Monte Carlo Analysis of the Dose
Enhancement in the Scalp of
Patients due to Titanium Plate
Backscatter during Post-Operative
Radiotherapy
Michael T. Hardin
Motivation
• It has been reported that patients receiving
post-operative radiotherapy are demonstrating
necrosis of the scalp superior to the placement
of titanium fixation plates.
• Healthy scalp tissue is approximately 3-5 mm
thick.
• Patients’ scalp tissue have become paper-thin
and fragile, requiring surgery.
• Synthes and Biomet titanium plates of
thickness 0.4 mm and 0.38 mm respectively.
Motivation
•
•
•
•
This is a clinically significant issue.
Why is this issue occurring?
What is the dose enhancement to the scalp?
How can this issue be remedied?
Theoretical Considerations
• Das & Kahn, 1989 divide the backscattered
dose at an interface into the dose due to:
(a) the backscattered photons
(b) the backscattering of secondary electrons set in
motion in the soft tissue medium above the
inhomogeneity
(c) the backscattering of secondary electrons set in
motion within the inhomogeneity
Das, I. J. & Kahn, F. M. (1989). Backscatter dose perturbation at high atomic number interfaces in megavoltage photon beams.
Medical Physics, 16, 367
Das, I. J. & Kahn, F. M. (1989). Backscatter dose perturbation at high atomic number interfaces in megavoltage photon beams.
Medical Physics, 16, 367
Theoretical Considerations
• The backscattered dose at interfaces in a
photon beam is primarily due to secondary
electron backscattering.
• The backscatter dose factor is nearly
independent of field size, photon energy, and
thickness above the inhomogeneity; saturates
quickly with thickness of the inhomogeneity.
Das, I. J. & Kahn, F. M. (1989). Backscatter dose perturbation at high atomic number interfaces in megavoltage photon beams.
Medical Physics, 16, 367
Titanium
Das, I. J. & Kahn, F. M. (1989). Backscatter dose perturbation at high atomic number interfaces in megavoltage photon beams.
Medical Physics, 16, 367
Clinical Measurements
• Kodak EDR2 and ISP Gafchromic MD-55 film
were used to quantify the dose enhancement.
• Titanium plates were placed inside the film
packets to replicate the clinical scenario.
• Different amounts of buildup were examined.
• Films were scanned and analyzed in RIT.
• Placement of the titanium plate.
• Phantom setup.
• Film processing and analysis in RIT.
• Results for EDR film ranged from 5% to 22%.
• Film processing and analysis in RIT.
• Results for Gafchromic film and Synthes plate
ranged from 8% to 13%.
• Results for Gafchromic film and Biomet plate
ranged from 0% to 13%.
Gafchromic Film
Relative Dose Increase vs Gantry Angle
15%
10%
Relative Dose Increase
Synthes
Biomet
5%
0%
0
15
30
45
60
75
90
105
120
-5%
-10%
-15%
Gantry Angle (Degrees)
135
150
165
180
195
Monte Carlo Modeling
• MCNPX was used to model and simulate the
transport of photons and electrons within a
box of water containing a thin titanium plate.
• Shimozato et al., 2010 found the maximum
relative differences between simulation and
TPS results on the entrance and exit sides of
the plate to be 23.1% and -12.7% respectively.
Shimozato, T., Yasui, K., Kawanami, R., Habara, K., Aoyama, Y., Tabushi, K., & Obata, Y. (2010). Dose distribution near thin titanium
plate for skull fixation irradiated by a 4-MV photon beam. Journal of Medical Physics, 35(2), 81-87.
• Dynamic 3D plotting in MCNP Visual Editor.
MCNPX
6 MV Fractional Depth Dose
1.000
0.900
Fractional Depth Dose
0.800
1.000
0.700
0.900
0.800
0.600
0.700
0.500
0.600
0.500
0.400
Water + Titanium
Water
0.400
0.300
0.300
0.200
0.200
0.100
0.000
0.100
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0.000
0
1
2
3
4
Depth (cm)
5
6
7
8
MCNPX
6 MV Dose Enhancement
30%
Percent Dose Enhancement
25%
20%
15%
Water + Titanium vs Water
10%
5%
0%
0
1
2
3
4
-5%
Depth (cm)
5
6
7
8
MCNPX
6 MV Fractional Depth Dose
1.000
0.900
Fractional Depth Dose
0.800
1.000
0.700
0.900
0.600
0.800
0.500
0.600
0.700
0.500
0.400
0.400
Water + Magnesium
0.300
0.300
Water
0.200
0.100
0.200
0.000
0.100
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0.000
0
1
2
3
4
Depth (cm)
5
6
7
8
MCNPX
6 MV Dose Enhancement
30%
Percent Dose Enhancement
25%
20%
15%
Water + Magnesium vs Water
Water + Titanium vs Water
10%
5%
0%
0
1
2
3
4
-5%
Depth (cm)
5
6
7
8
MCNPX • 6 MV
Relative Energy of Deposited Radiation
100
Cell Depth (cm)
Relative Energy
0.27
0.29
0.31
10
0.33
0.35
0.37
1
0.05
0.10
0.15
0.20
0.25
0.30
Energy Bin (MeV)
0.35
0.40
0.45
Conclusion
• The maximum relative dose enhancement on
the entrance and exit sides of the plate are
26.4% and 7.1% respectively with error < 1%.
• Future work involves research and development
of improved surgical fixation devices and/or
techniques. Additionally, what biological
mechanisms are responsible for this effect?
References
•
•
•
•
•
Das, I. J. & Kahn, F. M. (1989). Backscatter dose perturbation at high atomic
number interfaces in megavoltage photon beams. Medical Physics, 16, 367.
Feng, Y. (2006). A monte carlo simulation and deconvolution study of detector
response function for small field measurements. (Unpublished doctoral
dissertation). University of Cincinnati, Cincinnati, Ohio.
Kassing, W. M. (2001). A monte carlo investigation of the radiation dose
distribution in intravascular brachytherapy. (Unpublished doctoral dissertation).
University of Cincinnati, Cincinnati, Ohio.
Los Alamos National Laboratory. (2003). MCNP – A general monte carlo n-particle
transport code, version 5. Los Alamos, New Mexico: X-5 Monte Carlo Team.
Shimozato, T., Yasui, K., Kawanami, R., Habara, K., Aoyama, Y., Tabushi, K., & Obata,
Y. (2010). Dose distribution near thin titanium plate for skull fixation irradiated by a
4-MV photon beam. Journal of Medical Physics, 35(2), 81-87.