Dosimetric Impact of Density Variations In Wet Towel Bolus: Implications for Clinical Standardization
Abstract
Purpose
To recommend a standardized bolus procedure where traditional bolus is not clinically appropriate.
Methods
Towels (100cm2) in various arrangements were saturated with different amounts of water from 10cc to 30cc, scanned with a Siemens SOMATOM CT scanner and imported into Monaco TPS. Electron densities were evaluated. An anthropomorphic phantom was scanned and a clinically realistic vulvar plan was created in Monaco, using 0.8cm of density 1.0 bolus. A plan with a 10x10 field delivered to solid water with 0.5cm of 1.0 density bolus was also created. The plans were delivered with equivalent thicknesses of 20cc wet towel to simulate planned bolus, and MOSFETs in various locations recorded surface dose. The vulvar plan was then recalculated using the realistic density and thickness of 20cc wet towel.
Results
For a 100cm2 wet towel bolus, 20cc is the optimal amount of water to be fully wet without dripping and used in subsequent experiments and adopted as standard for clinical implementation. It has a relative electron density of 0.678. However, when layering more than two, the density was measured to be 0.788. The measured dose on the anthropomorphic phantom exceeded the bolus planned dose at all measured points, with an average of 112%. The measured dose measured on the solid water exceeded the planned dose at all measured points, with an average of 110%. The measured dose on the anthropomorphic phantom matched the wet towel planned dose at all measured points within 2%.
Conclusion
20cc of water per 100cm2 of towel should be used when creating wet towel bolus. If wet towel is indicated, planning using real thicknesses and standardized densities as recommended will create a more realistic surface dose in the TPS.