SU-E-T-41: An Experimental Study of High-Z Thin Layers for Surface Dose Enhancement

Abstract

Purpose: To experimentally evaluate the surface dose enhancement associated with thin layers of high‐Z materials in external beam radiotherapy for a potential use as a bolus or beam spoiler. Methods: Dose enhancement of high‐Z thin materials for surface dose in a 6MV beam was experimentally studied using 0.1 and 0.2 mm of Pb and Sn layers. The purity of each high‐Z layer is better than 99.6%. The high‐Z layer was positioned at the surface of the solid water phantom. A parallel plate ion chamber with window thickness of 0.03 mm was used to measure the doses in the downstream of the high‐Z thin layer. The results were compared with the dose measurement in the open field. The sizes of high‐Z layers were 15x15 cm2. The radiation fields were from 3×3 to 10×10 cm2. Results: The high‐Z layers significantly increase the surface dose for all field sizes. For 10×10 cm field, 0.1 mm Pb and Sn increase doses by 152% and 74% respectively and by 15% and 9% at the 2 mm depth. For 0.2 mm Pb and Sn layers the doses were increased by 198% and 111% at the skin, and 24% and 15% at 2 mm depth respectively. The dose increase at the surface by high‐Z layers is more remarkable at the smaller field sizes. At 3×3 cm2 field size, 0.2 mm Pb and Sb can respectively increase skin doses about 286% and 164%. It was also found that when the depth is greater than 1 cm, the doses with high‐z thin layer is close to that with the open field within 2% difference. Conclusion: High‐Z material thin layers such as Pb and Sn can significantly increase the surface doses without affecting the distant region; therefore they can be used as an alternative of conventional bolus.