Document Type

Conference Proceeding

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Publication Title

Medical Physics


Purpose: Electron beams are often used to treat superficial lesions of the lip, cheek, nose and ear. Lead is frequently used to block distal structures. It is customary to place an internal bolus of low atomic number in between the tissue and the lead to reduce electron backscatter from the lead. Space for the lead and the internal bolus is quite limited. The current method, recommended by TG25, for estimating the thickness of the lead plus internal bolus is not self-consistent and leads to a larger than necessary thickness. Furthermore, this method uses an older expression for the electron backscatter that is not applicable for energies less than 3 MeV at the lead interface. A new method is described here to provide a quick, accurate and self-consistent estimate of the minimum necessary thickness of the internal bolus and the lead for incident electron beam energies of 4, 6, 8, 9 and 10 MeV as a function of the thickness of the overlying tissue (plus external bolus if present). This method limits the dose enhancement at the tissue/bolus interface due to the underlying lead to 10%. Methods: The results of Monte Carlo calculations for the electron backscatter factor have been used to self-consistently calculate the minimum necessary internal bolus thickness. Results: A graph is provided thatallows the user to quickly read the necessary internal bolus and lead thickness given the incident electron energy and the thickness of the tissue to be treated. Conclusion: A very rapid and more accurate methodology has been developed to determine the most space economical internal bolus and lead thickness for electron treatments requiring internal shielding





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American Association of Physicists in Medicine 65th Annual Meeting & Exhibition, July 23-27, 2023, Houston, TX