Comparison of Measured and Calculated Exit Dose for Intracavitary Accelerated Partial Breast Irradiation with an Electronic Brachytherapy Source

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Abstracts / Brachytherapy 9 (2010) S23eS102

proportional to the square root of the photon fluence escaping the phantom. Note that the detection efficiency for the range of photon energies considered was essentially equal. Future measurements will be performed to validate the calculated results for 125I and 131Cs radionuclides. Results: Due to the higher photon energies of 125I and 131Cs, more photons were able to escape the LuciteÒ phantom without being absorbed. It was calculated that the number of photons that escape using 131Cs is 3.5 times greater than for 103Pd. For 125I it is 2.5 times greater than for 103Pd. It is evident from Figure 1 that the increased fluence reaching the gamma camera from 131Cs and 125I results in a superior CNR. Using this plot it can be demonstrated that to achieve the same CNR as achieved by a 103Pd seed at 5 cm depth in Lucite, the source would need to be at a depth of 16 cm for 131Cs, and 9.8 cm for 125I. One can also use the improved CNR exhibited by 125I or 131Cs to improve the spatial resolution of the device. The full width at half maximum spatial resolution of the gamma camera was found to be 4.9 mm for 103Pd, 2.7 mm for 131Cs and 3.2 mm for 125I for the same CNR of 10.

cm3 piece of VW and additional slabs of VW were added above the source holder to achieve source to surface distances of 3, 4, 5, and 6 cm. For each source to surface distance, TLD measurements were made with four backscatter conditions: no backscatter, FlexiShieldÒ, 15 cm of VW, and FlexiShieldÒ plus 15 cm of VW. The FlexiShieldÒ is a flexible silicone and tungsten sheet that is draped on the patient’s breast during accelerated partial breast irradiation (APBI) with the AxxentÒ source. Calibration TLDs were irradiated with 60Co. The intrinsic energy dependence of the TLDs for the AxxentÒ source relative to 60Co was determined with additional experiments and the appropriate correction was applied to the phantom measurements. Additional correction factors were applied to relate the dose measured by the TLD to the dose to water at the phantom surface. Each of the phantom geometries was modeled in the MCNP5 Monte Carlo code. The source was modeled as a point source with a realistic spectrum for the AxxentÒ source operating at 50 kVp. Also, TG-43 calculations were made for each source to surface distance using the published TG-43 parameters for the AxxentÒ source operating at 50 kVp. Results: The effect of backscatter on the TLD measured surface dose is summarized in Table 1, which shows the difference in the TLD measured dose for each backscatter condition relative to the TLD measured dose with no backscatter. Effect of Backscatter on TLD Measured Surface Doses

Figure 1: A graphical comparison of the CNR as a function of imaging time between 103Pd, 131Cs, and 125 I. Conclusions: Calculations suggest that the 131Cs radio-nuclide would be superior to 125I and 103Pd in terms of visualizing the source intraoperatively in a phantom with a gamma camera.

OR27 Presentation Time: 4:10 PM Comparison of Measured and Calculated Exit Dose for Intracavitary Accelerated Partial Breast Irradiation with an Electronic Brachytherapy Source Julie A. Raffi, M.S., Tina L. Pike, M.S., Stephen D. Davis, Ph.D., Larry A. DeWerd, Ph.D. Medical Physics, University of Wisconsin, Madison, WI. Purpose: To measure the exit dose for an electronic brachytherapy source with thermoluminescent dosimeters (TLDs) for several phantom geometries and to compare the TLD measured doses to Monte Carlo (MC) and TG-43 calculated doses. Materials and Methods: The Xoft AxxentÒ miniature x-ray source operating at 50 kVp was used for all of the measurements in this work. The phantom consisted of a combination of (30x30) cm2 and (35x35) cm2 Virtual WaterÔ (VW) slabs of varying thickness. A (30x30x2) cm3 VW holder was machined for the AxxentÒ source such that the center of the source was positioned at the center of the holder. The holder was placed on a (35x35x5)

Source to Surface Distance (cm)

Backscatter

Dose Difference vs. No Backscatter

3 3 3 3 4 4 4 4 5 5 5 5 6 6 6 6

none FlexiShield 15 cm VW FlexiShield none FlexiShield 15 cm VW FlexiShield none FlexiShield 15 cm VW FlexiShield none FlexiShield 15 cm VW FlexiShield

0% 3% 16% 4% 0% 4% 20% 6% 0% 4% 22% 4% 0% 3% 23% 4%

þ 15 cm VW

þ 15 cm VW

þ 15 cm VW

þ 15 cm VW

The TLD measured doses agreed well with the MC calculated doses when the appropriate corrections were applied. The TG-43 calculated doses agreed well with the measurements simulating full backscatter conditions (15 cm VW), but over-estimated the exit dose for the measurements made with no backscatter by up to 16% at 3 cm and by up to 23% at 6 cm. The addition of the FlexiShieldÒ increased the TLD measured dose by up to 4%, but did not compensate for the amount of backscatter assumed by TG-43 calculations. Conclusions: TG-43 calculations used for electronic brachytherapy treatment planning overestimate the exit skin dose for intracavitary APBI. TLDs can accurately measure the exit dose when the appropriate corrections are applied.

OR28

Presentation Time: 4:20 PM

Monte Carlo Simulations of the Primary Air-Kerma Strength Standard for Low-Energy Photon-Emitting Brachytherapy Sources Christopher S. Melhus, Ph.D., Mark J. Rivard, Ph.D. Radiation Oncology, Tufts Medical Center, Boston, MA. Purpose: Using the AAPM Task Group 43 Report dosimetry formalism, the dose-rate constant L for a given brachytherapy source is inversely proportional to air-kerma strength SK. Consensus L values average measured and calculated data. Previous work demonstrated that Monte Carlo transport physics and volume averaging can alter sK estimation by