AbstractID: 7852 Title: Shielding Modification for a Vault to Accommodate Isocenter Shift And Switching
From 6 to 18 MV
SHIELDING MODIFCATIONS FOR A VAULT TO ACCOMMODATE
ISOCENTER SHIFT AND SWITCHING FROM 6 TO 18 MV.
Purpose: This report describes how modifications were made to the primary and
secondary barriers of a vault designed for 6 MV to accommodate an 18 MV linac.
Materials and Methods: The 18 MV linac from another manufacturer required an
isocenter shift of 14.5 inches toward the maze of a low energy vault. Shielding
calculations for the primary and secondary barriers and the maze door were made
following the NCRP formalism and empirical transmission data for lead and concrete.
The impact of IMRT at 18MV was also considered. Two inches of lead was added to the
inner walls of the primary barrier and three inches of lead added to the top of the roof
area that overlapped the primary barrier in the vault. Five inches of lead were added to
the edge of the primary concrete barrier walls and roof on the maze side to account for
the isocenter shift. The secondary concrete barrier on the roof was increased from 8 to36
inches.
Results: A detailed radiation safety survey of the modified vault after installation of the
18 MV linac has confirmed the adequacy of the additional shielding for photons and
neutrons in controlled and public areas. Good agreement was found between the
predicted and measured instantaneous dose rates outside the primary and secondary
barriers. .
Conclusion: Upgrading a linac vault designed for 6 to 18 MV can be an economic
compromise compared with replacement