Therapy Shielding CalculationsTherapy Shielding Calculations

Melissa C. Martin, M.S., FACR, FACMPMelissa C. Martin, M.S., FACR, FACMP

American College of Medical PhysicsAmerican College of Medical Physics

21st Annual Meeting & Workshops21st Annual Meeting & Workshops

Scottsdale, AZScottsdale, AZ

June 13, 2004June 13, 2004

Therapy Shielding Design Traditionally Therapy Shielding Design Traditionally Relies on NCRP ReportsRelies on NCRP Reports

NCRP Report 49NCRP Report 49– Primary and secondary barrier calculation methodology Primary and secondary barrier calculation methodology

– Applicable up to Applicable up to 6060Cobalt and linacs up to 10 MVCobalt and linacs up to 10 MV

NCRP Report 51NCRP Report 51– Extended NCRP 49 methodology up to 100 MVExtended NCRP 49 methodology up to 100 MV

– Empirical shielding requirements for maze doorsEmpirical shielding requirements for maze doors

NCRP Report 79NCRP Report 79– Improved neutron shielding methodologyImproved neutron shielding methodology

NCRP Report 144NCRP Report 144– Update of NCRP 51 primarily aimed at non-medical facilitiesUpdate of NCRP 51 primarily aimed at non-medical facilities

Reports reflect progress inReports reflect progress in linac design and shielding research linac design and shielding researchReports reflect progress inReports reflect progress in linac design and shielding research linac design and shielding research

Revised NCRP Report in Drafting Stage byRevised NCRP Report in Drafting Stage byAAPM Task Group 57, NCRP SC 46-13AAPM Task Group 57, NCRP SC 46-13

Design of Facilities for Medical Radiation TherapyDesign of Facilities for Medical Radiation Therapy– 4 MV - 50 MV (including 4 MV - 50 MV (including 6060Co)Co)

Calculation scheme generally follows NCRP 49Calculation scheme generally follows NCRP 49

All shielding data (TVLs) reviewed and updatedAll shielding data (TVLs) reviewed and updated

Updated for intensity modulated radiation therapy (IMRT)Updated for intensity modulated radiation therapy (IMRT)

Improved accuracy of entrance requirementsImproved accuracy of entrance requirements– Both with and without the use of mazeBoth with and without the use of maze

Laminated barriers for high energy x-raysLaminated barriers for high energy x-rays– Photoneutron generation due to metal in primary barrierPhotoneutron generation due to metal in primary barrier

Goal: Improved accuracyGoal: Improved accuracyGoal: Improved accuracyGoal: Improved accuracy

BJR #11 megavoltage (MV) definition used hereBJR #11 megavoltage (MV) definition used here– British Journal of Radiology (BJR) Supplement No. 11British Journal of Radiology (BJR) Supplement No. 11

Comparison of BJR #11 and BJR #17 MV definitionsComparison of BJR #11 and BJR #17 MV definitions

Workload assumptions typically used for shielding designWorkload assumptions typically used for shielding design– Workload identified by symbol “W” in calculationsWorkload identified by symbol “W” in calculations

– For MV For MV 10 MV: W = 1000 Gy/wk at 1 meter from the target 10 MV: W = 1000 Gy/wk at 1 meter from the target

» Based on NCRP 49 Appendix C Table 2Based on NCRP 49 Appendix C Table 2

– For MV > 10: W = 500 Gy/wkFor MV > 10: W = 500 Gy/wk

» Based on NCRP 51 Appendix B Table 5Based on NCRP 51 Appendix B Table 5

Linear Accelerator Energy and WorkloadLinear Accelerator Energy and Workload

BJR #11 MV 4 6 10 15 18 20 24BJR #17 MV 4 6 10 16 23 25 30

Radiation Protection Limits for PeopleRadiation Protection Limits for People

Structural shielding is designed to limit exposure to peopleStructural shielding is designed to limit exposure to people– Exposure must not exceed a specific dose equivalent limitExposure must not exceed a specific dose equivalent limit

– Limiting exposure to unoccupied locations is not the goalLimiting exposure to unoccupied locations is not the goal

NCRP 116 design dose limit (P)NCRP 116 design dose limit (P)– 0.10 mSv/week for occupational exposure0.10 mSv/week for occupational exposure

– 0.02 mSv/week for the general public0.02 mSv/week for the general public

Typical international design dose limitsTypical international design dose limits– 0.12 mSv/week for controlled areas0.12 mSv/week for controlled areas

– 0.004 mSv/week for uncontrolled areas 0.004 mSv/week for uncontrolled areas 

NCRP 116 dose limit is a factor of 5 lower than NCRP 49 valueNCRP 116 dose limit is a factor of 5 lower than NCRP 49 valueNCRP 116 dose limit is a factor of 5 lower than NCRP 49 valueNCRP 116 dose limit is a factor of 5 lower than NCRP 49 value

Radiation Protection Limits for LocationsRadiation Protection Limits for Locations

Permissible dose outside vault depends on occupancyPermissible dose outside vault depends on occupancy

Occupancy factor (T):Occupancy factor (T):

Fraction of time a particular location may be occupiedFraction of time a particular location may be occupied

Maximum shielded dose (SMaximum shielded dose (Smaxmax) at protected location) at protected location

– Assuming occupancy factor T for protected locationAssuming occupancy factor T for protected location

Maximum shielded dose is traditionally referred to simply as P/TMaximum shielded dose is traditionally referred to simply as P/TMaximum shielded dose is traditionally referred to simply as P/TMaximum shielded dose is traditionally referred to simply as P/T

T

PS max

Occupancy Values from NCRP 49Occupancy Values from NCRP 49

Full occupancy for controlled areas by convention (T=1)Full occupancy for controlled areas by convention (T=1)

Full occupancy uncontrolled areas (T=1)Full occupancy uncontrolled areas (T=1)– Offices, laboratories, shops, wards, nurses stations, living Offices, laboratories, shops, wards, nurses stations, living

quarters, children’s play areas, and occupied space in nearby quarters, children’s play areas, and occupied space in nearby buildingsbuildings

Partial occupancy for uncontrolled areas (T=1/4)Partial occupancy for uncontrolled areas (T=1/4)– Corridors, rest rooms, elevators with operators, unattended Corridors, rest rooms, elevators with operators, unattended

parking lotsparking lots

Occasional for uncontrolled areas (T=1/16)Occasional for uncontrolled areas (T=1/16)– Waiting rooms, toilets, stairways, unattended elevators, janitor’s Waiting rooms, toilets, stairways, unattended elevators, janitor’s

closets, outside areas used only for pedestrian or vehicular trafficclosets, outside areas used only for pedestrian or vehicular traffic

Hourly Limit for Uncontrolled AreasHourly Limit for Uncontrolled Areas

0.02 mSv hourly limit for uncontrolled areas0.02 mSv hourly limit for uncontrolled areas

20 Gy/hr common assumption for calculation20 Gy/hr common assumption for calculation

Implies a lower limit for occupancy factorImplies a lower limit for occupancy factor– T T 20 / ( U W ) 20 / ( U W )

– T T 0.16 for higher energy accelerators (500 Gy / wk workload) 0.16 for higher energy accelerators (500 Gy / wk workload)

– T T 0.08 for lower energy accelerators (1000 Gy wk workload) 0.08 for lower energy accelerators (1000 Gy wk workload)

Not applied to low occupancy locations with no public Not applied to low occupancy locations with no public access access – e.g., unoccupied roof, machinery roome.g., unoccupied roof, machinery room

T = 1/10 rather than 1/16 typically used for exterior wallsT = 1/10 rather than 1/16 typically used for exterior wallsT = 1/10 rather than 1/16 typically used for exterior wallsT = 1/10 rather than 1/16 typically used for exterior walls

NCRP 134 Impact on Linac ShieldingNCRP 134 Impact on Linac Shielding

NCRP 134 distinguishes general employees from publicNCRP 134 distinguishes general employees from public– NCRP 134 maintains NCRP 116 limit of 0.02 mSv/wk for bothNCRP 134 maintains NCRP 116 limit of 0.02 mSv/wk for both

– Limit 25% of 0.02 mSv/wk from individual facility for general publicLimit 25% of 0.02 mSv/wk from individual facility for general public

Occupancy assumptions proposed for general publicOccupancy assumptions proposed for general public– T=1/40 for occasional occupancyT=1/40 for occasional occupancy

Equivalent to T=1/10 occasional for general employeesEquivalent to T=1/10 occasional for general employees– Similar to P/T required by hourly limit for primary barriersSimilar to P/T required by hourly limit for primary barriers

– Slightly increase from T = 1/16 used for secondary barriers Slightly increase from T = 1/16 used for secondary barriers

– T=1/16 still appropriate for locations with no public occupancyT=1/16 still appropriate for locations with no public occupancy

» e.g., machine rooms, unoccupied roofs, etc. e.g., machine rooms, unoccupied roofs, etc.

Impact increases if higher occupancy than T=1/40 adoptedImpact increases if higher occupancy than T=1/40 adoptedImpact increases if higher occupancy than T=1/40 adoptedImpact increases if higher occupancy than T=1/40 adopted

Basic Primary Barrier Calculation Basic Primary Barrier Calculation Unchanged from NCRP 49Unchanged from NCRP 49

Unshielded dose calculationUnshielded dose calculation

Attenuation in tenth-value layersAttenuation in tenth-value layers

Barrier thickness (tBarrier thickness (tcc) calculation) calculation

2pri

prid

UWS

eC TVLnTVLt )1(1

TP

Sn

pri

/log10

Margin in primary barrier thickness is recommended to Margin in primary barrier thickness is recommended to compensate for potential concrete density variationcompensate for potential concrete density variation

Margin in primary barrier thickness is recommended to Margin in primary barrier thickness is recommended to compensate for potential concrete density variationcompensate for potential concrete density variation

BD

A

D'

C'

A'

Maze

TargetRotational

Plane

* Target

Isocenter

1 ft

tC

dpri

C

Door

Primary Barrier Photon Tenth-Value Layers Primary Barrier Photon Tenth-Value Layers (mm) Come from a Variety of Sources(mm) Come from a Variety of Sources

Lead Concrete Steel Earth Borated PolyMV TVL1 TVLe TVL1 TVLe TVL1 TVLe TVL1 TVLe TVL1 TVLe0.20.250.30.40.5124

1.7 1.7 84 842.9 2.9 94 944.8 4.8 104 1048.3 8.3 109 10911.9 11.9 117 11726 26 147 14742 42 210 21053 53 292 292

61015

572 572648 648720 720

379 379379 379

182024

367 323410 377445 416462 432470 442483 457

56 5656 5656 5656 5656 5656 56

15 1519 1922 2229 2933 3354 5176 6991 91100 100104 104108 108109 109110 110110 110

135 135 84 84151 151 94 94167 167 104 104175 175 109 109188 188 117 117236 236 147 147336 336 210 210468 468 292 292

343 343

740 740 379 379752 752 390 390773 773 401 401

NCRP 49 NCRP 51

Anticipate upcoming NCRP report to review and update TVL dataAnticipate upcoming NCRP report to review and update TVL dataAnticipate upcoming NCRP report to review and update TVL dataAnticipate upcoming NCRP report to review and update TVL data

Nelson & LaRiviere Estimated from ConcreteMcGinley

Primary Barrier WidthPrimary Barrier Width

0.3 meter margin on each side of beam rotated 45 degrees0.3 meter margin on each side of beam rotated 45 degrees– Barrier width required assuming 40 cm x 40 cm field sizeBarrier width required assuming 40 cm x 40 cm field size

Field typically not perfectly square (corners are clipped)Field typically not perfectly square (corners are clipped)– 35 cm x 35 cm field size typically used to account for this 35 cm x 35 cm field size typically used to account for this

ftdw CC 0.124.0 '

C'

* Target

IsocenterTarget toNarrow Point

Distance(d C' )

w C

1 ft 1 ft

C C'

* Target

IsocenterTarget toNarrow Point

Distance(d C' )

w C

1 ft 1 ft

C

* Target

Isocenter

w C

1 ft 1 ft

Metal

Target toNarrow Point

Distance(d C' )

Slant Factor and Obliquity FactorSlant Factor and Obliquity Factor

Slant FactorSlant Factor– Path from target to protected location diagonally through barrierPath from target to protected location diagonally through barrier

» Incident angle Incident angle of line with respect to perpendicular of line with respect to perpendicular

– Required barrier thickness reduced by cos(Required barrier thickness reduced by cos())

» Same total distance through barrier to protected locationSame total distance through barrier to protected location

Scatter causes slant factor to underestimate exit doseScatter causes slant factor to underestimate exit dose– Multiplying thickness by obliquity factor compensates for thisMultiplying thickness by obliquity factor compensates for this

Lead Concrete SteelAngle 4 MV 10 MV 18 MV 4 MV 10 MV 18 MV 4 MV 10 MV 18 MV

0 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.0030 1.03 1.02 1.03 1.02 1.00 1.00 1.02 1.02 1.0445 1.07 1.07 1.10 1.07 1.04 1.04 1.07 1.07 1.0860 1.21 1.21 1.22 1.20 1.14 1.08 1.20 1.17 1.2070 1.44 1.47 1.52 1.47 1.28 1.22 1.48 1.42 1.45

Photoneutron Generation Due to Metal in Photoneutron Generation Due to Metal in Primary Barrier (Linacs Primary Barrier (Linacs 10 MV) 10 MV)

Dose-equivalent 0.3 m beyond barrier (McGinley)Dose-equivalent 0.3 m beyond barrier (McGinley)

– N is neutron production constant (Sv neutron per Gy workload)N is neutron production constant (Sv neutron per Gy workload)

» 1.9 x 10-3 for lead, 1.7 x 10-4 for steel at 18 MV (from McGinley)1.9 x 10-3 for lead, 1.7 x 10-4 for steel at 18 MV (from McGinley) Recent safety survey indicated somewhat higher 3.8 x 10-4 Recent safety survey indicated somewhat higher 3.8 x 10-4

value for steel at 18 MV is appropriatevalue for steel at 18 MV is appropriate

» N adjusted versus MV based on neutron leakage fraction vs MVN adjusted versus MV based on neutron leakage fraction vs MV

– F is field size (conventionally 0.16 mF is field size (conventionally 0.16 m22), t), t22 is metal thickness (m) is metal thickness (m)

– X-Ray attenuation prior to metal layer: 10^(-tX-Ray attenuation prior to metal layer: 10^(-t11 / TVL / TVLpp))

– Neutron attenuation after metal layer: 10^(-tNeutron attenuation after metal layer: 10^(-t33 / TVL / TVLNN))

NPTVLtTVLt

N

tt

FNUWS //

3

2

31 1010

305.02

Patient Photonuclear Dose Due to Metal in Patient Photonuclear Dose Due to Metal in Primary Barrier for MV > 10Primary Barrier for MV > 10

Metal in primary barrier can increase patient total body Metal in primary barrier can increase patient total body dose if MV > 10dose if MV > 10– Lead inside layer approximately doubles patient total body doseLead inside layer approximately doubles patient total body dose

– Increases risk of secondary cancerIncreases risk of secondary cancer

Concrete or borated polyethylene inside metal in primary Concrete or borated polyethylene inside metal in primary barrier is recommended if MV >10 barrier is recommended if MV >10 – Each inch of borated poly decreases patient dose from metal Each inch of borated poly decreases patient dose from metal

barrier photoneutron by approximately factor of 2barrier photoneutron by approximately factor of 2

Impact of IMRT on patient photonuclear dose is addressed Impact of IMRT on patient photonuclear dose is addressed laterlater

Avoid metal as inside layer of primary barrier if MV >Avoid metal as inside layer of primary barrier if MV > 1010Avoid metal as inside layer of primary barrier if MV >Avoid metal as inside layer of primary barrier if MV > 1010

Secondary BarrierSecondary Barrier

Patient scatter unshielded dosePatient scatter unshielded dose

– F is field size in cmF is field size in cm22

» typically 1600typically 1600

– a = scatter fraction = scatter fraction for 20 x 20 cm for 20 x 20 cm

beambeam

Leakage unshielded doseLeakage unshielded dose– Assumes 0.1% leakage fractionAssumes 0.1% leakage fraction

2sec

2

)400/(

dd

FWaS

scap

2sec

310

d

WSL

BD

A

C

D'

C'

A'

Maze

TargetRotational

Plane

* Target

Isocenterdsca

1 fttB

dsec

Door

Leakage Photon Tenth-Value Layers (mm) Leakage Photon Tenth-Value Layers (mm) Also Come from a Variety of SourcesAlso Come from a Variety of Sources

Nelson & LaRiviereNCRP 49Estimated

from ConcreteKleck & Varian

Average

Lead Concrete Steel Earth Borated PolyMV TVL1 TVLe TVL1 TVLe TVL1 TVLe TVL1 TVLe TVL1 TVLe4 53 53 292 292 91 91 468 468 292 2926 56 56 341 284 96 96 546 455 341 284

10 56 56 351 320 96 96 562 512 351 32015 56 56 361 338 96 96 578 541 361 33818 56 56 363 343 96 96 581 549 363 34320 56 56 366 345 96 96 586 552 366 34524 56 56 371 351 96 96 594 562 371 351

Neutron LeakageNeutron Leakage

Same form as photon leakage calculationSame form as photon leakage calculation

Based on dose-equivalent neutron leakage fraction vs MVBased on dose-equivalent neutron leakage fraction vs MV– 0.002%, 0.04%, 0.10%, 0.15% and 0.20% for 10, 15, 18, 20 and 24 MV0.002%, 0.04%, 0.10%, 0.15% and 0.20% for 10, 15, 18, 20 and 24 MV

– Based on Varian and Siemens neutron leakage dataBased on Varian and Siemens neutron leakage data

» Assumes quality factor of 10 for absorbed doseAssumes quality factor of 10 for absorbed dose

Shielded dose equivalent based on leakage neutron TVLsShielded dose equivalent based on leakage neutron TVLs– 211 mm for concrete211 mm for concrete

– 96 mm for borated polyethylene96 mm for borated polyethylene

Intensity Modulated Radiation Therapy Intensity Modulated Radiation Therapy (IMRT)(IMRT)

IMRT requires increased monitor units per cGy at isocenterIMRT requires increased monitor units per cGy at isocenter– Typical IMRT ratio is 5 MU per cGy, as high as 10 for some systems Typical IMRT ratio is 5 MU per cGy, as high as 10 for some systems

Percent workload with IMRT impacts shieldingPercent workload with IMRT impacts shielding– 50% typically assumed; 100% if vault is dedicated to IMRT50% typically assumed; 100% if vault is dedicated to IMRT

Account for IMRT by multiplying x-ray leakage by IMRT Account for IMRT by multiplying x-ray leakage by IMRT factorfactor– IMRT Factor = % IMRT x IMRT ratio + (1 - % IMRT)IMRT Factor = % IMRT x IMRT ratio + (1 - % IMRT)

– 3 is typical IMRT factor (50% workload with IMRT ratio of 5)3 is typical IMRT factor (50% workload with IMRT ratio of 5)

IMRT factor lower for neutrons if machine is dual energyIMRT factor lower for neutrons if machine is dual energy– e.g., 1.5 if dual energy linac with 50% of treatments below 10 MVe.g., 1.5 if dual energy linac with 50% of treatments below 10 MV

» Pessimistic since most IMRT is performed at 6 MV (next chart)Pessimistic since most IMRT is performed at 6 MV (next chart)

IMRT above 10 MV Significantly Increases IMRT above 10 MV Significantly Increases Patient Photonuclear Dose Patient Photonuclear Dose

Neutrons dominate patient total body dose for high energy Neutrons dominate patient total body dose for high energy linacslinacs– Neutron dose equivalent as high as ten times photon dose Neutron dose equivalent as high as ten times photon dose

» Potentially 1% of workload vs 0.1% photon leakagePotentially 1% of workload vs 0.1% photon leakage 0.05% required absorbed neutron dose x 20 quality factor0.05% required absorbed neutron dose x 20 quality factor

– Typical neutron dose equivalent is lower than requirementTypical neutron dose equivalent is lower than requirement

» 0.1 to 0.2% of workload 0.1 to 0.2% of workload

IMRT factor of 5 increases patient incidental dose 5XIMRT factor of 5 increases patient incidental dose 5X– Results in typical neutron total body exposure of 0.5 to 1.0% of WLResults in typical neutron total body exposure of 0.5 to 1.0% of WL

– Significantly increases risk of secondary cancerSignificantly increases risk of secondary cancer

Most IMRT is performed at 6 MV to mitigate increased secondary Most IMRT is performed at 6 MV to mitigate increased secondary cancer risk from photoneutronscancer risk from photoneutrons

Most IMRT is performed at 6 MV to mitigate increased secondary Most IMRT is performed at 6 MV to mitigate increased secondary cancer risk from photoneutronscancer risk from photoneutrons

Patient Scatter Significant Adjacent to Patient Scatter Significant Adjacent to Primary BarrierPrimary Barrier

Scatter traditionally neglected Scatter traditionally neglected for lateral barriersfor lateral barriers– Generally a good assumptionGenerally a good assumption

– 90 degree scatter has low 90 degree scatter has low energyenergy

Scatter is significant adjacent to Scatter is significant adjacent to primary barrier primary barrier – Calculations indicate Calculations indicate

comparable to leakagecomparable to leakage

– Slant thickness through barrier Slant thickness through barrier compensates for the increase in compensates for the increase in unshielded dose due to scatterunshielded dose due to scatter

» Barrier thickness Barrier thickness comparable to lateral is comparable to lateral is adequate for same P/Tadequate for same P/T

BD

A

C

D'

C'

A'

Maze

TargetRotational

Plane

*Target

Isocenterdsca

dsecSlant thickness

used to calculateattenuation

ScatterAngle

Door

1 ftActualbarrier

thickness

Patient Scatter Fraction for 400 cmPatient Scatter Fraction for 400 cm22 Field Field

Based on recent simulation work by Taylor et.al.Based on recent simulation work by Taylor et.al.

Scatter fraction increases as angle decreasesScatter fraction increases as angle decreases

Scatter fraction vs MV may increase or decreaseScatter fraction vs MV may increase or decrease– Tends to increase with MV at small scatter anglesTends to increase with MV at small scatter angles

– Decreases with increasing MV at large scatter anglesDecreases with increasing MV at large scatter angles

Angle (degrees)MV 10 20 30 45 60 90 135 1504 1.04E-02 6.73E-03 2.77E-03 2.09E-03 1.24E-03 6.39E-04 4.50E-04 4.31E-046 1.04E-02 6.73E-03 2.77E-03 1.39E-03 8.24E-04 4.26E-04 3.00E-04 2.87E-0410 1.66E-02 5.79E-03 3.18E-03 1.35E-03 7.46E-04 3.81E-04 3.02E-04 2.74E-0415 1.51E-02 5.54E-03 2.77E-03 1.05E-03 5.45E-04 2.61E-04 1.91E-04 1.78E-0418 1.42E-02 5.39E-03 2.53E-03 8.64E-04 4.24E-04 1.89E-04 1.24E-04 1.20E-0420 1.52E-02 5.66E-03 2.59E-03 8.54E-04 4.13E-04 1.85E-04 1.23E-04 1.18E-0424 1.73E-02 6.19E-03 2.71E-03 8.35E-04 3.91E-04 1.76E-04 1.21E-04 1.14E-04

Patient Scatter EnergyPatient Scatter Energy

Mean Scatter EnergyMean Scatter Energy

No standardized scatter Tenth-Value LayerNo standardized scatter Tenth-Value Layer– Primary MV rating based on peak MV in spectrum, not mean energyPrimary MV rating based on peak MV in spectrum, not mean energy

– Primary TVL at slightly higher MV (e.g, 50%) appears reasonablePrimary TVL at slightly higher MV (e.g, 50%) appears reasonable

» % increase little more than wild guess; more research is needed% increase little more than wild guess; more research is needed

Scatter Angle (degrees)MV 0 20 45 906 1.7 1.2 0.6 0.2510 2.8 1.4 0.6 0.2518 5.0 2.2 0.7 0.324 5.7 2.7 0.9 0.3

Ambiguity remains as to TVL to use for scatter Ambiguity remains as to TVL to use for scatter Ambiguity remains as to TVL to use for scatter Ambiguity remains as to TVL to use for scatter

Maze Calculation Likely Revised in Maze Calculation Likely Revised in Upcoming NCRP ReportUpcoming NCRP Report

New method identifies and evaluates specific mechanismsNew method identifies and evaluates specific mechanisms– Patient Scatter, Wall Scatter, Leakage scatterPatient Scatter, Wall Scatter, Leakage scatter

– Direct leakageDirect leakage

– Neutrons, capture gammasNeutrons, capture gammas

Mechanisms calculated at most stressing orientationMechanisms calculated at most stressing orientation– Scatter calculations multiplied by 2/3 to compensate for thisScatter calculations multiplied by 2/3 to compensate for this

Scatter energy relatively low at maze doorScatter energy relatively low at maze door– Primary 0.3 MV TVLs used for patient and wall scatter (2 bounces)Primary 0.3 MV TVLs used for patient and wall scatter (2 bounces)

– Primary 0.5 MV TVLs used for leakage scatter (1 bounce)Primary 0.5 MV TVLs used for leakage scatter (1 bounce)

– Scatter is significant typically only for low energy linacsScatter is significant typically only for low energy linacs

Goal: More-precise calculation avoiding over or under-shieldingGoal: More-precise calculation avoiding over or under-shieldingGoal: More-precise calculation avoiding over or under-shieldingGoal: More-precise calculation avoiding over or under-shielding

Maze: Patient ScatterMaze: Patient Scatter

Unshielded doseUnshielded dose

wherewhere

– 0.50.5 is 0.5 MV scatter fraction is 0.5 MV scatter fraction

» Second bounce fractionSecond bounce fraction

» 0.02 per m0.02 per m22 typically used typically used

– Other constants as before, e.g., Other constants as before, e.g.,

» a = patient scatter fractiona = patient scatter fraction

» F = field size in cm^2F = field size in cm^2

» h = room heighth = room height

23

22

21

5.0)400/(

PPP

Cp

ddd

AFWaS

BD

A

C

D'

A'

TargetRotational

Plane

*Target

Isocenter

dP2

dP3

dP1

w C

AC = w C h

Door

Maze: Wall ScatterMaze: Wall Scatter

Unshielded doseUnshielded dose

wherewhere– f f = patient transmission= patient transmission

– 11 = first reflection coefficient = first reflection coefficient

» 0.005 per m0.005 per m22 for 6 MV for 6 MV

» 0.004 per m0.004 per m22 for for 10 MV 10 MV

– AA11 = beam area (m = beam area (m22) at wall) at wall

– AAMM = Maze cross section (m = Maze cross section (m22))

» ddMM x room height x room height

23

22

21

5.011

SSS

MS

ddd

AAWfS

D

A

C

D'

A'

TargetRotational

Plane

* Target

Isocenter

dS1

dS2dM

dS3

Door

Maze: Leakage ScatterMaze: Leakage Scatter

Unshielded doseUnshielded dose

wherewhere– Constants as previously Constants as previously

defineddefined

22

21

1310

LL

CLS

dd

AWS

BD

A

C

D'

A'

TargetRotational

Plane

* Target

Isocenter

dL1

w C

dL2

AC = w C h

Door

Maze: Direct LeakageMaze: Direct Leakage

Unshielded doseUnshielded dose

Same as standard secondary Same as standard secondary photon leakage calculationphoton leakage calculation

Standard neutron leakage not Standard neutron leakage not typically usedtypically used– Use only if it exceeds the maze Use only if it exceeds the maze

neutron calculationneutron calculation

» e.g., if maze wall not e.g., if maze wall not sufficiently thicksufficiently thick

2

/3 '1010

L

TVLt

Ld

WS

D

B

A

C

A'

TargetRotationalPlane

* Target

Isocenter

C'

D'

dL

tD'

D

Door

Maze Neutron Calculation Based on Maze Neutron Calculation Based on Modified Kersey MethodModified Kersey Method

Unshielded dose equivalentUnshielded dose equivalent

wherewhere

– LLnn is neutron leakage fraction is neutron leakage fraction

» Same as used for secondary Same as used for secondary neutron leakage calculationneutron leakage calculation

– Modification to Kersey is Modification to Kersey is assuming first tenth-value assuming first tenth-value distance is 3 m instead of 5 mdistance is 3 m instead of 5 m

]5/)3(1[21

210

NdN

nNT

d

LWH

Upcoming NCRP report may recommend a more-complex Upcoming NCRP report may recommend a more-complex approach than thisapproach than this

Upcoming NCRP report may recommend a more-complex Upcoming NCRP report may recommend a more-complex approach than thisapproach than this

BD

A

C

D'

A'

TargetRotational

Plane

* Target

Isocenter

dN1

C'

Door

dN2

Maze Neutron ShieldingMaze Neutron Shielding

Modeled as 50% thermal neutrons and 50% fast neutronsModeled as 50% thermal neutrons and 50% fast neutrons

1 inch borated poly effectively eliminates all thermal 1 inch borated poly effectively eliminates all thermal neutronsneutrons

Fast neutron TVL is 2.4 inches for the first 4 inchesFast neutron TVL is 2.4 inches for the first 4 inches

Fast neutron TVL is 3.6 inches beyond 4 inches thicknessFast neutron TVL is 3.6 inches beyond 4 inches thickness

Maze Capture Gammas from ConcreteMaze Capture Gammas from Concrete

Gamma rays generated by neutron capture in the mazeGamma rays generated by neutron capture in the maze– Very significant for high energy linacsVery significant for high energy linacs

Unshielded dose is a factor of 0.2 to 0.5 of the neutron Unshielded dose is a factor of 0.2 to 0.5 of the neutron dose equivalent at the treatment room doordose equivalent at the treatment room door– Use the conservative factor (0.5)Use the conservative factor (0.5)

Capture gammas have moderate energy (3.6 MeV)Capture gammas have moderate energy (3.6 MeV)– TVL of 61 mm for leadTVL of 61 mm for lead

– Limited attenuation also provided by polyethylene (278 mm TVL) Limited attenuation also provided by polyethylene (278 mm TVL)

Dominates X-Ray dose at maze entrance for high energy linacsDominates X-Ray dose at maze entrance for high energy linacsDominates X-Ray dose at maze entrance for high energy linacsDominates X-Ray dose at maze entrance for high energy linacs

Direct-Shielded DoorDirect-Shielded Door

Neutron Door is simply a secondary barrierNeutron Door is simply a secondary barrier– Typically more layers and different materials than a wallTypically more layers and different materials than a wall

» Lead to attenuate leakage photonsLead to attenuate leakage photons

» Borated polyethylene to attenuate leakage neutronsBorated polyethylene to attenuate leakage neutrons Typically sandwiched between layers of leadTypically sandwiched between layers of lead

» Steel coversSteel covers

Specialized shielding procedure adjacent to doorSpecialized shielding procedure adjacent to door– Compensates for relatively small slant thickness in this locationCompensates for relatively small slant thickness in this location

– Vault entry toward isocenter similar to mazeVault entry toward isocenter similar to maze

– Vault entry away from isocenter is secondary barrierVault entry away from isocenter is secondary barrier

» But with specialized geometryBut with specialized geometry

Direct-Shielded Door: Far Side of EntranceDirect-Shielded Door: Far Side of Entrance

Extra material added to cornerExtra material added to corner– Lead to entrance wallLead to entrance wall

– Borated polyethylene or Borated polyethylene or concrete beyond wallconcrete beyond wall

Uses standard secondary Uses standard secondary barrier calculationbarrier calculation

Goal: provide same protection Goal: provide same protection as wall or door for path through as wall or door for path through corner corner

TargetRotationalPlane

Isocenter

TypicalGap0.5"

7.5"OverlapTypical

Door OverlapBeyond Far Sideof Entrance

Protected Point(1 ft beyond

door enclosure)

Isocenter toFar Side ofEntranceDistance

Isocenter to Door

SecondaryDistance

Direct-Shielded Door: Near Side of EntranceDirect-Shielded Door: Near Side of Entrance

Geometry similar to short mazeGeometry similar to short maze– Maze calculation can be used Maze calculation can be used

but is likely pessimisticbut is likely pessimistic

Requires less material than far Requires less material than far side of entranceside of entrance– Lower unshielded doseLower unshielded dose

– Lower energyLower energyTargetRotationalPlane

Isocenter

ProtectedPoint

(1 ft beyonddoor

enclosure)

dN1

*Target

dN2

7.5"TypicalDoorOverlap

TypicalGap0.5"

Shielding for Heating, Ventilation, and Air Shielding for Heating, Ventilation, and Air Conditioning (HVAC) DuctsConditioning (HVAC) Ducts

HVAC penetration is located at ceiling level in the vaultHVAC penetration is located at ceiling level in the vault– For vaults with maze, typically located immediately above doorFor vaults with maze, typically located immediately above door

– For direct-shielded doors, located in a lateral wall as far away from For direct-shielded doors, located in a lateral wall as far away from isocenter as possibleisocenter as possible

Ducts shielded with material similar to the door at entranceDucts shielded with material similar to the door at entrance

Material thickness 1/2 to 1/3 that required of the doorMaterial thickness 1/2 to 1/3 that required of the door– Path through material is at a very oblique angle due to penetration Path through material is at a very oblique angle due to penetration

location with slant factor between 2 and 3location with slant factor between 2 and 3

– Factor of at least 5 reduction in dose at head level (the protected Factor of at least 5 reduction in dose at head level (the protected location) vs. at the HVAC duct openinglocation) vs. at the HVAC duct opening

NCRP 49 recommends that shielding extend at least a factor NCRP 49 recommends that shielding extend at least a factor of three times the width of the HVAC penetrationof three times the width of the HVAC penetration

Photon SkyshinePhoton Skyshine

Unshielded doseUnshielded dose

wherewhere– (steradians) = 0.122(steradians) = 0.122

» for 40 x 40 cm beamfor 40 x 40 cm beam

Multiplying by additional factor Multiplying by additional factor of two is recommended of two is recommended

Primary TVLs used to calculate Primary TVLs used to calculate attenuationattenuation

22

21

3.10249.0

YYsky

dd

UWS

New construction seldom shields solely for skyshine due to New construction seldom shields solely for skyshine due to vigilance required to prevent unauthorized roof accessvigilance required to prevent unauthorized roof access

New construction seldom shields solely for skyshine due to New construction seldom shields solely for skyshine due to vigilance required to prevent unauthorized roof accessvigilance required to prevent unauthorized roof access

Floor

* Target

Isocenter

h

h

dY2

2 meters

dY1

Neutron SkyshineNeutron Skyshine

Unshielded doseUnshielded dose

wherewhere– = 2.71 (steradians) typical = 2.71 (steradians) typical

(target above isocenter)(target above isocenter)

– HHpripri is neutron dose-eq in beam is neutron dose-eq in beam

(0.00013, 0.002, 0.0039, 0.0043, and (0.00013, 0.002, 0.0039, 0.0043, and 0.014 times W for 10, 15, 18, 20, 0.014 times W for 10, 15, 18, 20, and 24 MV, respectively)and 24 MV, respectively)

Use factor is not applied since Use factor is not applied since neutrons in all orientationsneutrons in all orientations

Multiplying by additional factor of Multiplying by additional factor of two is recommended two is recommended

2

104.5 4

pri

sky

HH

Floor

* Target

Isocenter

Up to 20 meters lateral distance

Primary Goal of Upcoming NCRP Report is Primary Goal of Upcoming NCRP Report is Improved Shielding Calculation AccuracyImproved Shielding Calculation Accuracy

Very little impact for low energy acceleratorsVery little impact for low energy accelerators– Primary and secondary barrier calculation method unchangedPrimary and secondary barrier calculation method unchanged

– Very little impact to calculated shielding for given protection limitVery little impact to calculated shielding for given protection limit

Improved accuracy for high-energy acceleratorsImproved accuracy for high-energy accelerators– Avoids extra cost of over design due to pessimistic calculationsAvoids extra cost of over design due to pessimistic calculations

– Avoid extra cost of retrofitting if inaccurate calculations Avoid extra cost of retrofitting if inaccurate calculations underestimate required shieldingunderestimate required shielding

ReferencesReferences

Biggs, Peter J. “Obliquity factors for Biggs, Peter J. “Obliquity factors for 6060Co and 4, 10, 18 MV X Co and 4, 10, 18 MV X rays for concrete, steel, and lead and angles of incidence rays for concrete, steel, and lead and angles of incidence between 0º and 70º,” Health Physics. Vol. 70, No 4, 527-536, between 0º and 70º,” Health Physics. Vol. 70, No 4, 527-536, 1996.1996.

British Journal of Radiology (BJR) Supplement No. 11. British Journal of Radiology (BJR) Supplement No. 11. Central axis depth dose data for use in radiotherapy, 1972.Central axis depth dose data for use in radiotherapy, 1972.

Chibani, Omar and C.C. Ma. “Photonuclear dose calculations Chibani, Omar and C.C. Ma. “Photonuclear dose calculations for high-energy beams from Siemens and Varian linacs,” for high-energy beams from Siemens and Varian linacs,” Medical Physics, Vol 30, No. 8:1990-2000, August 2003. Medical Physics, Vol 30, No. 8:1990-2000, August 2003.

Kleck, J. “Radiation therapy facility shielding design.” 1998 Kleck, J. “Radiation therapy facility shielding design.” 1998 AAPM Annual MeetingAAPM Annual Meeting

References (Continued)References (Continued)

McGinley, P.H. Shielding Techniques for Radiation McGinley, P.H. Shielding Techniques for Radiation Oncology Facilities, 2nd ed. Madison, WI: Medical Physics Oncology Facilities, 2nd ed. Madison, WI: Medical Physics Publishing, 2002.Publishing, 2002.

National Council on Radiation Protection and National Council on Radiation Protection and Measurements. Measurements. Structural shielding design and evaluation Structural shielding design and evaluation for medical use of x-ray and gamma rays of energies up to for medical use of x-ray and gamma rays of energies up to 10 MeV.10 MeV. Washington, DC: NCRP, NCRP Report 49, 1976. Washington, DC: NCRP, NCRP Report 49, 1976.

National Council on Radiation Protection and National Council on Radiation Protection and Measurements. Measurements. Radiation protection design guidelines for Radiation protection design guidelines for 0.1-100 MeV particle accelerator facilities.0.1-100 MeV particle accelerator facilities. Washington, DC: Washington, DC: NCRP, NCRP Report 51, 1977.NCRP, NCRP Report 51, 1977.

References (Continued)References (Continued)

National Council on Radiation Protection and Measurements. National Council on Radiation Protection and Measurements. Neutron Contamination from Medical Accelerators.Neutron Contamination from Medical Accelerators. Bethesda, MD: NCRP, NCRP Report 79, 1984.Bethesda, MD: NCRP, NCRP Report 79, 1984.

Nelson, W.R., and P.D. LaRiviere. “Primary and leakage Nelson, W.R., and P.D. LaRiviere. “Primary and leakage radiation calculations at 6, 10, and 25 MeV,” Health Physics. radiation calculations at 6, 10, and 25 MeV,” Health Physics. Vol. 47, No. 6: 811-818, 1984.Vol. 47, No. 6: 811-818, 1984.

Rodgers, James E. “IMRT Shielding Symposium” AAPM Rodgers, James E. “IMRT Shielding Symposium” AAPM Annual Meeting, 2001.Annual Meeting, 2001.

Shobe, J., J.E. Rodgers, and P.L. Taylor. “Scattered fractions Shobe, J., J.E. Rodgers, and P.L. Taylor. “Scattered fractions of dose from 6, 10, 18, and 25 MV linear accelerator X rays in of dose from 6, 10, 18, and 25 MV linear accelerator X rays in radiotherapy facilities,” Health Physics, Vol. 76, No. 1, 27-35, radiotherapy facilities,” Health Physics, Vol. 76, No. 1, 27-35, 1999.1999.

References (Continued)References (Continued)

Taylor, P.L., J.E. Rodgers, and J. Shobe. “Scatter fractions Taylor, P.L., J.E. Rodgers, and J. Shobe. “Scatter fractions from linear accelerators with x-ray energies from 6 to 24 from linear accelerators with x-ray energies from 6 to 24 MV," Medical Physics, Vol. 26, No. 8, 1442-46, 1999.MV," Medical Physics, Vol. 26, No. 8, 1442-46, 1999.