Dosimetric Characterization of an Intensity-modulated X-Ray Brachytherapy System

An intensity-modulated X-ray brachytherapy system is being developed for various clinical applications. This new system makes it possible for clinical staff to control energy as well as dose rate for different tumor sites according to their sizes and radiobiological characteristics.

This system is mainly composed of an X-ray tube, guide tube collimation, and secondary (pseudo) target. Due to its configuration, convenient modulations of fluorescent X-ray energy and intensity are possible. To observe applicability of this novel system for various primary and secondary target combinations, Monte Carlo simulation using MCNP5 was performed, and air measurements were done. As a primary and pseudo-target combination, silver-molybdenum (Ag-Mo), tungsten-neodymium (W-Nd), and tungsten-erbium (W-Er) were used for the calculation for dose profile. Specifically, a dose distribution was calculated around each of these target combinations. Dose distributions as a function of target angles were also calculated. The Ag-Mo combination was analyzed for Cartesian coordinates of xy, xz, and yz planes of the pseudo-target to observe dose distribution as a function of the angle of secondary target.

The results showed that radial dose fall-off of Ag-Mo was greater than commercially available brachytherapy sources (¹⁰³ Pd and¹²⁵ I) due to its low characteristic X-ray energy.

Dose distribution variance should be considered in beam modulation for clinical application. Dynamic movement of the pseudo-target is feasible and remains as a subject for future research.