Air-kerma strength determination of a new directional (103)Pd source

Abstract

Purpose: A new directional (103)Pd planar source array called a CivaSheet™ has been developed by CivaTech Oncology, Inc., for potential use in low-dose-rate (LDR) brachytherapy treatments. The array consists of multiple individual polymer capsules called CivaDots, containing (103)Pd and a gold shield that attenuates the radiation on one side, thus defining a hot and cold side. This novel source requires new methods to establish a source strength metric. The presence of gold material in such close proximity to the active (103)Pd region causes the source spectrum to be significantly different than the energy spectra of seeds normally used in LDR brachytherapy treatments. In this investigation, the authors perform air-kerma strength (S(K)) measurements, develop new correction factors for these measurements based on an experimentally verified energy spectrum, and test the robustness of transferring S(K) to a well-type ionization chamber.

Methods: S(K) measurements were performed with the variable-aperture free-air chamber (VAFAC) at the University of Wisconsin Medical Radiation Research Center. Subsequent measurements were then performed in a well-type ionization chamber. To realize the quantity S(K) from a directional source with gold material present, new methods and correction factors were considered. Updated correction factors were calculated using the MCNP 6 Monte Carlo code in order to determine S(K) with the presence of gold fluorescent energy lines. In addition to S(K) measurements, a low-energy high-purity germanium (HPGe) detector was used to experimentally verify the calculated spectrum, a sodium iodide (NaI) scintillating counter was used to verify the azimuthal and polar anisotropy, and a well-type ionization chamber was used to test the feasibility of disseminating S(K) values for a directional source within a cylindrically symmetric measurement volume.

Results: The UW VAFAC was successfully used to measure the S(K) of four CivaDots with reproducibilities within 0.3%. Monte Carlo methods were used to calculate the UW VAFAC correction factors and the calculated spectrum emitted from a CivaDot was experimentally verified with HPGe detector measurements. The well-type ionization chamber showed minimal variation in response (<1.5%) as a function of source positioning angle, indicating that an American Association of Physicists in Medicine (AAPM) Accredited Dosimetry Calibration Laboratory calibrated well chamber would be a suitable device to transfer an S(K)-based calibration to a clinical user. S(K) per well-chamber ionization current ratios were consistent among the four dots measured. Additionally, the measurements and predictions of anisotropy show uniform emission within the solid angle of the VAFAC, which demonstrates the robustness of the S(K) measurement approach.

Conclusions: This characterization of a new (103)Pd directional brachytherapy source helps to establish calibration methods that could ultimately be used in the well-established AAPM Task Group 43 formalism. Monte Carlo methods accurately predict the changes in the energy spectrum caused by the fluorescent x-rays produced in the gold shield.

FIG. 1.

A schematic of a sample CivaSheet™ array of nine dots. The dots are all oriented such that the shields are on the same side and are embedded in a flat bioabsorbable membrane. Fenestrations manufactured in the material are shown as white circles. Please note that the illustration is not to scale.

FIG. 2.

A schematic showing the basic components of a CivaDot in a cross-sectional view. The schematic illustrates the region of ¹⁰³Pd and the gold layer for shielding. The cold direction corresponds to the side of the dot with the gold shield. Please note that the illustration is not to scale.

FIG. 3.

Schematic of the measurement setup with a CivaDot in its fixed measurement position with the UW VAFAC. Please note that the illustration is not to scale.

FIG. 4.

Schematic of a CivaDot in its PMMA window-frame holder for UW VAFAC measurements. Please note that the illustration is not to scale.

FIG. 5.

Diagram of the CivaDot holder for use with a Standard Imaging HDR1000 Plus well-type ionization chamber. The CivaDot shown in the figure is held by securing the surrounding bioabsorbable membrane in a hinged frame holder. Please note that the illustration is not to scale.

FIG. 6.

Results of the HPGe spectra measurements of a typical CivaDot (shown in black) and a specially constructed dot without the gold shield (shown in gray) normalized to maximum count.

FIG. 7.

Shown in black are the results of the HPGe differential measurements for a typical CivaDot and a special dot without the gold shield. Also plotted in gray is the mcnp6 Monte Carlo calculated difference spectrum for the CivaDots.

FIG. 8.

In-air fluence data from a CivaDot measured with a NaI detector are plotted in black. The mcnp6-predicted fluence distribution is plotted in gray. Notice the significant asymmetry due to the presence of the gold shield.

FIG. 9.

Results of the change in ionization current with a CivaDot secured in a custom holder within a Standard Imaging HDR1000 Plus well-type ionization chamber as a function of angle. The values for a given dot are normalized to the average of the five readings for each dot. The fifth reading was acquired at the initial zero degree orientation to evaluate repeatability.