently computed by Model-based algorithms • Dm,m must be reported along with TG43 • Dw,m can also be reported but method must be specified:
• e.g. large cavity theory, small cavity theory • Could be energy and target size dependent (voxel, cells, …)
!
#1: #2:
2- Cross section assignments � Accurate tissue segmentation, sources and
applicators needed: identification (ρe ,Zeff) � e.g. in breast: adipose and glandular tissue have
significantly different (ρe ,Zeff); dose will be different
� If this step is not accurate è incorrect dose � Influences dosimetry and dose outcome studies � Influences dose to organs at risk
Recommendation - segmentation � Extract electron density from CT calibra>on (see TG53, TG66 …) � Use the density from CT for each voxel
� Use recommended >ssue composi>ons � Organ-‐based (contoured) assignments
� Prostate from Woodard et al, BJR 59 (1986) 1209-‐18 � All others from ICRU-‐46 composi>on
� From CT calibra>on: breast, adipose, muscle and bone
Recommendation - segmentation � If artifacts (e.g. from metals)
� Override the density using the recommended default organ/tissue density (TG-186 table)
� Assign tissue composition based on organ contours
Recommendation - segmentation � If no CT (US and MRI)
� Use contoured organs with recommended tissue compositions � For 192Ir, water is a good approximation for soft tissues only. � Air, lung, bone, … should be assigned correctly
� Use accurate source and applicators geometry and
composition
3- Specific commissioning process
� MBDCA specific tasks � Currently, only careful comparison to Monte Carlo with or
w/o experimental measurements can fully test the advanced features of these codes
� This is not sustainable for the clinical physicists
Recommendation - Commissioning
• Two parts process
• Level 1: MBDCA should fall back to TG43 in well controlled conditions • Full scatter: R-r ≥ 5 cm or 20 cm • All water
192Ir Test Geometry for MBDCA Water
20 cm at least
LEVEL 2
� MBDCA specific tasks � Monte Carlo remains the gold standard for comparison
� Might not be appropriate for all clinics
Need Standardized MBDCA Benchmarks � Excellent reference HDR 192Ir benchmarks in MedPhys
� Acuros BrachyVision
Petrokokkinos et al., MedPhys 38, 1981-1992 (2011)
3- Specific commissioning process
� AAPM/ESTRO/ABG working group to tackle this issue Luc Beaulieu (chair), Frank-André Siebert (vice-chair) Facundo Ballaster, Åsa Carlsson-Tedgren, Annette Haworth, Goeff Ibbott, Firas Mourtada, Panagiotis Papagiannis, Mark J Rivard, Ron Sloboda and Frank Verhaegen. Strategy: Registry of validated cases with reference doses calculations. We will try to involved the vendor as much as possible to make the cases compatible with all TPS.
TG-186 Recommendations • The full TG-186 report has a detailed rational
supporting the various recommendations
• Following the recommendations should ensure uniformity of implementation across centers
• NOTE: there is one MBDCA commercial system and it is for 192Ir only at this time.
Clinical relevance of MBDCA � More accurate dose calculation
� Impact on prescription, dose to OARs, … � Current dose-outcome relationships could be wrong!
� Enable better treatment approaches � Directional sources, mixed sources, shielded
applicators, …
Conclusion • Advanced dose calculation is a necessary step for
better brachytherapy treatments
• Change in dose calculation standard is not new (e.g. lung EBRT)
• Transition period • Revisiting dose-outcomes, dose prescription
• The future of brachytherapy is exciting
Merci!
[email protected] http://physmed.fsg.ulaval.ca
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