Dose Enhancement Effect of a Raster Based Surface Applicator for Large Field Surface Brachytherapy

Abstract

Purpose: Typical delivery for brachytherapy surface dose use uniform channels in a HAM applicator, specially molded applicators with non-uniform channels, or Leipzig style cones. A unique method using a robotic arm to move a source in raster fashion at fixed distance over a surface has been developed. In this method, the source is placed inside a spherical tip which contacts the treatment surface, which allows a lateral air gap. For HDR systems using low energy, such as the 50 kV Xoft electronic brachytherapy system, air gaps will modify the deposited dose relative to homogenous water computations. This study examines the effects of the air gap in raster delivered large fields at various depths. Materials and Methods: The Xoft 50 kV S700 source was modeled at the center of a 10 mm radius water equivalent sphere in contact with a flat water equivalent surface. Dose at and below the surface was computed in Matlab using TG-43 parameters. The air gap was modeled using a shifted radial dose function, g(r’), where r ‘= r – t(air), where the air gap, t(air), is the path length from sphere to surface. The 3D dose for a single dwell position was used to create square field sizes ranging from 4 to 20 cm using uniformly spaced dwell points of 5 mm step sizes. The air shifted dose was compared to an unshifted source in an infinite water bath. Calculations were repeated for sphere tip sizes of 5 mm radius and 20 mm radius. Results: For a single dwell position, the dose with depth down the axis between the source and the tangent of the sphere and surface is unchanged. For calculation points away from this central axis, a dose enhancement effect is noted which increases nearly linearly with distance. For example at the surface a 2-fold dose increase is predicted at 2.5 cm from the central axis (14.7% with air gap vs. 7.7 % without) and a 10-fold increase is noted at 10 cm distance (10.3% with air gap, 1.0% without air gap). For large fields, surface dose is increased by the air gap, which increases with field size. For 4, 10 and 20 cm fields the surface dose is enhanced by the air gap by 46%, 56% and 83% relative to the infinite water calculations. Although the depth dose for a single source position remains unchanged, the relative depth dose decreases with square field size. For a 10x10 cm field, the surface normalized depth dose at 5 mm depth drops by 20 %, from 62.5% with no air gap to 50.2% with an air gap. At 10 mm depth the normalized PDD drops by 26%, from 42.3% to 31.4%. This air gap enhanced depth dose reduction, or beam softening effect, increases with field size. Reduction of the spherical tip from 10 mm radius to 5 mm radius increased the dose enhancement and depth dose reduction effects, while increasing the tip size to 20 mm radius produced the opposite result. Conclusions: A robotic raster type delivery system has the potential to deliver any field shape or size. Unlike multi-channel applicators, which are limited in size to applicators on hand, a raster method is limited only by the reach or size of the robotic raster mechanism. The air gap for a spherical tip enmhanced the dose at the surface for a 50 kV source. The enhancement effect increases with field size and reduced spherical tip size. A decrease in depth dose is observed for smaller tip sizes or larger field sizes.