6/10mv linear accelerators on the second floor of a high ... · orientation factor for calculation...

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

Shielding Report #1: 21/10/2013


V300

Beam Tapers: Floor Shielding

[4a] Secondary Barrier Calculation - Point P; Leakage X-RAYS [4b] Patient Scatter

Ratio of monitor units per cGy for leakage no unit MU/cGy 3 3

Leakage dose per day at 1 metre Gy/day 360 360 Dose rate at isocentre

Leakage dose per year (for leakage calculation) Gy/y 90000 90000 Scatter Factor 1.20E-03 0.00115

60 deg

scatter

Orientation Factor for calculation point Op 1.0 1.0 U=1 for sec barrier Beam Area (cm2) 1600 1600 cm^2

Distance from source isocentre m Ls 7.08 7.08 metres from Iso-C Usage Factor 0.66 0.66

60 degree

scatter

Concrete to be added mm 0 0 mm concrete Dpri (m) 1 1 m

Steel to be added mm 0 0 mm steel Dsec (m) 7.08 7.08 m

Concrete V-250 to be added mm 0 0 mm concrete V-250 Ref Beam Area F (cm2) 400 400 cm^2

Concrete V-300 to be added mm 254 254 mm concrete V-300

Lead to be added mm 0 0 mm lead

BPE to be added mm 0 0 mm BPE

Obliquity angle (to the barrier normal) degrees Phi 39 39 degrees

Monitor units per minute MU/min Mu 800 800

Effective Dose rate at 1 metre Gy/min DR DR0 x F x (Mu/Mnorm) 0.008 0.008 Effective Dose rate at 1m 8 8 Gy/min

Unattenuated dose rate at point P Gy/hr Dru DR x 60 / Ls^2 0.0096 0.0096 Unattenuated dose rate 0.0460 0.0440 Gy/hr

Equivalent concrete thickness of barrier mm Te (C+Ta+(SxTVLcs/TVLss))/cos(Phi) 546 536 Equivalent concrete 546 536 mm

Slant thickness of barrier mm 702 689 Slant thickness of barrier 702 689 mm

Oblique thickness of barrier (MRP method) mm 624 612 Oblique thickness (MRP) 624 612 mm

Oblique thickness of barrier (DG method) mm 605 586 Oblique thickness DG 605 586 mm

Attenuation factor (secondary) As exp(-ln(10) x Te/ TVLcs) 1.05E-02 1.42E-02 Attenuation factor (scatter) 1.59E-03 2.81E-03

Instantaneous dose rate uSv/hr Dinst Dru x As x 10^6 100.54 136.32 Instantaneous dose rate 73.25 123.61 uSv/h

Dose per hour in this modality uSv Dhr Dinst x R x Op \ MU/cGy factor 6.284 8.520 Dose per hour in this modality 1.007 1.700 uSv/h

Fraction of intended planned workload used per mode fmode 1 1

Fraction of intended planned workload used

per mode 1 1

Time Averaged Dose Rate (TADR) uSv/hr Dta fmode(1)*Dhr[6MV]+fmode(2)*Dhr[10MV] 14.80400 ***

Time Averaged Dose Rate

(TADR) 2.707 *** uSv/h

Total Dose per year (12 hour days) uSv/y 44412 uSv/year Total Dose per year 12h 8121 uSv/year

Occupancy factor for area T 1 *** Occupancy factor for area 1 ***

TADR with occupancy (TADR2000) uSv/hr Dta2000 14.80 *** TADR2000 2.707 *** uSv/h

Annual Effective Dose (secondary 2000 hours) uSv Dann(sec) 29608 uSv/year Annual Dose 2000 hours) 5414 uSv/year

[5] Secondary Barrier Calculation - Point P: NEUTRONS

Ratio of monitor units per cGy for leakage no unit MU/cGy 3 3

Leakage dose per day at 1 metre Gy/day 360 360

Leakage dose per year (for leakage calculation) Gy/y 90000 90000

Neutron Leakage Fraction (%) % (n)Gy/(p) Gy 0 0.003 %

Total neutron dose from direct neutron dose equivalent factor 0 2

Neutron Quality Factor (c.1MeV) Sv/Gy (n)Sv/(n)Gy 10 10

Neutron dose per year at 1 metre Sv/y 0.0 54.0

Orientation Factor for calculation point Op 1.0 1.0

Distance from source isocentre m Ls 7.08 7.08

Thickness of shielding BPE mm Ls 0 0

Thickness of shielding concrete mm Ls 0 0

Thickness of shielding concrete V-250 mm Ta 0 0

Thickness of shielding concrete V-300 mm 254 254

Obliquity degrees Phi 39 39

Obliquity modifying factor (BIR formula) 1.13 1.13 applied in this case

Attenuation Factor BPE 1.00E+00 1.00E+00

Attenuation Factor concrete 1.00E+00 1.00E+00

Attenuation Factor concrete V-250&V-300 4.42E-02 1.85E-02 Uses combined TVL for both materials

Effective Dose rate at point of interest uSv/y N/A 19948 uSv/year

Dose per hour in this modality uSv/hr N/A 6.649

Occupancy factor for area T N/A 1.00

TADR with occupancy (TADR2000) uSv/hr N/A 6.649

Instantaneous Dose Rate uSv/hr N/A 106.39 uSv/h

Annual Effective Dose (2000h) uSv/year N/A 13298 uSv/year


P8 (L2,4,6) - through wall 8 Gy/min - leakage barrier 0.1 0.1 0.011 23 34

P9 (L1) - into corridor 8 Gy/min - leakage barrier 3.8 1.9 0.115 231 346

P9 (L2-6) - adjacent bunker 8 Gy/min - leakage barrier 4.7 2.4 0.144 288 431

P10 (L1) 8 Gy/min - primary barrier 19.2 7.2 0.118 237 356

P10 (L2-6) 8 Gy/min - primary barrier 23.4 8.8 0.144 289 433





Column 1 (outside) 8 Gy/min - leakage barrier 5.2 4.2 0.394 788 1183

X1 -> P13 (missing steel next column) 8 Gy/min - leakage barrier 21.8 16.1 1.466 2933 4399

P13 (X1->P13 + P12(L2-6) ) 8 Gy/min - leakage barrier 22.8 17.0 1.550 3099 4649

P14 (as proposed) 8 Gy/min - leakage barrier 366.3 289.1 24.160 48320 72481

P14 (with Pb+BPE section) 8 Gy/min - leakage barrier 7.5 7.4 0.478 955 1433

P18 8 Gy/min - leakage barrier 2.2 1.7 0.154 309 463

Column 3 (control room) 8 Gy/min - leakage barrier 4.4 3.4 0.323 645 968

Column 3 (25 mm extra steel) 8 Gy/min - leakage barrier 2.1 1.7 0.163 325 488

X1 -> P15 (missing steel next column) 8 Gy/min - leakage barrier 4.2 3.3 0.299 599 898

P15 (X1->P15 + P8(L2-6) ) 8 Gy/min - leakage barrier 4.4 3.5 0.311 622 933

X4 -> P16 8 Gy/min - leakage barrier 225.6 174.0 14.366 28732 43098

X4 -> P16 with extra shielding Pb+ BPE 8 Gy/min - leakage barrier 2.5 2.5 0.175 351 526

X5 -> P16 with extra shielding Pb+BPE 8 Gy/min - leakage barrier 3.5 3.4 0.238 477 715

P16 (with extra shielding Pb+BPE) 8 Gy/min - leakage barrier 6.0 5.8 0.414 828 1241

X4 -> P17 (missing steel) 8 Gy/min - leakage barrier 2.2 1.7 0.156 313 469

P8extended (L5) - through door 8 Gy/min - leakage barrier 4.5 4.5 0.334 668 1001

P17 (X4->P17+P8ext(L5) ) 8 Gy/min - leakage barrier 6.7 6.2 0.490 980 1471

X4 -> P17 (extended steel) 8 Gy/min - leakage barrier 1.1 0.9 0.081 162 243

P17 (X4->P17ext steel +P8ext(L5) ) 8 Gy/min - leakage barrier 5.6 5.4 0.415 830 1244


P14: neutron dose is 28% of the total, scatter 11%, leakage 61%

Shielding Report #1amended: added one extra V300 block adjacent to door surround

Shielding Report #1amended with one extra block: 24/01/2014

SUMMARY SECTION

Wall A - primary barrier IDR (beam)

IDR (one

min.) TADR Occupancy Total Dose

TADR200

0

(µSv/h) (µSv/h) (µSv/h) Factor per Year (µSv) (µSv/y)

[2] Primary Barrier - X-rays 0.00 0.00 0.000 1 0.0 0.0

[3] Primary Bariier - Neutrons 0.00 0.00 0.000 1 0.0 0.0

TOTAL 0.0 0.0 0.000 0 0

SUMMARY SECTION

Wall A - secondary barrier IDR (beam)

IDR (one

min.) TADR Occupancy Total Dose

TADR200

0

(µSv/h) (µSv/h) (µSv/h) Factor per Year (µSv) (µSv/y)

[4a] Secondary Barriers - Leakage X-Rays 3.89 3.89 0.397 1 1191 794

[4b] Secondary Barriers - Scatter X-Rays 0.01 0.01 0.000 1 1 1

[5] Secondary Barriers - Neutrons 14.72 14.72 0.920 1 2760 1840

TOTAL 18.6 18.6 1.32 3952 2635

SUMMARY TOTALS

LOCATION: P16: X4->P16 from Iso-C

Maximum IDR (beam on) calculated 18.62 µSv/h

Maximum IDR (averaged over one minute) 18.61 µSv/h

Maximum TADR calculated 1.317 µSv/h

TADR2000 (combined) 2635 µSv/year

Total Dose per year (full occupancy 12h days) 3952 µSv/year

Dose Constraint for location (with occupancy factor) 300 µSv/year

Occupancy Factor 0.2 --

Total allowed dose per year in location (100%

occupancy) 1500 µSv/year

Meet design constraints No -- Only East Head Position

Shielding Report #1amended: 24/01/2014

APPENDIX #5 SUMMARY SHEET Version Date: 24/01/2014 Shielding Plan Date: Amendment

Part: 1 SK121813-02

Location Condition

Max (Beam

on) Averaged Max TADR2000 Total Dose

IDR IDR TADR (combined) (Full occ.)

µSv/h µSv/h µSv/h µSv/year µSv/year

X4 -> P16 HEAD EAST ONLY 8 Gy/min - leakage barrier 12.1 11.6 0.456 913 1369

X4->P16 8 Gy/min - leakage barrier 18.6 18.6 1.317 2635 3952

X5 -> P16 8 Gy/min - leakage barrier 13.1 13.1 1.200 2399 3599

P16 (combined X4->P16 & X5->P16) 8 Gy/min - leakage barrier 31.8 31.7 2.517 5034 7551

Shielding Report #1amended: 24/01/2014

X1->P15 8 Gy/min - leakage barrier 5.5 4.2 0.383 766 1150 1.0

X2->P15 8 Gy/min - leakage barrier 0.1 0.1 0.010 20 30 1.0

P15 (X3->P15 + X4->P15) 8 Gy/min - leakage barrier 5.6 4.4 0.393 787 1180 1.0

X4->P16+X5->P16 (Veritas RPs) 8 Gy/min - leakage barrier 9.2 8.0 0.606 1213 1819 0.2

X4->P16+X5->P16 (Alternative RPs) 8 Gy/min - leakage barrier 10.6 8.6 0.722 1443 2165 0.2

X4-P16 (Head - East) 8 Gy/min - leakage barrier 6.9 6.7 0.537 1075 1612 0.2

X4 -> P17 (through extra block) 8 Gy/min - leakage barrier 1.7 1.4 0.121 243 364 0.2

P8extended (L5) - through door 8 Gy/min - leakage barrier 4.5 4.5 0.334 668 1001 0.2

P17 (X4->P17+P8ext(L5) ) 8 Gy/min - leakage barrier 6.2 5.8 0.455 910 1365 0.2

Door 1 8 Gy/min - leakage barrier 7.4 7.3 0.547 1095 1642 0.2

X3->P19, Borough Wing) 8 Gy/min - leakage barrier 1.0 0.9 0.078 156 234 0.5

X4->P19, Borough Wing) 8 Gy/min - leakage barrier 0.8 0.8 0.067 133 200 0.5

X5->P19, Borough Wing 8 Gy/min - leakage barrier 1.5 1.2 0.103 207 310 0.5

P19 (X3, X4 and X5) 8 Gy/min - leakage barrier 3.3 2.8 0.248 496 745 0.5

C1 8 Gy/min - primary barrier 39.5 14.8 0.259 521 782 0.1



L1 8 Gy/min - leakage barrier 3.8 1.9 0.126 252 379 0.1

L2 8 Gy/min - leakage barrier 0.2 0.1 0.013 26 39 0.1

F1 8 Gy/min - primary barrier 16.6 6.2 0.249 549 824 0.1

F2 (alt. ray-path) 8 Gy/min - primary barrier 52.3 19.6 0.659 1327 1990 0.1

F3 8 Gy/min - primary barrier 11.2 4.2 0.164 333 499 0.1

FL1 8 Gy/min - leakage barrier 7.8 3.8 0.267 533 800 0.1

FL2 8 Gy/min - leakage barrier 6.5 5.1 0.475 950 1425 0.1

FL2 (void space) 8 Gy/min - leakage barrier 21.6 16.2 1.507 3015 4522 0.1

Duct Penetration1 8 Gy/min - leakage barrier 24.6 11.9 0.781 1562 2343 0.1

Duct Penetration2 8 Gy/min - leakage barrier 0.4 0.4 0.038 76 114 0.1

At corner top wall facing Borough Wing 8Gy/min - leakage barrier 10.3 9.4 0.978 1955 2933 0.5

COLOUR HIGHLIGHT KEY Changes DG comments on 27/2/2014 Veritas report

Satisfactory - no issues identified FL2: void space accounted for - see Appendix #2

Advisory - there may be actions attached L3: over wall shielding towards Boruhg Wing - shielding deficit

Warning - serious issues identified

25mm steel shim

Oblique Ray path through structural beam deficit: shim added

Solution: 3xV300+1xV250 blocks on door surrounds

Extra 127mm V250

block

Problems solved….

Build Stage: Primary Barrier Taper Level 3 above Linacs

Build Stage: Primary Taper above Linac

Build Stage: Level 3 Linac Shielding

Build Stage: Level 2 Linac Floor Shielding

Build Stage: Level 2 Walls

Build Stage: Proximity of other buildings

Build Stage: Level 2 Walls

Build Stage: Level 2 Walls packed ‘grout gap’

Build Stage: Door Frames

Build Stage: Door Frame/Exterior Window

Build Stage: Angled Physics Ports

Build Stage: unfilled door casing

Build Stage: 11tonne door

Build Stage: Level 2 Outside Walkway – occupancy Vs distance to near buildings?

Conclusions• Check assumptions made in design

• Detailed calculation always required, especially with a novel design challenge i.e. on the 2nd floor with adjacent buildings

• Beware of neutron dose with high density/high Z aggregated concrete blocks, it becomes significant factor

• High density materials can reduce barrier thicknesses when space is at premium

• Regular inspection during build essential to head off problems later

References

• [1] McGinley, PH, Shielding techniques for radiation oncology facilities, Medical Physics Publishing, Wisconsin, 1998

• [2] NCRP Report No.151, Structural Shielding, Design and Evaluation for megavoltage X-and Gamma Ray Radiotherapy Facilities

• [3] IPEM Report No.75, The design of radiotherapy treatment room facilities, IPEM, York, 1997

• [4] Biggs, PJ, Styczynski, JR, Do angles of obliquity apply to 30° scattered radiation from megavoltage beams?, Health Physics, 98(4), 425-432, 2008

6/10mv linear accelerators on the second floor of a high ... · orientation factor for calculation...

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