Session Item

The INTRABEAM system (Carl Zeiss Meditec AG) is an electronic brachytherapy device designed for intraoperative radiotherapy (IORT) applications. Despite its benefits and extended use for common diseases as brain and breast cancers, the INTRABEAM x-ray source has not been characterized according to the AAPM TG-43 specifications for brachytherapy sources. This restricts its modeling in commercial treatment planning systems (TPSs), with the consequence that the doses to organs at risk (OARs) are unknown. Knowledge of these doses is typically important when dose distributions need to be compared and combined with external beam dose distributions. The aim of this work is to characterize the INTRABEAM source according to the TG-43 brachytherapy dosimetry protocol.

The dose distribution in water around the INTRABEAM source was determined with Monte Carlo (MC) calculations using egs_brachy, a user code of EGSnrc. MC statistical uncertainties were in the range 0.1% to 0.4% at 1 to 5 cm from the source tip in its longitudinal axis. For the validation of the MC model, depth dose calculations in water along the source longitudinal axis were compared with measurements in two different setups: (1) using a water phantom provided by the source manufacturer and a soft x-ray ionization chamber (PTW 34013) and (2) with a customized setup using a Wellhöfer water tank and synthetic diamond detectors (microDiamond PTW TN60019), with low volume averaging effects and uncertainties from the detector geometry. The calculated radial dose function for the INTRABEAM is compared with published data for the Xoft Axxent® (a subsidiary of iCAD, Inc. Nashua, NH) source, and several commonly used HDR and LDR brachytherapy sources.

Measurements in water with the ionization chamber agreed with the MC model calculations within uncertainties. These combined uncertainties vary with depth in water and have an approximate value of 3.1% at 1 cm from the source tip. The use of the microDiamond yielded local percent differences within uncertainties in points of steeper dose gradients. The radial dose function (Figure 1) presents a steep fall-off close to the INTRABEAM source (< 1 cm) with a gradient higher than that of conventional brachytherapy radionuclides (¹⁹²Ir, ¹⁰³Pd, and ¹²⁵I), but it is partially flattened at larger distances with a similar fall-off as the Xoft source. The simulated 2D anisotropy values (Figure 2) were mainly uniform along θ = 0° for r > 1 cm, and gradually decreased towards θ ≈ 120°. For regions close to the source, the behavior was strongly affected by the beam attenuation in the elements of the source walls.