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An Experimental Study of High-Z Thin Layers for Surface Dose Enhancement

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H Zhang

H Zhang1*, P Zhou1, n wakai1,2, I Das1, (1)Department of Radiation Oncology, Indiana University school of medicine, Indianapolis, IN, (2) Department of Radiation Oncology, Osaka University school of medicine, Osaka, Japan

SU-E-T-41 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

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 3x3 to 10x10 cm².
Results: The high-Z layers significantly increase the surface dose for all field sizes. For 10x10 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 3x3 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.

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