Study of encapsulated {sup 170}Tm sources for their potential use in brachytherapy
- Department of Atomic, Molecular and Nuclear Physics, University of Valencia, E-46100 Burjassot (Spain) and IFIC, CSIC, University of Valencia, E-46100 Burjassot (Spain)
Purpose: High dose-rate (HDR) brachytherapy is currently performed with {sup 192}Ir sources, and {sup 60}Co has returned recently into clinical use as a source for this kind of cancer treatment. Both radionuclides have mean photon energies high enough to require specific shielded treatment rooms. In recent years, {sup 169}Yb has been explored as an alternative for HDR-brachytherapy implants. Although it has mean photon energy lower than {sup 192}Ir, it still requires extensive shielding to deliver treatment. An alternative radionuclide for brachytherapy is {sup 170}Tm (Z=69) because it has three physical properties adequate for clinical practice: (a) 128.6 day half-life, (b) high specific activity, and (c) mean photon energy of 66.39 keV. The main drawback of this radionuclide is the low photon yield (six photons per 100 electrons emitted). The purpose of this work is to study the dosimetric characteristics of this radionuclide for potential use in HDR-brachytherapy. Methods: The authors have assumed a theoretical {sup 170}Tm cylindrical source encapsulated with stainless steel and typical dimensions taken from the currently available HDR {sup 192}Ir brachytherapy sources. The dose-rate distribution was calculated for this source using the GEANT4 Monte Carlo (MC) code considering both photon and electron {sup 170}Tm spectra. The AAPM TG-43 U1 brachytherapy dosimetry parameters were derived. To study general properties of {sup 170}Tm encapsulated sources, spherical sources encapsulated with stainless steel and platinum were also studied. Moreover, the influence of small variations in the active core and capsule dimensions on the dosimetric characteristics was assessed. Treatment times required for a {sup 170}Tm source were compared to those for {sup 192}Ir and {sup 169}Yb for the same contained activity. Results: Due to the energetic beta spectrum and the large electron yield, the bremsstrahlung contribution to the dose was of the same order of magnitude as from the emitted gammas and characteristic x rays. Moreover, the electron spectrum contribution to the dose was significant up to 4 mm from the source center compared to the photon contribution. The dose-rate constant {Lambda} of the cylindrical source was 1.23 cGy h{sup -1} U{sup -1}. The behavior of the radial dose function showed promise for applications in brachytherapy. Due to the electron spectrum, the anisotropy was large for r<6 mm. Variations in manufacturing tolerances did not significantly influence the final dosimetry data when expressed in cGy h{sup -1} U{sup -1}. For typical capsule dimensions, maximum reference dose rates of about 0.2, 10, and 2 Gy min{sup -1} would then be obtained for {sup 170}Tm, {sup 192}Ir, and {sup 169}Yb, respectively, resulting in treatment times greater than those for HDR {sup 192}Ir brachytherapy. Conclusions: The dosimetric characteristics of source designs exploiting the low photon energy of {sup 170}Tm were studied for potential application in HDR-brachytherapy. Dose-rate distributions were obtained for cylindrical and simplified spherical {sup 170}Tm source designs (stainless steel and platinum capsule materials) using MC calculations. Despite the high activity of {sup 170}Tm, calculated treatment times were much longer than for {sup 192}Ir.
- OSTI ID:
- 22096668
- Journal Information:
- Medical Physics, Vol. 37, Issue 4; Other Information: (c) 2010 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA); ISSN 0094-2405
- Country of Publication:
- United States
- Language:
- English
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