Program Information

Construction and Evaluation of a Neutron Wall to Shield a 15 MV Linac in a Low-Energy Vault

M Speiser*, F Hager, R Foster, T Solberg, UT Southwestern Medical Center, DALLAS, TX

SU-E-T-267 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

Purpose: To design and quantify the shielding efficacy of an inner Borated Polyethylene (BPE)
wall for a 15 MV linac in a low energy vault.
Methods: A Varian TrueBeam linac with a maximum photon energy of 15 MV was installed in a
smaller, preexisting vault. This vault originally housed a low-energy machine and did not have
sufficient maze length recommended for neutron attenuation. Effective dose rate calculations
were performed using the Modified Kersey's Method as detailed in NCRP Report No. 151 and
found to be unacceptably high. An initial survey following the machine installation confirmed
these calculations. Rather than restrict the linac beam energy to 10 MV, BPE was investigated
as a neutron moderating addition. An inner wall and door were planned and constructed using
4'x8'x1" thick 5% BPE sheets. The resulting door and wall had 2" of BPE; conduits and duct
work were also redesigned and shielded. A survey was conducted following construction of the
wall.
Results: The vault modification reduced the expected effective dose at the vault door from 36.23
to 0.010 mSv/week.
Conclusion: As specific guidelines for vault modification are lacking, this project quantitatively
demonstrates the potential use of BPE for vault modification. Such modifications may provide a
low-cost shielding solution to allow for the use of high energy modes in smaller treatment vaults.

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