Investigation of inter-source and intra-source spectral variations in an electronic brachytherapy source using...

The Xoft electronic brachytherapy source is commonly used to treat superficial lesions and tumors located at shallow depths. However, uncertainties in material composition and geometry, mainly arising due to the manual assembly of the x-ray tube components, contribute to inter-source variability in the tube output spectrum. In addition, aging of the x-ray tube may lead to intra-source variability in the tube spectrum.

To investigate the inter- and intra-source variability of the spectrum for the Xoft S7500 model experimentally and through simulations, as well as to study the impact of this variability on dosimetry.

The Amptek X123 CdTe x-ray spectrometer was used to measure the spectrum of the Xoft S7500 source model. First, the spectrometer was calibrated using ¹³⁷Cs, ¹⁵²Eu, and ⁵⁷Co. Second, the source output spectrum was measured on the side and at the tip of the source at a distance of 15.5 cm from the source tip. Throughout the measurement, the source was fixed to the optical table using a 3D-printed holder and was aligned using a laser beam. The intra-source variability was studied by investigating the spectrum emitted by one single source measured in five trials. Similarly, the inter-source variability was investigated by measuring five different sources of the S7500 model at the tip and the side. The symmetry of the output was investigated by comparing the side and tip spectra. The measurements were then compared with the simulated spectra using a previously developed E-Brachy software package, taking into account the range of variation in material composition reported by the manufacturer. Finally, to determine whether or not the uncertainty in the material composition and the output spectrum affect the dosimetry by a clinically significant amount, we compared the depth dose curves created by the upper and lower limits of the range of uncertainty in the material composition.

The calibration line was obtained to correspond the detector channels to the energy in keV using linear regression. Escape peak and background corrections were performed on the spectrum, and the counts in the K-edges of cadmium (26.7 keV) and tellurium (31.8 keV) were readjusted so that the silver peaks were more clearly resolved. Intra-source variability demonstrated consistent peak resolution for tungsten, yttrium, and silver, with a coefficient of variation (CV) ranging from 0.5% to 9.8%. Inter-source variability highlighted significant differences between tip and side measurements, with up to 13.5% variation. Simulated spectra revealed the impact of material composition on the characteristic peak intensities, with the intensity of the peaks in the measured spectra lying between simulated spectra in the case of higher and lower percentage limits of yttrium and silver present in the material composition. Depth-dose simulations showed minimal differences (<1.2%) between material compositions.

This study comprehensively compared the measured and simulated spectra of the Xoft S7500 source model. The differences in the depth-dose curves are within 2% as recommended by the AAPM TG-56⁸ requirements, and the variations in the spectra for the source model S7500 lie within the recommended range.