Mark J. Rivard, PhD
Tufts University School of Medicine New England Medical Center, Boston, MA
Brachytherapy Dosimetry Parameters for the Xoft Electronic Brachytherapy Source
CIRMS 2005, October 25th, 2005
Full Disclosure
The Xoft Inc. AXXENT™ devices included is not yet FDA-approved for clinical use.
Dr. Rivard serves as a consultant for Xoft Inc.
BackgroundBrachytherapy: Application of radiation therapy at short distances. Radiotherapy surface application (plesiotherapy) or within the human body.
How to accurately administer brachytherapy for electronic brachytherapy sources?Once source strength & calibrations are in-hand, what are the next steps towards clinical implementation?
Brachytherapy Dosimetry Geometry
2-D Brachytherapy Dosimetry Formalism, AAPM TG-43U1 (2004)
D(r,θ) = SK Λ gL(r) GL(r,θ) F(r,θ)GL(r0,θ0)
D(r,θ) dose rate to water at point P(r,θ)SK air kerma strengthΛ dose rate constant
gL(r) radial dose functionGL(r,θ) geometry function (line source approximation)F(r,θ) 2-D anisotropy function
AXXENT BrachytherapyDosimetry Formalism
D(r,θ) = SK Λ gP(r) r02 F(r,θ)
r2
-simplified geometry function due to L < 1 mm-accurate application of 2-D anisotropy function
Xoft Depth-Dose Characteristics Source designed using MCNP to emulate 192Ir dose characteristics
1.E-04
1.E-03
1.E-02
1.E-01
1.E+00
1.E+01
1.E+02
1.E+03
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0
Radius (cm)
Dos
e R
ate
(cG
y/m
in)
50 kV50 kV MCNP5Pd 103I 125Ir 192
Similar dose rate to 10 Ci HDR 192Ir over region of interest
(Gy/
min
ute)
AXXENT BrachytherapyDosimetry Formalism
D(r,θ) = SK Λ gP(r) r02 F(r,θ)
r2
radial dose function
• PTW 34013 soft x-ray chamber on an XYZ stage in water
• Monte Carlo model using MCNP5 with EPDL97-based mcplib04 cross-section library, mono-energetic electrons incident on target, modeling included all source components
• Measurements have SD ≤ 5% at each operating voltage, and agree with MCNP5 within ± 5% from 1 to 7 cm
Xoft Radial Dose Functions
AXXENT BrachytherapyDosimetry Formalism
D(r,θ) = SK Λ gP(r) r02 F(r,θ)
r2
2-D anisotropy function
2-D Anisotropy Function, F(r,θ)• At a 3 cm radius, ratios of MCNP-to-measured values at each angle
average 1.014 with a SD of 5.6% for 40, 45 and 50 kVp
• At a 5 cm radius, ratios of MCNP-to-measured values at each angle average 1.035 with a SD of 4.9% for 40, 45 and 50 kVp
• F(r,θ) varies slowly with depth for r > 2 cm
90°
180°
0°
AXXENT BrachytherapyDosimetry Formalism
D(r,θ) = SK Λ gP(r) r02 F(r,θ)
r2
pre-Tx air kerma strength measurements
Agreement Between Meas. & Calcs.Radiochromic film compared to RTP output - favorable results(Sou-Tung Chiu-Tsao)
D (Gy)34 Red17 Orange10.2 Yellow6.8 Green5.1 Blue3.4 Dark Blue1.7 Magenta
Film 40A01
D (Gy)34 Red17 Orange10.2 Yellow6.8 Green5.1 Blue3.4 Dark Blue1.7 Magenta
Film 40A01
D (Gy)34 Red17 Orange10.2 Yellow6.8 Green5.1 Blue3.4 Dark Blue1.7 Magenta
Film 40A01
X-Ray Source
D (Gy)
34 Red
17 Orange
10.2 Yellow
6.8 Green
5.1 Blue
3.4 Dark Blue
1.7 Magenta
40 kV Probe in BrachyVision®
D (Gy)
34 Red
17 Orange
10.2 Yellow
6.8 Green
5.1 Blue
3.4 Dark Blue
1.7 Magenta
40 kV Probe in BrachyVision®
Good agreement betweenionization chamber results
and Monte Carlo calcs
All Xoft case shown are simulated. Isodose lines represent dose as determined by treatment planning system using Xoft TG-43 parameters.CTs of Xoft balloon in water bath are superimposed on patient CT images. In no cases have balloons been implanted in actual patients.
Transverse Plane #1
MammoSite / Ir-192 Xoft / 50 kV*
Transverse Plane #2
MammoSite / Ir-192 Xoft / 50 kV*
All Xoft case shown are simulated. Isodose lines represent dose as determined by treatment planning system using Xoft TG-43 parameters.CTs of Xoft balloon in water bath are superimposed on patient CT images. In no cases have balloons been implanted in actual patients.
All Views, MammoSite / Ir-192
All Xoft case shown are simulated. Isodose lines represent dose as determined by treatment planning system using Xoft TG-43 parameters.CTs of Xoft balloon in water bath are superimposed on patient CT images. In no cases have balloons been implanted in actual patients.
All Views, Xoft
All Xoft case shown are simulated. Isodose lines represent dose as determined by treatment planning system using Xoft TG-43 parameters.CTs of Xoft balloon in water bath are superimposed on patient CT images. In no cases have balloons been implanted in actual patients.
Clinical Implementation of eBxNo NRC radioactive materials license needed
Consider AAPM Task Group Reports and Guidance–TG-43 Brachytherapy Dosimetry Formalism–TG-56 Code of Practice for Brachytherapy–TG-59 High Dose Rate Tx Delivery
Comparisons of measured and calculated dose distributions, development of consensus dataset as established for radionuclide-based Bx
Need development of eBx NIST calibration standard for consistent clinical implementation
Clinical Implementation of eBx
Clinical Implementation of eBx
Safety ?
Efficacy ?
Accuracy ?
Consistency ?
Clinical Implementation of eBx
Safety ?
Efficacy ?
Accuracy ?
Consistency ?
Summary• Dose distributions at 40, 45, and 50 kVp have been comprehensively
characterized using ionization chambers and Monte Carlo modeling
• Dose distributions distilled into clinical brachytherapy dosimetry parameters using hybrid formalism compatible with current treatment planning systems
• Additional measurements and calculations needed to validate results
• Accurate measure and standardization of source strength via a primary standards laboratory (e.g., NIST) will help facilitate clinical implementation.
• Other manufacturers are considering pursuing eBx …
