Calculated and measured brachytherapy dosimetry parameters in water for the Xoft Axxent X-Ray Source: an electronic brachytherapy source.

A new x-ray source, the model S700 Axxent X-Ray Source (Source), has been developed by Xoft Inc. for electronic brachytherapy. Unlike brachytherapy sources containing radionuclides, this Source may be turned on and off at will and may be operated at variable currents and voltages to change the dose rate and penetration properties. The in-water dosimetry parameters for this electronic brachytherapy source have been determined from measurements and calculations at 40, 45, and 50 kV settings. Monte Carlo simulations of radiation transport utilized the MCNP5 code and the EPDL97-based mcplib04 cross-section library. Inter-tube consistency was assessed for 20 different Sources, measured with a PTW 34013 ionization chamber. As the Source is intended to be used for a maximum of ten treatment fractions, tube stability was also assessed. Photon spectra were measured using a high-purity germanium (HPGe) detector, and calculated using MCNP. Parameters used in the two-dimensional (2D) brachytherapy dosimetry formalism were determined. While the Source was characterized as a point due to the small anode size, < 1 mm, use of the one-dimensional (1D) brachytherapy dosimetry formalism is not recommended due to polar anisotropy. Consequently, 1D brachytherapy dosimetry parameters were not sought. Calculated point-source model radial dose functions at gP(5) were 0.20, 0.24, and 0.29 for the 40, 45, and 50 kV voltage settings, respectively. For 1<r<7 cm, measured point-source model radial dose functions were typically within 4% of calculated results. Calculated values for F(r, theta) for all operating voltages were within 15% of unity along the distal end (theta=0 degree), and ranged from F(1 cm, 160 degrees) = 0.2 to F(15 cm, 175 degrees) = 0.4 towards the catheter proximal end. For all three operating voltages using the PTW chamber, measured dependence of output as a function of azimuthal angle, psi, was typically on average +/-3% for 0 degree < or = psi < or = 360 degrees. Excluding an energy response function, measurements of normalized photon energy spectra were made for three operating voltages, and were typically within 2% agreement with the normalized Monte Carlo calculated spectra. In general, the model S700 Source exhibited depth dose behavior similar to low-energy photon-emitting low dose rate sources 125I and l03Pd, yet with capability for variable and much higher dose rates and subsequently adjustable penetration capabilities. This paper presents the calculated and measured in-water brachytherapy dosimetry parameters for the model S700 Source at the aforementioned three operating voltages.