Transcript abdulhamid-chaikh-grenoble-university-hospital

Analysis by Monte-Carlo simulations of
the characteristics of nano and micro
dosimeters for real time measurements
in radiotherapy and medical physics
Abdulhamid ChaikhPhD, Jacques BalossoMD,PhD
Grenoble University Hospital, Department of Radiation Oncology
University Grenoble-Alpes, Grenoble, France
Micaela CunhaPhD, Etienne TestaPhD, Michaël BeuvePhD
Université de Lyon, Université Lyon 1, CNRS, France
IN2P3, UMR 5822, IPNL, F-69622 Villeurbanne, France
E-mail: [email protected]
A.Chaikh et al®, August 11-13-2015 Frankfurt
Context
Introduction and purpose
Materials and methods
Results and discussion
Conclusion and perspectives
2
Introduction
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Radiotherapy & medical physics
The goal of radiotherapy is to deliver a radiation dose to treat the cancer using XRay generators :
 Maximizing the Tumor Control Probability
 Minimizing Probabilities of Normal Tissue Complications (organs at risk)
 By using multiple “cross fired” beams varying from a technique to another
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A treatment plan must be calculated and validated, two methods are available:
 Physical model: Using physical quantities and statistics (DVH)
 Measurements : “In vivo dosimetry” in real time using a dosimeter
o Currently : at beam entrance using a macro semiconductor placed on the patient skin
o More recently : an implantable micro dosimeter placed in the target volumes
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Tolerance constrain between planned dose and measured dose: ± 5%
3
Introduction
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In-vivo dosimetry with implanted micro
dosimeter
Example of dose measurement using a µ-dosimeter : DorGaN project -France
IrradiationOff
ON
Irradiation
Optical fiber
Fiber connector
µ -dosimeter (900 µm)
Gallium Nitride
Patient
Linked
Optical fiber
Bi-channel
Photo detection
Dose (Gy)
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Purpose
Available mico/macro dosimeters for
radiotherapy
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Purpose
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•
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Why do we need an implantable
µ/nano dosimeter?
In practice radiotherapy:
 93% of “In-vivo dosimetry” is based on diodes in France (ASN, 2013)
 Entrance dose
The available dosimeters are imperfect and need a correction factors
Ideal in-vivo dosimeter desired
 Implanted micro / nano dosimeter
 High accuracy and high precision < 5%
 Reproducibility < 2%
 No correction factors
Intended clinical use:
 Real-time absolute dose monitoring
at target volume in the patient
 Toward in-situ dosimetry and dose
guided radiotherapy
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Materials
& methods
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Monte-Carlo simulations
Monte-Carlo simulation method, widely used for radiotherapy:
 Modeling linear accelerator in medical physics
 Tracks individual particle histories (photon /electrons)
 Dose calculation
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Materials
& methods
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Monte-Carlo simulations
Monte-Carlo simulations were carried out to:
 Evaluate the influence of the dosimeter size on the
measured dose
 Characterize the size of micro / nano dosimeter for
radiotherapy
o As small as possible
o With high accuracy ( < 5%) and high
reproducibility
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Principle :
 Estimate the level of dose fluctuations
 Determine the probability p (%) of error in dose
measurements
http://www.wienkav.at/kav/kfj/91033454/physik/emc/emc.htm
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Materials
& methods
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Simulations of micro/nano dosimeter
Simulation of the irradiation of a water volume with photons
 X = Y = Z: 50 to 200 µm
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Cylindrical targets simulate the dosimeters :
 Placed in the irradiated water volume (“water space”)
 Density equivalent to water
 The target length was set as equal to the diameter :
 Smallest radius (nm): < 1 µm
 Intermediate radius (µm) : 1 µm to 9 µm
 Largest radius (µm): ≥ 10 µm
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Materials
& methods
Simulations of micro/nano dosimeter
Water
volume
Y (µm)
Source
modelling
Dosimeter
Beam
60Cobalt
Z (µm)
X (µm)
• Scheme of the transversal view of the irradiated water
phantom
• The nano dosimeter (circle) is placed
• The dots represent the energy transfer points after the
interaction of the electrons with the medium
dosimeter
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Materials
& methods
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60Cobalt
Modelization of doses in the targets
source was simulated to generate the photon beam:
 Irradiate the water phantom with 1.3 MeV photons
 Doses simulated using only electrons generated by Compton effect
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The simulated doses were :
 Lower dose : 0.1Gy – 1Gy: delivered dose by one beam
 Intermediate dose : 1Gy – 2Gy: daily fraction on clinical routine
 Higher dose : > 2 Gy : hypofractionated treatment plans
electron
60Cobalt
Deposited dose
Scattering
photon
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Materials
& methods
Measurements of deposited doses in the
targets
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Using the concept of micro-dosimetry :
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The specific energy “zt” in the target is defined as the cumulated energy transferred by
the radiation over the mass “mt” of the target :
 zt = ε / mt
 <zt> is the mean specific energy in the targets over many irradiation
configurations with the same dose D
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The probability (p%) that a measurement yields a value outside of confidence
intervals :
 [<z>- γ *<z> ; <z> + γ *<z>]
 γ varied from to 3% to 10%
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Results &
discussion
Influence of dosimeter size on the
measured doses
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Largest radius : 10 µm-dosimeter
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The effect of fluctuations is less significant than in the other cases
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The distributions of specific energy are Gaussian curves
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The zt values at the peak match the average specific energy in the targets
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The relative width of the distributions decreases as the irradiation dose increases
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The increase in the dose resulted in a higher number of energy-transfer points and thus
in a reduction of the relative statistical fluctuations
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Note: <zt > corresponds to 86 - 87% of the irradiation dose since a part of the
energy is converted to heat and is not considered
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Results &
discussion
Influence of dosimeter size on the
measured doses
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Intermediate radius: 1 µm < r < 10 µm dosimeter
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The dose distribution is no longer symmetrical, showing a tail at higher values of
specific energy
As the irradiation dose increases, the distribution peak shifts to higher values of
specific energy, closer to the value of <zt>
As the dose and radius increase the distribution of energy tends to a Gaussian curve
Probability distribution of specific energies :
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•
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1 µm, 0.1 Gy
1 µm, 1 Gy
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Results &
discussion
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Influence of dosimeter size on the
measured doses
For the smallest radius : r ≤ 0.1 µm nano-dosimeter
 The effect of fluctuations is very significant: very large range of specific
energies a nano-dosimeter may receive
 The dosimeter is very likely to receive no energy at all
 The shape of the distribution is:
o Characterized by one photon interaction
o Independent of the irradiation dose
0.1 µm, 0.1 Gy
Probability distribution of specific energies
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Results &
discussion
Influence of dosimeter size on the
measured doses
• Dose effect with a small radius of 0.3 μm

Lower doses ≤ 0.3 Gy :
o Structure close to the one of 0.1 μm
dosimeters
 Dose values ≥ 1 Gy :
o The shape is similar to that of 1 μm
dosimeters
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Probability distribution of specific energies
0.3 µm, 0.1 Gy
0.3 µm, 3 Gy
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Results &
discussion
Influence of dosimeter size on the
measured doses
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Higher delivered dose using micro dosimeter
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Conditions:
 Irradiated dose 10 Gy
 Radius : 10 µm
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Gaussian curve
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Measured dose in the target 8.1 Gy
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Error of measurements  20 %
10 µm, 10 Gy
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Results &
discussion
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Probability of dose measurements p (%)
Probability p (%) to obtain dose measurements outside the range
[<z> -γ * <z> ; <z> + γ * <z>] with γ varing from 3 to 10%
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For the same radius:
 A smaller dose results in a higher p %,
which in turn decreases for a larger γ
For the same dose:
 p % decreases as the radius increases
 p % is lower for a larger interval
around <z>
In particular :
 p % is equal to zero when “r =10 μm”,
“D= 10 Gy” and γ is “5% or 10%”
 This means that in these cases all the
specific energies are contained in the
interval considered.
0.1 Gy
0.3 Gy
1 Gy
3 Gy
10 Gy
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Conclusion &
perspectives
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Characterization of µ/nano dosimeter
Characterization of the size of an implantable dosimeter at µ and nano scales for
clinical use with radiation oncology
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The specific energy probability distributions is strongly dependent on :
 Target size radius
 Delivered dose level
 A dose value < 0.3 Gy, none of the dosimeter radii would allow for a reproducible
measurement of the irradiation dose
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The best results obtained
 With a µ-dosimeter “r = 10 µm”
 Distributions of energy is close to Gaussian curve
 But still ~ 20 % of the measurements would be outside the interval confidence
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Conclusion &
perspectives
Characterization of µ/nano dosimeter
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The ability of the dosimeter to yield measurements is dependent on
 Size
 Deposited dose
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Strong correlation between the accuracy of measured doses and the dosimeter size
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An excessively small radius renders the measurements chaotic and not statisticallyreproducible, even for a dose as high as 10 Gy
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A target radius of 10 μm may allow for a better reproducibility of the
measurements in a wider range of doses
• Recommended radius of dosimeter for radiotherapy “r > 10 µm” to
satisfy the dose tolerance of ± 5%
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Reference
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[1] Abdulhamid Chaikh, Michaël BEUVE, Jacques BALOSSO. Nanotechnology in Radiation Oncology.
Int J Cancer Ther Oncol 2015 ; 3(2): 3217; DOI: 10.14319/ijcto.32.17
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[2] Abdulhamid Chaikh, Arnaud GAUDU, Jacques Balosso. Monitoring methods for skin dose in
interventional radiology. Int J Cancer Ther Oncol; 3(1):03011. DOI: 10.14319/ijcto.0301.1.
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[3] Chaikh A, Balosso J, Giraud JY, Wang R, Pittet P, Luc GN. Characterization of GaN dosimetry for
6MV photon beam in clinical conditions. Radiation measurements; 2014: 392-395.
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[4] http://www.wienkav.at/kav/kfj/91033454/physik/emc/emc.htm
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[5] Gervais B, Beuve M, Olivera G H, Galassi M E. Numerical simulation of multiple ionization and high
LET effects in liquid water radiolysis. Radiat. Phys. Chem 2006; 75(4):493-513.
Acknowledgements
• The authors acknowledge the financial support of the French
National Research Agency (ANR-11-TECS-018)
Remerciements :
• France HADRON
• The PRIMES “LabEx”
• Dr. Patrick Pittet
• Dr. Jean Yves Giraud
A.Chaikh et al®, August 11-13-2015 Frankfurt
Thank you for your attention
[email protected]