Embed Size (px)
Citation preview
1st New Mexico Workshop on Monte Carlo for Particle Therapy Treatment PlanningAlbuquerque, New Mexico
16-18 May 2011
Target Definition and Treatment Planning in Particle Therapy using
Monte Carlo Error Propagation Analysis
Gonzalo Cabal, Oliver Jäkel, Jürgen Debus
Monte Carlo MethodsIn Medical Physics
Radiation Transport(Fluence, flux, absorbed dose,
particle spectra, LET spectra, ...)
Monte Carlo MethodsIn Medical Physics
Radiation Transport(Fluence, flux, absorbed dose,
particle spectra, LET spectra, ...)
Stochastic methods for plan optimization
Monte Carlo MethodsIn Medical Physics
Radiation Transport(Fluence, flux, absorbed dose,
particle spectra, LET spectra, ...)
Stochastic methods for plan optimization
Uncertainty propagation
Motivation and GoalPTV definition in particle therapy is currently using
concepts borrowed from conventional photon therapy.
The quality of particle therapy TP is particularly sensible to uncertainties. Expansions rules should be well understood.
We propose a method for target definition which we call Dynamic Target Definition.
SummaryBrief review on target definition in radiation therapy
A Monte Carlo error propagation analysis and target expansion
Show some clinically motivated examples
Conclusions
7
ICRU reports 50 and 62
1993 1999
8
GTVThe GTV is the gross tumour volume
The GTV is the gross palpable or visible/demonstrable extent and location of the malignant growth.
GTV
9
CTVThe CTV is the clinical target volume
The CTV is a tissue volume that contains a GTV and/or subclinical microscopic malignant disease, which has to be eliminated. This volume thus has to be treated adequately.
CTV
10
ITVThe ITV is the internal target volume
The ITV consists of the CTV plus an “internal margin” (IM)
The IM is designed to compensate for expected physiologic movements and variations in size, shape and position of the CTV.
ITV
11
PTVThe PTV is the planning target volume
The PTV is a geometrical concept used for treatment planning, and it is defined to select appropriate beam arrangements to ensure that the prescribed dose is actually delivered to the CTV.
The PTV includes the ITV plus a set-up margin to account for set-up uncertainties and machine tolerancesPTV
ICRU Definitions of Volumes
Purdy, J., Sem. Rad Onc. Vol 14, No 1, 2004
“However, exactly how these margins should be combined is still not at all made clear.”
ICRU 62: CTV-PTV margin= IM uncertainties & SM uncertainties
Expansion Recipes
Van Herk, M., Errors and margins in radiotherapy, Sem.Rad.Onc. Vol 14, No 1, 2004. n1, 2004
14
Even more volumes...The treated volume is the tissue volume that is planned to receive at least a dose selected and specified by the radiation oncology team as being appropriate to achieve the purpose of the treatment.
The irradiated volume is the tissue volume that receives a dose that is considered significant in relation to normal tissue tolerance.
15
Even more volumes...
OAR's are organs-at-risk
OAR’s are normal tissues whose radiation sensitivity may significantly influence treatment planning and/or prescribed dose.
PRV: planning organ at risk volume(similar concept like PTV)
16
Conformity index
A conformity index (CI) can be employed when the PTV is fully enclosed by the treated volume, then being the quotient of the treated volume and the volume of the PTV.
CI = treated volume / PTV
CI is ≥ 1 and ideally = 1
“... researchers are encourage to continue to seek improved methods of specifying volumes for reducing clinical and geometrical uncertainties.”
Purdy, J.,Current ICRU Definitions of Volumes: Limitations and Future Directions.Sem. Rad Onc. Vol 14, No 1, 2004
“Protons require margins in depth to take range uncertainties into account, as well as the more traditional lateral uncertainties. At present, these margins are often quite substantial, especially in the lung whose low density results in the physical extent of the depth overshoot being some four to five times greater than would be the case in, say, muscle. A combination of technical, imaging, and procedural advances need to be developed to reduce this problem to an acceptable level.”
Gotein, M., Magical Protons?, IJROBP, Vol 70, Issue 3, 2008,p. 654-656
“We need error bars!!”H. Suit, at PTCOG 48, Discussion after T.Lomax talk
On safety margins
18
Dynamic Target Definition(DTD)
Bayesian approach to uncertainty assessment
Monte Carlo Error Propagation Analysis
Expansion rule
Materials & Methods
Isodoses are shownin red (95%), orange (90%), green (50%),
Yellow(25%) and blue(10%).Normalized to prescribed dose.
Consider systematic range errors in a simple phantom:
Three materials of the RING PHANTOMWith WEPL values: 1.6 (outer ring), 0.3 (inner ring) and 1.0 (tumor inner core)
Volume with dose greaterthan 90% prescribed dose
Volume with dose greater than 95% prescribed dose
Is target coverage good enough?
Target coverage
for an ideal “error-free”
model
To deal with uncertainties and guarantee target coverage :Target Expansion
For each voxel a probability density function (PDF) will be calculated.
•This information will be used for target expansion
How do the errors in the HU-range calibration translate into dose uncertainties? Sampling from a uniform probability dist. with the following relative variances:
“Bone material” → 10%“Soft tissue” → 5%“Lung tissue” → 20%
Modeling the HU-range calibration errors
Histogram of relative dose values of a voxel in the SOBP region
Is target coverage good enough? Uncertainties in the range translate into uncertainties in the dose
at each voxel.
Set of voxels having a histogram with a standard deviation (STD) less than s% and mean deviation m % from the prescribed dose
CTV CTV
Voxels with a STD and mean deviation greater than a predefined threshold (m=5% and s=3% in the figure)
will not be considered to have a robust dose coverage
tCTV(m,s)
Is target coverage good enough? Uncertainties in the range translate into uncertainties in the dose
at each voxel.
Set of voxels having a histogram with a standard deviation (STD) less than s% and mean deviation m % from the prescribed dose
CTV CTV
Voxels with a STD and mean deviation greater than a predefined threshold (m=5% and s=3% in the figure)
will not be considered to have a robust dose coverage
From Dose uncertainties to target expansion
LbLr
=averagevalueofWEPLalongLraveragevalueofWEPLalongLb
Define tV(m,s) as:Volume made of voxels having a dose histogram with a relative uncertainty lower than s% (of the prescribed dose) and a mean deviation from the prescribed dose lower than m%. Define tCTV(m,s)=tV ∩ CTV Define PTV(m,s) as:Expansion aiming to get that every voxel of the CTV will have a histogram with an STD lower than s% and a mean deviation of the dose lower than m%
CTV
tCTV(m,s)
PTV(m,s)
Lb
Lr
Construct PTV(m,s) the following way:1. The distance from a point in the border of the tCTV to the border of the CTV in the direction normal to the surface of the tCTV is Lr.2. Following this direction we expand the volume a distance Lb, given by:
3. Plan for PTV and assess a new tCTV(m,s). Iterate until tCTV=CTVPTV volume generated in one iteration
for the example shown previously
Is target coverage good enough?tCTV(5,3) when planned on CTV (no expansion)
tCTV(5,3) when planned on a 5 mm isotropic PTV expansion
tCTV(5,3) when planned on PTV(5,3) after one iteration
The generated PTV increases the robustness of the plan towards target coverage. Simultaneously the PTV size is smaller compared to the isotropic expansion which translates into lower integral doses.
DVH of the mean dose distributions CTV and Integral (Isotropic vs Proposed expansion)
A clinical example
• For this example both HU-to-WEPL calibration error and set up (SU) uncertainties were modeled. A two field SFUD technique was applied.
• SU uncertainties were sampled from a normal distribution with a mean value of 0,a σ = 1 mm in the AP direction and σ = 2 mm in the LR direction.
• Following a Bayesian approach, HU-WEPL calibration errors were modeled as: Uniform probability density function with mean values = 0 and with different variances for bone, soft and low density tissue.
A clinical example
2 horizontal opposing beams oblique beams: +60° / -60°
• PTV contour and size are beam arrangement specific!
• These arrangements that are less affected by uncertainties and will produce smaller PTV
• PTV size becomes a dynamic measure of robustness
A clinical example
Dto, +90°/-90°PTV size=1004 voxels
90% of CTV voxels covered
Fixed 6 mm margin, +60°/-60° PTV size= 1004 voxels
83% of CTV voxels covered
Proposed margin expansion, +60°/-60° PTV size=909 voxels
98% of CTV voxels covered
Dto, +90°/-90°PTV size= 1140 voxels
95% of CTV voxels covered
RobustRobustConformityConformity
29
Robust Conformity indexes (RCI)
The robust conformity indexes (RC Indexes) acknowledges that robustness and conformity are concepts that can not be separated from each other. There is no sense in talking about plans that are conformal yet, not robust.Robustness should be the very first requirement of any plan.
R = tCTV / CTV (ideally =1)C= (tV-tCTV)/CTV (ideally =0)
We want to approach tV=tCTV=CTV
A clinical example
Dto, +90°/-90°CI=1.11
R=0.9 C=0
Fixed 6 mm margin, +60°/-60° CI=1.11
R=0.83 C=0
Proposed margin expansion, +60°/-60° CI=1.14
R=0.98 C=0;
Dto, +90°/-90°CI=1.09
R=0.95 C=0
RobustRobustConformityConformity
A clinical example
Nominal dose distributions
Mean dose distributions
Fixed 6 mm margin PTV(5,3)
From a Bayesian perspective, we can now assess the uncertainty in thedose at each voxel!
Dose uncertainty maps for a 6 mm margin expansion (LEFT)and the for the method described below (RIGHT)
A clinical example
Fixed 6 mm margin PTV(5,3)
There is a pleasure in recognizing old things from a new point of view.” Feynman, R., Space time approach to Non-Rel. QM, Rev. Mod. Phys, (20) 1948
Range and SU uncertainties translate into dose uncertainties and have a high value in the distal end of the particle trajectory.
A clinical example
A clinical example
Dto, +90°/-90°
Fixed 6 mm margin, +45°/-45° Proposed margin expansion, +45°/-45°
Dto, +90°/-90°
A clinical example
Dto, +90°/-90°PTV size=1451 voxels
R=0.64 C=0.1
Fixed 6 mm margin, +45°/-45° PTV size= 1451 voxels
R=0.84 C=0.23
Proposed margin expansion, +45°/-45° PTV size=1316 voxels
R=0.99 C=0
Dto, +90°/-90°PTV size= 1352 voxels
R=0.9 C=0
RobustRobustConformityConformity
What about IMPT?
What about IMPT?
What about IMPT?
What about IMPT?
Compact subvolumedid not exist for std valueslower than 9% !!!
What about IMPT?
The CTV-PTV expansion method failed to The CTV-PTV expansion method failed to assure the CTV robust coverageassure the CTV robust coverage we we
demandeddemanded
What about IMPT?
The CTV-PTV expansion method failed to The CTV-PTV expansion method failed to assure the CTV robust coverageassure the CTV robust coverage we we
demandeddemanded
Albertini F, Hug EB and Lomax AJ (2010) Is there a benefit to define safety margins for Intensity Modulated Proton Therapy Plans? Scientific Session PTCOG49 meeting
What about IMPT?
There is a benefit in defining margins in IMPT treatments, however, generally speaking a margin alone might not be sufficient to assure CTV coverage.
Ideally, Target Definition should be done in parallel with a robust optimization alg. (Unkelbach J, et al, PMB 52 (2007), 2755-2773) for beam spot modulation and a beam angle selection procedure(Cabal G, et al, An algorithm for optimizing beam angle configuration in particle therapy, PTCOG48, Heidelberg, Germany).
ConclusionsWe have developed a method for PTV definition in particle therapy.
The method produces a volume that reduces the sensibility of CTV coverage to errors and uncertainties as compare to isotropic expansions.
The obtained volumes are usually smaller compared to isotropic expansions.
We recognize treatment planning as a complex process where the right target expansion, beam spot modulation and beam arrangement are co-dependent and should be done in parallel.
Dynamic Target Definition together with alg. for robust optimization and beam angle selection offers a solution to this problem.
Thank You !
