Effect of wetness level on the suitability of wet gauze as a substitute for Superflab as a bolus material for use with 6 mv photons.

Despite the availability of commercial tissue equivalent bolus material, wet gauze has an application in radiation therapy to provide superior conformance to irregular contours. Wet gauze bolus has the potential to reduce air gaps between the bolus and surface, which could decrease surface dose if sufficiently large to disrupt electronic equilibrium. Wet gauze bolus is often fabricated and wetness judged qualitatively. We assessed the effect of specific gauze wetness levels, quantified in terms of physical density, at various field sizes with respect to their effectiveness as bolus material compared with Superflab. For large fields, > 7 x 7 cm2 in this study, wet gauze sponges with a physical density of 1.02 g/cm3 performed essentially identical to Superflab; at a smaller field size the wet gauze was slightly less effective, likely due to the heterogeneity of the gauze-water matrix. Gauze that was wetter, with a physical density of 1.2 g/cm3, or less wet, with a physical density of 0.75 g/cm3 was not as effective either due to enhanced photon absorption in the wetter sponges, or less effective establishment of electronic equilibrium in the less wet sponges. The presence of an air gap under Superflab led to reduced surface dose, especially for small fields and large air gaps. Thus, if Superflab use leads to poor contact with the skin, wet gauze having a physical density of 1.02 g/cm3 can be used as a substitute. Judging the water content of wet gauze subjectively is not acceptable as over- or under wetness can lead to decreased effectiveness of the bolus material.