SU-GG-T-28: Physical Principles of Intensity Modulated Electronic Brachytherapy (IMEB)

Abstract

Purpose: To describe a new collimator design and methodology for controlling the effective intensity of radiation produced by a microminiature x-ray source which allows the modulation of dose distributions for electronic brachytherapy applications. Method and Materials: A close fitting, two-section adjustable lead collimator consisting of a lower cylindrical band and an upper cap was constructed for a 50kV electronic brachytherapy source (Xoft Inc., Fremont, CA) to provide controllable beam hardening and attenuation. Micro-indexing the source through an indwelling catheter using variable step sizes, in combination with adjustment of the collimator gap width, modifies the resulting depth-doses. Results: Depth dose increases with wider collimator aperture settings and for decreasing step sizes. For a multi-stepped plan using step sizes of 1mm, the relative dose at 1cm depth was 0.50 for a gap of 0.5mm and 0.74 for a gap of 2.0mm. For a multi-stepped plan with a constant 2mm aperture size, the relative dose at 1cm depth increased from 0.70 for a step size of 2.5mm to 0.74 for a step size of 1.0mm. Conclusion: Appropriate depth dose adjustment by microindexing and variable collimation of an electronic brachytherapy source facilitates the development of IMEB plans that can be made to conform to irregularly shaped tumor cavities or those closely bounded by critical structures. Conflict of Interest: Partial financial support was provided by Xoft, Inc.