Shielding Calculation for LINAC

(1)

Therapy Shielding Calculations

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

American College of Medical Physics

21st Annual Meeting & Workshops

Scottsdale, AZ

June 13, 2004

(2)

Therapy Shielding Design Traditionally

Relies 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}6060_{Cobalt and linacs up to 10 MV}_{Cobalt and linacs up to 10 MV}

– Extended NCRP 49 methodology up to 100 MV_{Extended NCRP 49 methodology up to 100 MV} – Empirical shielding requirements for maze doors_{Empirical shielding requirements for maze doors}

– Improved neutron shielding methodology_{Improved neutron shielding methodology}

– Update of NCRP 51 primarily aimed at non-medical facilities_{Update of NCRP 51 primarily aimed at non-medical facilities}

Reports reflect progress in

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

Reports reflect progress in

(3)

Revised NCRP Report in Drafting Stage by

AAPM 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 maze_{Both 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 barrier_{Photoneutron generation due to metal in primary barrier}

Goal: Improved accuracy

(4)

■ BJR #11 megavoltage (MV) definition used hereBJR #11 megavoltage (MV) definition used here

– British Journal of Radiology (BJR) Supplement No. 11_{British 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 calculations_{Workload 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 2_{Based on NCRP 49 Appendix C Table 2} – For MV > 10: W = 500 Gy/wk_{For MV > 10: W = 500 Gy/wk}

» Based on NCRP 51 Appendix B Table 5_{Based on NCRP 51 Appendix B Table 5}

Linear Accelerator Energy and Workload

BJR #11 MV 4 6 10 15 18 20 24

(5)

Radiation 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 limit_{Exposure must not exceed a specific dose equivalent limit} – Limiting exposure to unoccupied locations is not the goal_{Limiting 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 exposure_{0.10 mSv/week for occupational exposure} – 0.02 mSv/week for the general public_{0.02 mSv/week for the general public}

■ Typical international design dose limitsTypical international design dose limits

– 0.12 mSv/week for controlled areas_{0.12 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 value

(6)

Radiation 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 occupied

■ Maximum shielded dose (SMaximum shielded dose (S_max_max) at protected location) at protected location

– Assuming occupancy factor T for protected location_{Assuming occupancy factor T for protected location}

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

T

P

(7)

Occupancy 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

buildings

■ 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 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 traffic

(8)

Hourly 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

– e.g., unoccupied roof, machinery room_{e.g., unoccupied roof, machinery room}

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

(9)

NCRP 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 both_{NCRP 134 maintains NCRP 116 limit of 0.02 mSv/wk for both}

– Limit 25% of 0.02 mSv/wk from individual facility for general public_{Limit 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 occupancy_{T=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 barriers_{Similar 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 occupancy_{T=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 adopted

(10)

Basic Primary Barrier Calculation

Unchanged from NCRP 49

■ Unshielded dose calculationUnshielded dose calculation

■ Attenuation in tenth-value layersAttenuation in tenth-value layers

■ Barrier thickness (tBarrier thickness (t_c_c) calculation) calculation

2 pri pri d U W S = e C TVL n TVL t = ₁ + ( −1)       = T P S n pri / log₁₀

Margin in primary barrier thickness is recommended to

compensate for potential concrete density variation

Margin in primary barrier thickness is recommended to

compensate for potential concrete density variation

B D A D ' C ' A ' M a z e T a r g e t R o t a t i o n a l P l a n e

*

T a r g e t I s o c e n t e r 1 f t t_C d _{p r i} C D o o r

(11)

Primary Barrier Photon Tenth-Value Layers

(mm) Come from a Variety of Sources

Lead Concrete Steel Earth Borated Poly

MV TVL1 TVLe TVL1 TVLe TVL1 TVLe TVL1 TVLe TVL1 TVLe 0.2 0.25 0.3 0.4 0.5 1 2 4 1.7 1.7 84 84 2.9 2.9 94 94 4.8 4.8 104 104 8.3 8.3 109 109 11.9 11.9 117 117 26 26 147 147 42 42 210 210 53 53 292 292 6 10 15 572 572 648 648 720 720 379 379 379 379 18 20 24 367 323 410 377 445 416 462 432 470 442 483 457 56 56 56 56 56 56 56 56 56 56 56 56 15 15 19 19 22 22 29 29 33 33 54 51 76 69 91 91 100 100 104 104 108 108 109 109 110 110 110 110 135 135 84 84 151 151 94 94 167 167 104 104 175 175 109 109 188 188 117 117 236 236 147 147 336 336 210 210 468 468 292 292 343 343 740 740 379 379 752 752 390 390 773 773 401 401 NCRP 49 NCRP 51

Anticipate upcoming NCRP report to review and update TVL data

(12)

Primary 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 size_{Barrier 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}

ft d w_C = 0.4 2 _C_' + 1.0 C '

*

T a r g e t I s o c e n t e r T a r g e t t o N a r r o w P o in t D is t a n c e ( d_{C '}) w _C 1 f t 1 f t C _{C '}

*

T a r g e t I s o c e n t e r T a r g e t t o N a r r o w P o in t D is t a n c e ( d_{C '}) w _C 1 f t 1 f t C

*

T a r g e t I s o c e n t e r w _C 1 f t 1 f t M e t a l T a r g e t to N a r r o w P o i n t D is t a n c e ( d_{C '})

(13)

Slant Factor and Obliquity Factor

■ Slant FactorSlant Factor

– Path from target to protected location diagonally through barrier_{Path from target to protected location diagonally through barrier} » Incident angle _{Incident angle}θ_θ of line with respect to perpendicular of line with respect to perpendicular

– Required barrier thickness reduced by cos(_{Required barrier thickness reduced by cos(}θ_θ))

» Same total distance through barrier to protected location_{Same 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 this_{Multiplying thickness by obliquity factor compensates for this}

Lead Concrete Steel

Angle 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 1.00 30 1.03 1.02 1.03 1.02 1.00 1.00 1.02 1.02 1.04 45 1.07 1.07 1.10 1.07 1.04 1.04 1.07 1.07 1.08 60 1.21 1.21 1.22 1.20 1.14 1.08 1.20 1.17 1.20 70 1.44 1.47 1.52 1.47 1.28 1.22 1.48 1.42 1.45

(14)

Photoneutron Generation Due to Metal in

Primary Barrier (Linacs

≥

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 appropriate

value for steel at 18 MV is appropriate

» N adjusted versus MV based on neutron leakage fraction vs MV_{N 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}_{), t}

2

2 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 / TVL_p_p))

– Neutron attenuation after metal layer: 10^(-tNeutron attenuation after metal layer: 10^(-t₃₃ / TVL / TVL_N_N)) N P t TVL TVL t N t t F N U W S / / 3 2 3 1 10 10 305 . 0 2 − − + + =

(15)

Patient Photonuclear Dose Due to Metal in

Primary Barrier for MV > 10

■ Metal in primary barrier can increase patient total body dose Metal in primary barrier can increase patient total body dose

if MV > 10

– Lead inside layer approximately doubles patient total body dose_{Lead inside layer approximately doubles patient total body dose} – Increases risk of secondary cancer_{Increases 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

– 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 2

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

later

Avoid metal as inside layer of primary barrier if MV >

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

Avoid metal as inside layer of primary barrier if MV >

(16)

Secondary Barrier

■ Patient scatter unshielded dosePatient scatter unshielded dose

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

» typically 1600_{typically 1600}

– _a_{= scatter fraction}_{= scatter fraction}

for 20 x 20 cm beam for 20 x 20 cm beam

■ Leakage unshielded doseLeakage unshielded dose

– Assumes 0.1% leakage fraction_{Assumes 0.1% leakage fraction} 2 sec 2 ) 400 / ( d d F W a S sca p = 2 sec 3 10 d W S_L − = B D A C D ' C ' A ' M a z e T a r g e t R o t a t i o n a l P l a n e

*

T a r g e t I s o c e n t e r d _{s c a} 1 f t t_B d _{s e c} D o o r

(17)

Leakage Photon Tenth-Value Layers (mm)

Also Come from a Variety of Sources

Nelson & LaRiviere

NCRP 49 Kleck & Varian_Average _{from Concrete}Estimated

Lead Concrete Steel Earth Borated Poly MV TVL1 TVLe TVL1 TVLe TVL1 TVLe TVL1 TVLe TVL1 TVLe

4 53 53 292 292 91 91 468 468 292 292 6 56 56 341 284 96 96 546 455 341 284 10 56 56 351 320 96 96 562 512 351 320 15 56 56 361 338 96 96 578 541 361 338 18 56 56 363 343 96 96 581 549 363 343 20 56 56 366 345 96 96 586 552 366 345 24 56 56 371 351 96 96 594 562 371 351

(18)

Neutron 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 MV_{0.002%, 0.04%, 0.10%, 0.15% and 0.20% for 10, 15, 18, 20 and 24 MV} – Based on Varian and Siemens neutron leakage data_{Based on Varian and Siemens neutron leakage data}

» Assumes quality factor of 10 for absorbed dose_{Assumes 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 concrete_{211 mm for concrete}

(19)

Intensity Modulated Radiation Therapy

(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 IMRT_{50% 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

factor

– 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 MV_{e.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)}

(20)

IMRT above 10 MV Significantly Increases

Patient Photonuclear Dose

■ Neutrons dominate patient total body dose for high energy Neutrons dominate patient total body dose for high energy

linacs

– 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 leakage_{Potentially 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 requirement_{Typical 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 WL_{Results in typical neutron total body exposure of 0.5 to 1.0% of WL} – Significantly increases risk of secondary cancer_{Significantly increases risk of secondary cancer}

Most IMRT is performed at 6 MV to mitigate increased secondary

cancer risk from photoneutrons

Most IMRT is performed at 6 MV to mitigate increased secondary

cancer risk from photoneutrons

(21)

Patient Scatter Significant Adjacent to

Primary Barrier

■ Scatter traditionally neglected Scatter traditionally neglected

for lateral barriers

– Generally a good assumption_{Generally a good assumption}

– 90 degree scatter has low 90 degree scatter has low energy

energy

■ Scatter is significant adjacent to Scatter is significant adjacent to

primary barrier

– Calculations indicate _{Calculations indicate} comparable to leakage comparable to leakage

– Slant thickness through barrier _{Slant thickness through barrier} compensates for the increase in compensates for the increase in unshielded dose due to scatter unshielded dose due to scatter

» Barrier thickness _{Barrier thickness}

comparable to lateral is comparable to lateral is adequate for same P/T adequate for same P/T

B D A C D ' C ' A ' M a z e T a r g e t R o t a t i o n a l P l a n e

*

T a r g e t I s o c e n t e r d _{s c a} d _{s e c} S la n t t h ic k n e s s u s e d to c a lc u l a te a t t e n u a ti o n S c a t t e r A n g l e θ D o o r 1 f t A c t u a l b a r r ie r t h ic k n e s s

(22)

Patient Scatter Fraction for 400 cm

22

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 angles_{Tends to increase with MV at small scatter angles} – Decreases with increasing MV at large scatter angles_{Decreases with increasing MV at large scatter angles}

Angle (degrees)

MV 10 20 30 45 60 90 135 150

4 1.04E-02 6.73E-03 2.77E-03 2.09E-03 1.24E-03 6.39E-04 4.50E-04 4.31E-04 6 1.04E-02 6.73E-03 2.77E-03 1.39E-03 8.24E-04 4.26E-04 3.00E-04 2.87E-04 10 1.66E-02 5.79E-03 3.18E-03 1.35E-03 7.46E-04 3.81E-04 3.02E-04 2.74E-04 15 1.51E-02 5.54E-03 2.77E-03 1.05E-03 5.45E-04 2.61E-04 1.91E-04 1.78E-04 18 1.42E-02 5.39E-03 2.53E-03 8.64E-04 4.24E-04 1.89E-04 1.24E-04 1.20E-04 20 1.52E-02 5.66E-03 2.59E-03 8.54E-04 4.13E-04 1.85E-04 1.23E-04 1.18E-04 24 1.73E-02 6.19E-03 2.71E-03 8.35E-04 3.91E-04 1.76E-04 1.21E-04 1.14E-04

(23)

Patient 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 energy_{Primary MV rating based on peak MV in spectrum, not mean energy} – Primary TVL at slightly higher MV (e.g, 50%) appears reasonable_{Primary TVL at slightly higher MV (e.g, 50%) appears reasonable}

» % increase little more than wild guess; more research is _{% increase little more than wild guess; more research is}

needed

Scatter Angle (degrees)

MV 0 20 45 90

6 1.7 1.2 0.6 0.25

10 2.8 1.4 0.6 0.25

18 5.0 2.2 0.7 0.3

24 5.7 2.7 0.9 0.3

Ambiguity remains as to TVL to use for scatter

(24)

Maze Calculation Likely Revised in

Upcoming NCRP Report

■ New method identifies and evaluates specific mechanismsNew method identifies and evaluates specific mechanisms

– Patient Scatter, Wall Scatter, Leakage scatter_{Patient Scatter, Wall Scatter, Leakage scatter} – Direct leakage_{Direct leakage}

– Neutrons, capture gammas_{Neutrons, capture gammas}

■ Mechanisms calculated at most stressing orientationMechanisms calculated at most stressing orientation

– Scatter calculations multiplied by 2/3 to compensate for this_{Scatter 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 linacs_{Scatter is significant typically only for low energy linacs}

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

(25)

Maze: Patient Scatter

■ Unshielded doseUnshielded dose

■ wherewhere

– α_α_0.5_0.5 is 0.5 MV scatter fraction is 0.5 MV scatter fraction

» Second bounce fraction_{Second 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^2_{F = field size in cm^2}

» h = room heighth = room height 2 3 2 2 2 1 5 . 0 ) 400 / ( P P P C p d d d A F W a S = α B D A C D ' A ' T a r g e t R o t a t i o n a l P l a n e

*

T a r g e t I s o c e n t e r d _{P 2} d _{P 3} d P 1 w _C A _C = w _C h D o o r

(26)

Maze: Wall Scatter

where

– f _f= patient transmission= patient transmission

– α_α₁₁ = first reflection coefficient = first reflection coefficient

» 0.005 per m0.005 per m22_{for 6 MV}_{for 6 MV}

» 0.004 per m_{0.004 per m}22_for_for≥≥ _{10 MV}_{10 MV}

– AA₁₁ = beam area (m = beam area (m22_{) at wall}_{) at wall}

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

» dd_M_M x room height x room height 2 3 2 2 2 1 5 . 0 1 1 S S S M S d d d A A W f S = α α D A C D ' A ' T a r g e t R o t a t i o n a l P l a n e

*

T a r g e t I s o c e n t e r d _{S 1} d _{S 2} d _M d _{S 3} D o o r

(27)

Maze: Leakage Scatter

where

– Constants as previously _{Constants as previously} defined defined 2 2 2 1 1 3 10 L L C LS d d A W S α − = B D A C D ' A ' T a r g e t R o t a t i o n a l P l a n e

*

T a r g e t I s o c e n t e r d _{L 1} w _C d _{L 2} A _C = w _C h D o o r

(28)

Maze: Direct Leakage

■ Same as standard secondary Same as standard secondary

photon leakage calculation

■ Standard neutron leakage not Standard neutron leakage not

typically used

– Use only if it exceeds the maze Use only if it exceeds the maze neutron calculation

neutron calculation

» e.g., if maze wall not e.g., if maze wall not sufficiently thick sufficiently thick 2 / 3 _' 10 10 L TVL t L d W S D − − = B A C A ' T a r g e t R o t a t i o n a l P la n e

*

T a r g e t I s o c e n t e r C ' D ' d _L t_{D '} θ D D o o r

(29)

Maze Neutron Calculation Based on

Modified Kersey Method

■ Unshielded dose equivalentUnshielded dose equivalent

where

– LL_n_n is neutron leakage fraction is neutron leakage fraction

» Same as used for secondary _{Same as used for secondary} neutron leakage calculation neutron 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 m distance is 3 m instead of 5 m ] 5 / ) 3 ( 1 [ 2 1 10 2 − + = N d N n NT d L W H

Upcoming NCRP report may recommend a more-complex approach

than this

Upcoming NCRP report may recommend a more-complex approach

than this than this B D A C D ' A ' T a r g e t R o t a t i o n a l P l a n e

*

T a r g e t I s o c e n t e r d _{N 1} C ' D o o r d _{N 2}

(30)

Maze 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

neutrons

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

(31)

Maze 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 linacs_{Very significant for high energy linacs}

■ Unshielded dose is a factor of 0.2 to 0.5 of the neutron dose Unshielded dose is a factor of 0.2 to 0.5 of the neutron dose

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 lead_{TVL 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 linacs

(32)

Direct-Shielded Door

■ Neutron Door is simply a secondary barrierNeutron Door is simply a secondary barrier

– Typically more layers and different materials than a wall_{Typically more layers and different materials than a wall} » Lead to attenuate leakage photons_{Lead to attenuate leakage photons}

» Borated polyethylene to attenuate leakage neutrons_{Borated polyethylene to attenuate leakage neutrons}

■ Typically sandwiched between layers of leadTypically sandwiched between layers of lead

» Steel covers_{Steel covers}

■ Specialized shielding procedure adjacent to doorSpecialized shielding procedure adjacent to door

– Compensates for relatively small slant thickness in this location_{Compensates for relatively small slant thickness in this location} – Vault entry toward isocenter similar to maze_{Vault entry toward isocenter similar to maze}

– Vault entry away from isocenter is secondary barrier_{Vault entry away from isocenter is secondary barrier} » But with specialized geometry_{But with specialized geometry}

(33)

Direct-Shielded Door: Far Side of Entrance

■ Extra material added to cornerExtra material added to corner

– Lead to entrance wall_{Lead to entrance wall}

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

■ Uses standard secondary Uses standard secondary

barrier calculation

■ Goal: provide same protection Goal: provide same protection

as wall or door for path through

corner corner T a r g e t R o t a t i o n a l P l a n e I s o c e n t e r T y p i c a l G a p 0 . 5 " 7 . 5 " O v e r l a p T y p i c a l D o o r O v e r l a p B e y o n d F a r S i d e o f E n t r a n c e P r o t e c t e d P o i n t ( 1 f t b e y o n d d o o r e n c l o s u r e ) I s o c e n te r t o F a r S i d e o f E n t r a n c e D i s ta n c e I s o c e n t e r to D o o r S e c o n d a r y D i s ta n c e

(34)

Direct-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 pessimistic

but is likely pessimistic

■ Requires less material than far Requires less material than far

side of entrance

– Lower unshielded doseLower unshielded dose

– Lower energy_{Lower energy}

T a r g e t R o t a t i o n a l P l a n e I s o c e n t e r P r o t e c t e d P o i n t ( 1 f t b e y o n d d o o r e n c l o s u r e ) d _{N 1}

*

T a r g e t d _{N 2} 7 . 5 " T y p i c a l D o o r O v e r l a p T y p i c a l G a p 0 . 5 "

(35)

Shielding for Heating, Ventilation, and Air

Conditioning (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 door_{For 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 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 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 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 penetration

(36)

Photon Skyshine

where

– _Ω_Ω (steradians) = 0.122(steradians) = 0.122

» for 40 x 40 cm beam_{for 40 x 40 cm beam}

■ Multiplying by additional factor Multiplying by additional factor

of two is recommended

■ Primary TVLs used to calculate Primary TVLs used to calculate

attenuation attenuation 2 2 2 1 3 . 1 0249 . 0 Y Y sky d d U W S = Ω

New construction seldom shields solely for skyshine due to

vigilance required to prevent unauthorized roof access

New construction seldom shields solely for skyshine due to

vigilance required to prevent unauthorized roof access

F l o o r

*

T a r g e t I s o c e n t e r h h d _{Y 2} 2 m e t e r s d _{Y 1} Ω

(37)

Neutron Skyshine

where

– _Ω_Ω = 2.71 (steradians) typical = 2.71 (steradians) typical (target above isocenter)

(target above isocenter)

– HH_pri_pri is neutron dose-eq in beam is neutron dose-eq in beam (0.00013, 0.002, 0.0039, 0.0043, (0.00013, 0.002, 0.0039, 0.0043, and 0.014 times W for 10, 15, 18, and 0.014 times W for 10, 15, 18, 20, and 24 MV, respectively)

20, and 24 MV, respectively)

■ Use factor is not applied since Use factor is not applied since

neutrons in all orientations

■ Multiplying by additional factor Multiplying by additional factor

of two is recommended of two is recommended π 2 10 4 . 5 × 4 Ω = − pri sky H H F l o o r

*

T a r g e t I s o c e n t e r Ω U p t o 2 0 m e t e r s l a t e r a l d i s t a n c e

(38)

Primary Goal of Upcoming NCRP Report is

Improved Shielding Calculation Accuracy

■ Very little impact for low energy acceleratorsVery little impact for low energy accelerators

– Primary and secondary barrier calculation method unchanged_{Primary and secondary barrier calculation method unchanged} – Very little impact to calculated shielding for given protection limit_{Very 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 calculations_{Avoids 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 shielding

(39)

References

■ 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

between 0º and 70º,” Health Physics. Vol. 70, No 4, 527-536,

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.

■ Chibani, Omar and C.C. Ma. “Photonuclear dose Chibani, Omar and C.C. Ma. “Photonuclear dose

calculations for high-energy beams from Siemens and

Varian linacs,” 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 Meeting

(40)

References (Continued)

■ McGinley, P.H. Shielding Techniques for Radiation Oncology McGinley, P.H. Shielding Techniques for Radiation Oncology

Facilities, 2nd ed. Madison, WI: Medical Physics 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

10 MeV.

10 MeV. Washington, DC: NCRP, NCRP Report 49, 1976. Washington, DC: NCRP, NCRP Report 49, 1976.

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.

(41)

References (Continued)

Measurements.

Measurements. Neutron Contamination from Medical Neutron Contamination from Medical

Accelerators.

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.

Vol. 47, No. 6: 811-818, 1984.

■ Rodgers, James E. “IMRT Shielding Symposium” AAPM Rodgers, James E. “IMRT Shielding Symposium” AAPM

Annual Meeting, 2001.

■ Shobe, J., J.E. Rodgers, and P.L. Taylor. “Scattered 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 radiotherapy facilities,” Health Physics, Vol. 76,

No. 1, 27-35, 1999.

(42)

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

MV," Medical Physics, Vol. 26, No. 8, 1442-46, 1999.