6/10MV Linear accelerators on the second floor of a high rise building: the Roman boat and the patient experience • New Kings Health Partners Cancer Centre nearing completion: 13 storeys tall • Best available use of space on crowded inner London site • Bunkers designed for 2 nd floor • Roman boat in ground left undisturbed and still ‘excavate-able’; • Areas adjacent to Linacs on 2 nd floor have maximised natural light and views to enhance patient experience David Gallacher, Consultant Physicist Guy’s and St.Thomas’ NHS Foundation Trust
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6/10MV Linear accelerators on the second floor of a high ... · Orientation Factor for calculation point Op 1.0 1.0 Distance from source isocentre m Ls 7.08 7.08 Thickness of shielding
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6/10MV Linear accelerators on the second floor of a high rise building: the Roman boat and the patient experience
• New Kings Health Partners Cancer Centre nearing completion: 13 storeys tall
• Best available use of space on crowded inner London site
• Bunkers designed for 2nd floor• Roman boat in ground left undisturbed
and still ‘excavate-able’;• Areas adjacent to Linacs on 2nd floor have
maximised natural light and views to enhance patient experience
David Gallacher, Consultant Physicist
Guy’s and St.Thomas’ NHS Foundation Trust
KHPCC Level2: 6 Linac Bunkers at 6/10MV
The solution?
• Use dense aggregate proprietary blocks
• Blocks have iron and other added materials to increase mass density
• V250 blocks = 4000kg.m-3
• V300 blocks = 5000kg.m-3
• Enable shield thickness to be reduced
• Also, service plant areas immediately above on 3rd
floor, with reduced height and low occupancy
• Service area below, on 1st floor, also with reduced height and low occupancy
Data: Shielding Materials (Veritas)• Supplied Data at 10/6MV Primary X-Rays
• V250 – TVL=238/211mm
• V300 – TVL=187/165mm
• Standard Concrete* – TVL=389/343mm
• Lead – TVL=56/56mm
• Supplied Data at 10/6MV Leakage X-Rays
• V250 – TVL=188/170mm
• V300 – TVL=147/135mm
• Standard Concrete* - TVL=305/279mm
• Lead – TVL=46/46mm*at concrete density of 2350kg.m-3
Shielding Materials (Veritas)• Supplied Data for Neutrons
• V250 – TVL=165mm
• V300 – TVL=165mm
• Standard Concrete* – TVL (fast neutrons from head leakage)=210mm[1]
• BPE – TVL=97mm
Note that the TVL for neutron shielding is not greatly lower than that for standard concrete *at concrete density of 2350kg.m-3
• [1] McGinley, PH, Shielding techniques for radiation oncology facilities, Medical Physics Publishing, Wisconsin, 1998
Calculations I
• X-Rays: use standard techniques: NCRP151, IPEM75
• t= TVL1 + (n-1)TVLe
• Allows for 1st TVL change, build up depth, equilibrium
• For composite barriers after the initial barrier thickness applied at TVL1 all subsequent thicknesses applied using respective TVLe values
• Calculate ‘effective thickness’ of concrete for further calculations and oblique ray path correction
Calculations II
• X-Rays and neutrons: use ‘mean ray path’ method for primary and leakage X-rays
• t(effective,MRP)=t(1+cos(θ))/(2cos(θ))• For oblique incidence at barriers the full slant
thickness was used by supplier: this was contested, usual method is to:
• (a) add 1-2 HVL to barrier thickness calculated for slant;
• (b) use MRP method;• (c ) use Biggs slant correction factors• (b) and (c ) are practically equivalent for 6/10MV X-
rays
Calculations III
• Neutrons: use 0.003% (nGy/photon Gy)for 10MV
• Neutrons: use quality factor=10 (applies for 0.5-1MeV neutrons)
• Leakage workload = 3 x applied beam W/L
• Assume inverse square from treatment head, but apply x2 for scattered neutrons in treatment room as per NCRP51, equivalent to ‘add one HVL’ rule
V300V250
Lead
SteelBPE
BPE
Composite Primary Beam Shield
Composite Primary Beam Shields: Second Floor KHPCC