Commissioning and clinical implementation of
Monte Carlo treatment planning system for
electron beams
Joanna E.Cygler
The Ottawa Hospital Regional Cancer Centre, Ottawa, Canada
Carleton University Dept. of Physics, Ottawa, Canada
University of Ottawa, Dept. of Radiology. Ottawa, Canada
The Ottawa
L’Hopital
Hospital
d’Ottawa
Regional Cancer Centre
Educational Objectives
• Appreciate the need for MC based treatment
planning systems
• Understand the effect of different types of
heterogeneities (geometry and density) on dose
distribution
• Understand how to set user control parameters in
the TPS to achieve optimum results (minimum
statistical noise, minimum distortion of real dose
distribution )
• Learn and understand differences between water
tank and real patient anatomy based monitor unit
values
J.E. Cygler, ACMP 2009
Rationale for Monte Carlo dose
calculation for electron beams
• Difficulties of commercial pencil beam based
algorithms
– Monitor unit calculations for arbitrary
SSD values – large errors*
– Dose distribution in inhomogeneous media
has large errors for complex geometries
* can
be circumvented by entering separate virtual
machines for each SSD – labour consuming
J.E. Cygler, ACMP 2009
Rationale for Monte Carlo dose
calculation for electron beams
6.2 cm
15
Relative Dose
9 MeV
10
Measured
Pencil beam
Monte Carlo
depth = 6.2 cm
depth = 7 cm
5
0
-10
/tex/E TP /abs/X TS K 09S .O R G
-5
0
Horizontal Position /cm
5
10
98-10-21
Ding, G. X., et al, Int. J. Rad. Onc. Biol Phys. (2005) 63:622-633
J.E. Cygler, ACMP 2009
Commercial implementations
• MDS Nordion 2001*
- First commercial Monte Carlo treatment planning for
electron beams
– Implementation of Kawrakow’s VMC++ Monte Carlo
dose calculation algorithm (2000)
– Handles electron beams from all clinical linacs
• Varian Eclipse eMC 2004
– Based on Neuenschwander’s MMC dose calculation
algorithm (1992)
– Handles electron beams from Varian linacs only
*presently Nucletron
J.E. Cygler, ACMP 2009
Monte Carlo calculations in
commercial TPS
•
Divide the beam into treatmentindependent and treatmentdependent components
•
Simulate treatment-independent
components
–
characterize phase space
distribution with a beam model
•
Simulate transport through the
patient anatomy – dose distribution
Courtesy of Varian
after Janssen et al4
J.E. Cygler, ACMP 2009
Description of Nucletron Electron
Monte Carlo DCM
Fixed applicator
with optional,
arbitrary inserts
Calculates absolute
dose per monitor
unit (Gy/MU)
510(k) clearance (June 2002)
J.E. Cygler, ACMP 2009
User input data Nucletron TPS
Treatment unit specifications:
• Position and thickness of jaw collimators and MLC
• For each applicator scraper layer:
Thickness
Position
Shape (perimeter and edge)
Composition
• For inserts:
Thickness
Shape
Composition
J.E. Cygler, ACMP 2009
User input data Nucletron TPS cont
Dosimetric data for beam characterization
• Without applicators:
–
–
X and Y in-air profiles (8x8, 8x20, 8x35, 35x35,
SDD = 70 & 90 cm)
Central axis Depth Dose in water for various
field sizes
• With applicators:
– Central axis depth dose and profiles in water
– Absolute dose at the calibration point
Dosimetric data for verification
– Central axis depth doses and profiles for various
field sizes
J.E. Cygler, ACMP 2009
Varian Macro Monte Carlo
• PDF table look-up for “kugels” or spheres
instead of analytical and numerical
calculation
• CT images pre-processes
– Homogenous areas
large spheres
– In/near heterogeneous areas
small spheres
• Database with probable outcome for every
combination of:
– 30 incident energy values (0.2-25 MeV)
– 5 materials (air, lung, water, Lucite and solid
bone)
– 5 sphere sizes (0.5-3.0 mm)
EGSnrc used to create beam data
J.E. Cygler, ACMP 2009
User input data - Varian eMC
Open field measurements (no applicator)
• Depth-dose curves in water at the source-tophantom distance (SPD) = 100 cm
• Absolute dose, expressed in [cGy/MU], at a
specified point on the depth dose curve
•
Profile in air at source-detector distance,
SDD = 95 cm for the wide open field without an
applicator, e.g. 40x40 cm2.
J.E. Cygler, ACMP 2009
User input Varian eMC cont.
For each energy/applicator combination:
• PDD in water at SSD = 100 cm
•
Absolute dose, expressed in [cGy/MU], at a specified
point on the depth dose curve
No head geometry details required, since at this time
Eclipse works only for Varian linac configuration
J.E. Cygler, ACMP 2009
Clinical implementation of
treatment planning software
• Beam data acquisition and fitting
• Software commissioning tests
• Clinical implementation
– procedures for clinical use
– possible restrictions
– staff training
A physicist responsible for TPS implementation should
have a thorough understanding of how the system works.
J.E. Cygler, ACMP 2009
User controlled calculation
parameters
• Nucletron
– User can define number of histories used in
calculation (in terms of particle #/cm2)
• Varian
– User can define:
• Maximum number of histories used in
calculation >/=0
• statistical uncertainty (within the high dose
volume)
• Calculation grid size (voxel size)
• Smoothing method and level
• Random number generator seed
J.E. Cygler, ACMP 2009
Software commissioning tests: goals
• Setting user control parameters in the TPS
to achieve optimum results (minimum
statistical noise, accuracy vs. speed of
calculations)
– Number of histories
– Voxel size
– Smoothing
• Understand differences between water
tank and real patient anatomy based
monitor unit values
J.E. Cygler, ACMP 2009
Software commissioning tests
• Homogeneous water phantom
• Inhomogeneous phantoms (Cygler et al, Phys. Med. Biol., 32,
1073 (1987) and Ding G.X.et al, Med. Phys., 26, 2571-2580, 1999)
– Shiu et al, Med.Phys. 19, 623—36, 1992;
– Boyd et al, Med. Phys., 28, 950-8, 2001
• All scans done with a high (1 mm) resolution
• Criteria for acceptability (Van Dyk et al, Int. J. Rad. Oncol.
Biol. Phys., 26, 261-273,1993;
Fraass, et al, AAPM TG 53: Quality assurance for clinical
radiotherapy treatment planning,” Med. Phys. 25, 1773–1829 1998)
J.E. Cygler, ACMP 2009
Typical Experimental setup
Electron applicator
•RFA300
(Scanditronix)
dosimetry system
Water tank
•p-type electron
diode
Diode
detector
•Scan resolution =
1mm
In-air or in water beam profiles
J.E. Cygler, ACMP 2009
Homogeneous water phantom tests
• Standard SSD 100 cm and extended SSD
– Open applicators – PDD and profiles
– Square and circular cut-outs
• Oblique incidence
– GA = 150 and 300
• MU tests – SSD 100 and extended SSD
– All open applicators
– Square, rectangular, circular, some irregular
cutouts
J.E. Cygler, ACMP 2009
Inhomogeneous phantoms
• Low and high density inhomogeneities
– 1 D (slab) geometry
– 2 D (ribs) geometry
– 3 D (small cylindrical) geometry
• Complex (trachea and spine) geometry
J.E. Cygler, ACMP 2009
Effect of statistical uncertainty
on MC calculations
120
Measurement
eMC − 1%
eMC − 3%
100
20 MeV, SSD=100 cm,
homogeneous water phantom.
Central axis PDD measured and
eMC (Eclipse) calculated for
1% and 3% statistical
precision, 2.5 mm voxel size, no
smoothing.
Dose (%)
80
60
40
20
0
0
20
40
60
80
100
120
Depth (mm)
Courtesy of R. Popple, Med Phys. 33, 1540- 51, 2006
J.E. Cygler, ACMP 2009
Lateral profiles at various depths,
SSD=100 cm, Nucletron TPS
20 MeV, 10x10cm2 applicator, SSD=100cm.
Homogeneous water phantom. Cross-plane profiles at
various depths. MC with 10k and 50k/cm2.
110
110
100
100
90
90
80
80
70
meas.@2cm
60
calc.@2cm
50
meas.@3.0cm
calc.@3.0cm
40
meas.@d=3cm
calc.@d=3cm
calc.@d=3cm,50k
meas.@d=7.8cm
calc.@d=7.8cm
70
Dose / cGy
Dose / cGy
9 MeV, 10x10cm2 applicator, SSD=100cm. Homogeneous
water phantom,cross-plane profiles at various depths. MC
with 10k/cm2.
60
50
calc.@d=7.8cm,50k
meas.@d=9cm
calc.@d=9cm
calc.@d=9cm,50k
40
meas.@4.0cm
30
30
calc.@4.0cm
20
20
10
10
0
0
-10
-5
0
Off - axis / cm
5
10
-10
-5
0
5
10
Off - axis / cm
J.E. Cygler, ACMP 2009
Overall mean and variance of MC/hand
monitor unit deviation, Nucletron TPS
0.45
0.40
Our data
0.35
Mean=-0.003
0.30
Variance=0.0129
fraction
Theory
0.25
0.20
0.15
0.10
0.05
0.00
-0.05
-0.04
-0.03
-0.02
-0.01
0
0.01
0.02
0.03
0.04
0.05
1- MC/hand
Cygler et al. Med. Phys. 31 (2004) 142-153
J.E. Cygler, ACMP 2009
Air cylinder
900 MHz, CPU time for 50k/cm2
14:01-20 MeV , 9:31 – 9MeV
Voxel size 0.39 cm, 47 slices
20 MeV, Air cylinder,10x10cm2 applicator,
SSD=100cm. Cross-plane profiles.
MCwith 10k/cm2.
9 MeV, Air cylinder,10x10cm2 applicator,
SSD=100cm. Cross-plane profiles.
MCwith 10k/cm2.
140
120
120
100
meas@d=2.1cm
Dose / cGy
calc.@d=2.1cm
calc.@d=2.1cm,50k
80
meas.@d=4cm
calc.@d=4cm
60
calc.@d=4cm,50k
meas.@d=5cm
40
80
Dose / cGy
100
meas.@d=2.1cm
40
calc.@d=5cm,50k
20
0
0
-6
-4
-2
calc.@d=6cm,50k
calc.@d=5cm
20
-8
calc.@d=2.1cm
calc.@d=2.1cm,50k
meas.@d=6cm
calc.@d=6cm
60
0
2
Off - axis / cm
4
6
8
-8
-6
-4
-2
0
2
4
6
8
Off - axis / cm
J.E. Cygler, ACMP 2009
Results MC tests:voxel size
9
MeV
Air
cylinder
140
0.19 cm
130
120
dose / cGy
0.39 cm
110
100
90
80
Meas 0.1 cm
70
60
-1
-0 .5
0
0 .5
1
o f f - a x is / c m
Cygler et al. Med. Phys. 31 (2004) 142-153
J.E. Cygler, ACMP 2009
Eclipse eMC no smoothing
Voxel size = 2 mm
Air
Air
Bone
Bone
120
110
120
depth= 4.7cm
18 MeV
110
100
90
90
Relative Dose
100
RelativeDose
4.7 cm
80
depth = 6.7 cm
70
60
50
depth = 7.7 cm
40
18 MeV
depth = 4.7 cm
80
70
60
50
40
30
20
Measured
eMC
30
Measured
eMC
20
10
10
0
0
-6
-4
-2
0
2
Off-axisX position /cm
4
6
-6
-4
-2
0
2
4
Off-axisY position /cm
Ding, G X., et al (2006). Phys. Med. Biol. 51 (2006) 2781-2799.
J.E. Cygler, ACMP 2009
6
Eclipse eMC
Effect of voxel size and smoothing
Air
Air
Bone
Bone
110
2mmandnosmoothing
18 MeV
110
Relative Dose
100
90
80
70
2mmandwith3Dsmoothing
60
5 mm and with 3D smoothing
50
120
Relative Dose
120
4.7 cm
90
80
70
60
50
40
30
30
depth = 4.9 cm
5 mm and with
3D smoothing
100
40
20
2 mm and with 3D smoothing
20
depth = 4.9 cm
10
10
2mmandnosmoothing
18 MeV
0
0
-6
-4
-2
0
Off-axisX position /cm
2
4
6
-6
-4
-2
0
2
4
Off-axisY position /cm
Ding, G X., et al (2006). Phys. Med. Biol. 51 (2006) 2781-2799.
J.E. Cygler, ACMP 2009
6
Dose-to-water vs. dose-to-medium
Hard bone cylinder
2cm
1 cm diameter and 1 cm length
110
110
9 MeV
Bone cylinder is replaced
by water-like medium but
with bone density
100
90
90
80
80
70
70
Dose
Dose
100
depth= 2
60
50
40
60
50
20
Bone
cylinder
location
40
Measured
eMC
30
BEAM/dosxyz
simulation
30
20
10
10
0
0
-8
-6
-4
-2
0
2
4
6
0
8
2
3
4
5
Central AxisDepth /cm
Off-axis distance /cm
100
Ding, G X., et al
Phys. Med. Biol. 51
(2006) 2781-2799
1.14
depth = 3
90
9 MeV
80
1.13
depth= 4
70
60
SPR
Dose
1
Measured
eMC
50
40
1.12
30
Water/Bone stopping-power ratios
1.11
20
10
0
1.10
-8
-6
-4
-2
0
2
4
Off-axisdistance /cm
6
8
0
1
2
3
4
5
depth in water /cm
J.E. Cygler, ACMP 2009
9 MeV – hard bone Dm vs. Dw
Dose profiles
within the bone
Location of hard
bone
J.E. Cygler, ACMP 2009
9 MeV – clinical hard bone
Dm vs. Dw
(b)
(a)
Location of
bone
(c)
J.E. Cygler, ACMP 2009
Results: Clinical – soft bone
• The maximum difference in dose is 1.8%, in
agreement with soft bone phantom study
and consistent with stopping power ratio
Dose-to-Water
Medium
Dose-to-
J.E. Cygler, ACMP 2009
Results: Clinical – soft bone
% dose
• The maximum difference in dose is 1.8%,
which is in agreement with the phantom
studies
across patient / cm
J.E. Cygler, ACMP 2009
Treatment Planning Procedure
The physician:
• outlines CTV and/or GTV
The dosimetrist / physicist:
• models the beam (energy, custom cutout)
– CTV covered by 90% isodose line
The software calculates :
• absolute dose distribution in cGy
• number of monitor units, MU
J.E. Cygler, ACMP 2009
Clinical implementation issues
• Bolus fitting (no air gaps)
• Lead markers (wires, lead shots) used in
simulation
• Monitor unit calculations
– Water tank or real patient anatomy
– Dose to water or dose to medium
• Workload
J.E. Cygler, ACMP 2009
Example of poorly fitting bolus
J.E. Cygler, ACMP 2009
How to correct poorly fitting bolus
J.E. Cygler, ACMP 2009
Good clinical practice
• Murphy’s Law of computer software
“All software contains at least one bug”
• Independent checks
J.E. Cygler, ACMP 2009
MU MC vs. hand calculations
Monte Carlo
Hand Calculations
Real physical dose
calculated on a patient
anatomy
Rectangular water
tank
Inhomogeneity
correction included
No inhomogeneity
correction
Arbitrary beam angle
Perpendicular beam
incidence only
J.E. Cygler, ACMP 2009
9 MeV, full scatter phantom
(water tank)
RDR=1 cGy/MU
J.E. Cygler, ACMP 2009
Lateral
scatter missing
Real contour / Water tank =
=234MU / 200MU=1.17
J.E. Cygler, ACMP 2009
MU real patient vs.water tank
MC / Water tank= 292 / 256=1.14
J.E. Cygler, ACMP 2009
MU-real patient vs. water tank
Impact on DVH
120.0
PTV-MUMC
100.0
PTV-MUWT
%volume
80.0
LT eye-MUMC
LT eye-MU-WT
60.0
RT eye-MUMC
40.0
RT eye-MU-WT
20.0
0.0
0.0
10.0
20.0
30.0
40.0
50.0
60.0
dose / Gy
J.E. Cygler, ACMP 2009
Posterior cervical lymph node
irradiation - impact on DVH
45.0
customized
40.0
35.0
30.0
PTV / cm
3
conventional
25.0
20.0
15.0
10.0
Jankowska et al, Radiotherapy & Oncology, 2007
5.0
0.0
0.0
5.0
10.0
15.0
20.0
25.0
30.0
dose / Gy
J.E. Cygler, ACMP 2009
Internal mammary nodes
MC / Water tank= 210 / 206=1.019
J.E. Cygler, ACMP 2009
Timing – Nucletron TPS
Theraplan Plus
•
•
•
•
•
•
10x10 cm2 applicator
50k histories/cm2
Anatomy - 41 CT slices
Voxels 3 mm3
Pentium 4 Xenon 2.2 GHz
Calculation time
– 1.5 min. for 6 MeV beam
– 8.5 minutes for 20 MeV beam
Faster than pencil beam!
J.E. Cygler, ACMP 2009
Timing – Nucletron TPS
Oncentra MasterPlan
Anatomy - 41 CT slices
Voxels 3 mm3
10x10 cm2 applicator
50k histories/cm2
9 MeV Timer Results:
Init = 0.586793 seconds
Calc = 204.069 seconds
Fini = 0.262845 seconds
Sum = 204.919 seconds
20 MeV Timer Results:
Init = 0.628385 seconds
Calc = 331.551 seconds
Fini = 0.232272 seconds
Sum = 332.412 seconds
J.E. Cygler, ACMP 2009
Timing – Varian Eclipse
Eclipse MMC, Varian single CPU Pentium IV
XEON, 2.4 GHz
10x10 cm2, applicator, water phantom,
cubic voxels of 5.0 mm sides
6, 12, 18 MeV electrons,
3, 4, 4 minutes, respectively
Chetty et al.: AAPM Task Group Report No. 105: Monte Carlobased treatment planning, Med. Phys. 34, 4818-4853, 2007
J.E. Cygler, ACMP 2009
Timing – Pinnacle3
dual processor 1.6 GHz Sun workstation, 16 GB RAM.
Overall uncertainty
2%
1%
0.5%
Patient
# histories
CPU time
(min)
#
histories
CPU time
(min)
# histories
CPU time
(h)
1(cheek)
3.4x106
4.8
1.4x107
20
1.6x108
3.9
2 (ear)
1.7x106
2.1
6.5x106
8.1
7.1x107
1.5
3
(breast)
3.3x106
7.1
1.4x107
29.9
1.5x108
5.4
4 (face)
1.1x107
32.1
4.7x108
134.5
5.2x108
24.5
Fragoso et al.: Med. Phys. 35, 1028-1038, 2008
J.E. Cygler, ACMP 2009
New developments
J.E. Cygler, ACMP 2009
Energy modulated electron radiation
therapy
• IMRT using photon beams is a widely used
treatment modality and has become feasible with
the introduction of the MLC.
• Energy modulated electron therapy (EMET) using
Monte Carlo dose calculations is a promising new
technique that enhances the treatment planning
and delivery of dose to superficially located
tumors.
J.E. Cygler, ACMP 2009
MLC for electron beams
Pictures courtesy of C. Ma
C-M Ma, et al ,Phys. Med. Biol. 45 (2000) 2293–2311
J.E. Cygler, ACMP 2009
MLC for electron beams
EMET with FLEC
• Medical Physics Group at McGill University
(Al-Yahiya, Verhaegen and Seuntjens)
recently proposed and studied the
feasibility of a practical “few-leaf electron
collimator” (FLEC) for delivering energy
modulated electron therapy (EMET).
K. Al-Yahiya et al, Phys. Med. Biol. 50, (2005), 847-857
J.E. Cygler, ACMP 2009
The Few Leaf Electron Collimator
The compact design of the FLEC makes it suitable to be
attached to a clinical electron applicator. FLEC can be
automated and remotely controlled.
K. Al-Yahiya et al, Phys. Med. Biol. 50, (2005), 847-857
J.E. Cygler, ACMP 2009
MERT –dose distributions
Ma et al.PMB 48 (2003) 909-924
patient planned using
a) wedged tangential
photon beams
b) IMRTtangential
beams
c) four field IMRT
d) eight field MERT
Isodose lines shown:
55, 52.5, 50, 45, 40,
25, 15 and 5 Gy.
J.E. Cygler, ACMP 2009
Dose Volume Histograms
Ma et al. PMB 48 (2003) 909-924
DVH for the target, lung and heart for:
a) the three photon beam plans
b) the three IMRTplans for the chest wall patient
• Healthy tissues (non-target volumes) receive the least dose with MERT high dose
regions are significantly reduced in normal tissues
•MERT reduces both max cardiac and lung doses by >20 Gy relative to tangents
• Almost no volume receives dose greater than 20 Gy in MERT.
J.E. Cygler, ACMP 2009
•Electron range increases low lung dose
Conclusions
• Commercial MC based TP system are available
– easy to implement and use
– MC specific testing required
• Fast and accurate 3-D dose calculations
• Single virtual machine for all SSDs
• Large impact on clinical practice
– CT based planning for most sites – workload
increase; accuracy improved
– More attention to technical issues needed
– Dose-to-medium calculated
– MU based on real patient anatomy (including
contour irregularities and tissue
heterogeneities)
• Requirement for well educated physics staff
J.E. Cygler, ACMP 2009
Acknowledgements
George X. Ding
George Daskalov
Gordon Chan
Robert Zohr
Elena Gil
Indrin Chetty
Margarida Fragoso
Richard Popple
Charlie Ma
Jan Seuntjens
Support of Nucletron and Varian is
gratefully acknowledged
J.E. Cygler, ACMP 2009
Thank you
J.E. Cygler, ACMP 2009
References
1.
2.
3.
4.
5.
Kawrakow, I. “VMC++ electron and photon Monte Carlo
calculations optimized for radiation treatment planning”,
Proceedings of the Monte Carlo 2000 Meeting, (Springer,
Berlin, 2001) pp229-236
Neuenschwander H and Born E J 1992 A Macro Monte
Carlo method for electron beam dose calculations Phys.
Med. Biol. 37 107 – 125
Neuenschwander H, Mackie T R and Reckwerdt P J 1995
MMC—a high-performance Monte Carlo code for electron
beam treatment planning Phys. Med. Biol. 40 543–74
Janssen, J. J., E. W. Korevaar, L. J. van Battum, P. R.
Storchi, and H. Huizenga. (2001). “A model to determine
the initial phase-space of a clinical electron beam from
measured beam data.” Phys Med Biol 46:269–286.
Traneus, E., A. Ahnesjö, M. Åsell.(2001) “Application and
Verification of a Coupled Multi-Source Electron Beam
Model for Monte Carlo Based Treatment Planning,”
Radiotherapy and Oncology, 61, Suppl.1, S102.
J.E. Cygler, ACMP 2009
References cont.
6.
7.
8.
9.
Cygler, J. E., G. M. Daskalov, and G. H. Chan, G.X. Ding.
(2004). “Evaluation of the first commercial Monte Carlo
dose calculation engine for electron beam treatment
planning.” Med Phys 31:142-153.
Ding, G. X., D. M. Duggan, C. W. Coffey, P. Shokrani, and J.
E. Cygler. (2006). “First Macro Monte Carlo based
commercial dose calculation module for electron beam
treatment planning-new issues for clinical consideration.”
Phys. Med. Biol. 51 (2006) 2781-2799.
Popple, RA., Weinberg, R., Antolak, J., (2006)
“Comprehensive evaluation of a commercial macro Monte
Carlo electron dose calculation implementation using a
standard verification data set”. Med Phys 33:1540-1551
Fragoso, M., Pillai, S., Solberg, T.D., Chetty, I., (2008)
“Experimental verification and clinical implementation of a
commercial Monte Carlo electron beam dose calculation
algorithm”. Med Phys 35:1028-1038.
J.E. Cygler, ACMP 2009