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Design and Implementation of Internal Shielding to Reduce Electron Back Scattering

M Xu1*, R Foster1, S Mihalcik1,2, (1) Northwestern Medicine Cancer Centers, Geneva and Warrenville, IL, (2) Radiation Oncology Consultants, Ltd., Oak Brook, IL

Presentations

PO-GePV-P-32 (Sunday, 7/25/2021)   [Eastern Time (GMT-4)]

Purpose: The objective was to report implementation of internal shielding to reduce electron back scattering in mouth-lip treatment.

Methods: A customized mouth guard was designed to reduce electron back scattering (EBS) to protect the normal organs beyond the irradiated area. The mouth stent was made by a commercially available BiteLok by Klarity with lead foil to shield the transmission radiation. EZ-bolus pellets were coated surrounding lead to attenuate the electron back scattering in the interface of lead and internal normal tissue. The delivered dose was verified by OSLD in vivo dosimetry confirmed the prescribed dose.

Results: An incident electron beam of 6 MeV was used to treat the upper lip of 1.0-cm thickness. Additional 1.0-cm thick bolus was applied to the lip surface to ensure the skin dose achieved. The energy Em at 2.0-cm depth was 1.76 MeV. The minimum lead thickness for shielding Em was 0.9 mm. Based upon the characterization on the interface of lead and soft material, EBS was estimated around 67%. The depth upstream in tissue for EBS approximately was estimated to be 5.9-mm range for a 10% transmission of EBS. These obtained thickness values of lead and EZ-bolus provide fundamental parameters in the mouth guard/stent design. An OSLD in vivo dosimetry further verified 1.8% deviation in measured and prescribed doses in the device used.

Conclusion: The commercial BiteLok and EZ-bolus can be utilized to make an internal shielding device as a mouth guard to shield electron irradiation and reduce EBS. The design can be implemented quickly and reliably in clinic electron beam setup. In vivo dosimetry further verifies the shielding effects met design goal on the transmitted dose through lead and residual EBS.

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