Therapy Shielding Calculations

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 49
• Primary and secondary barrier calculation
  methodology
• Applicable up to 60Cobalt and linacs up to 10 MV
• NCRP Report 51
• Extended NCRP 49 methodology up to 100 MV
• Empirical shielding requirements for maze doors
• NCRP Report 79
• Improved neutron shielding methodology
• NCRP Report 144
• Update of NCRP 51 primarily aimed at non-medical
  facilities

Reports reflect progress in linac design and
shielding research
3
Revised NCRP Report in Drafting Stage byAAPM
Task Group 57, NCRP SC 46-13

• Design of Facilities for Medical Radiation
  Therapy
• 4 MV - 50 MV (including 60Co)
• Calculation scheme generally follows NCRP 49
• All shielding data (TVLs) reviewed and updated
• Updated for intensity modulated radiation therapy
  (IMRT)
• Improved accuracy of entrance requirements
• Both with and without the use of maze
• Laminated barriers for high energy x-rays
• Photoneutron generation due to metal in primary
  barrier

Goal Improved accuracy
4
Linear Accelerator Energy and Workload

• BJR 11 megavoltage (MV) definition used here
• British Journal of Radiology (BJR) Supplement No.
  11
• Comparison of BJR 11 and BJR 17 MV definitions
• Workload assumptions typically used for shielding
  design
• Workload identified by symbol W in calculations
• For MV ? 10 MV W 1000 Gy/wk at 1 meter from
  the target
• Based on NCRP 49 Appendix C Table 2
• For MV gt 10 W 500 Gy/wk
• Based on NCRP 51 Appendix B Table 5

5
Radiation Protection Limits for People

• Structural shielding is designed to limit
  exposure to people
• Exposure must not exceed a specific dose
  equivalent limit
• Limiting exposure to unoccupied locations is not
  the goal
• NCRP 116 design dose limit (P)
• 0.10 mSv/week for occupational exposure
• 0.02 mSv/week for the general public
• Typical international design dose limits
• 0.12 mSv/week for controlled 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
  occupancy
• Occupancy factor (T)
• Fraction of time a particular location may be
  occupied
• Maximum shielded dose (Smax) at protected
  location
• Assuming occupancy factor T for protected location

Maximum shielded dose is traditionally referred
to simply as P/T
7
Occupancy Values from NCRP 49

• Full occupancy for controlled areas by convention
  (T1)
• Full occupancy uncontrolled areas (T1)
• Offices, laboratories, shops, wards, nurses
  stations, living quarters, childrens play areas,
  and occupied space in nearby buildings
• Partial occupancy for uncontrolled areas (T1/4)
• Corridors, rest rooms, elevators with operators,
  unattended parking lots
• Occasional for uncontrolled areas (T1/16)
• Waiting rooms, toilets, stairways, unattended
  elevators, janitors closets, outside areas used
  only for pedestrian or vehicular traffic

8
Hourly Limit for Uncontrolled Areas

• 0.02 mSv hourly limit for uncontrolled areas
• 20 Gy/hr common assumption for calculation
• Implies a lower limit for occupancy factor
• T ? 20 / ( U W )
• T ? 0.16 for higher energy accelerators (500 Gy /
  wk workload)
• T ? 0.08 for lower energy accelerators (1000 Gy
  wk workload)
• Not applied to low occupancy locations with no
  public access
• 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
  public
• 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
• Occupancy assumptions proposed for general public
• T1/40 for occasional occupancy
• Equivalent to T1/10 occasional for general
  employees
• Similar to P/T required by hourly limit for
  primary barriers
• Slightly increase from T 1/16 used for
  secondary barriers
• T1/16 still appropriate for locations with no
  public occupancy
• e.g., machine rooms, unoccupied roofs, etc.

Impact increases if higher occupancy than T1/40
adopted
10
Basic Primary Barrier Calculation Unchanged from
NCRP 49

• Unshielded dose calculation
• Attenuation in tenth-value layers
• Barrier thickness (tc) calculation

Margin in primary barrier thickness is
recommended to compensate for potential concrete
density variation
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
6
10
15
18
20
24
NCRP 49
NCRP 51
Nelson LaRiviere
Estimated from Concrete
McGinley
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
  degrees
• Barrier width required assuming 40 cm x 40 cm
  field size
• Field typically not perfectly square (corners are
  clipped)
• 35 cm x 35 cm field size typically used to
  account for this

13
Slant Factor and Obliquity Factor

• Slant Factor
• Path from target to protected location diagonally
  through barrier
• Incident angle q of line with respect to
  perpendicular
• Required barrier thickness reduced by cos(q)
• Same total distance through barrier to protected
  location
• Scatter causes slant factor to underestimate exit
  dose
• Multiplying thickness by obliquity factor
  compensates for this

14
Photoneutron Generation Due to Metal in Primary
Barrier (Linacs ? 10 MV)

• Dose-equivalent 0.3 m beyond barrier (McGinley)
• 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)
• Recent safety survey indicated somewhat higher
  3.8 x 10-4 value for steel at 18 MV is
  appropriate
• N adjusted versus MV based on neutron leakage
  fraction vs MV
• F is field size (conventionally 0.16 m2), t2 is
  metal thickness (m)
• X-Ray attenuation prior to metal layer 10(-t1 /
  TVLp)
• Neutron attenuation after metal layer 10(-t3 /
  TVLN)

15
Patient Photonuclear Dose Due to Metal in Primary
Barrier for MV gt 10

• Metal in primary barrier can increase patient
  total body dose if MV gt 10
• Lead inside layer approximately doubles patient
  total body dose
• Increases risk of secondary cancer
• Concrete or borated polyethylene inside metal in
  primary barrier is recommended if MV gt10
• 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 later

Avoid metal as inside layer of primary barrier if
MV gt 10
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Secondary Barrier

• Patient scatter unshielded dose
• F is field size in cm2
• typically 1600
• a scatter fraction for 20 x 20 cm
  beam
• Leakage unshielded dose
• Assumes 0.1 leakage fraction

17
Leakage Photon Tenth-Value Layers (mm) Also Come
from a Variety of Sources
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
Estimated from Concrete
Kleck Varian Average
Nelson LaRiviere
NCRP 49
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Neutron Leakage

• Same form as photon leakage calculation
• Based 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
• Based on Varian and Siemens neutron leakage data
• Assumes quality factor of 10 for absorbed dose
• Shielded dose equivalent based on leakage neutron
  TVLs
• 211 mm for concrete
• 96 mm for borated polyethylene

19
Intensity Modulated Radiation Therapy (IMRT)

• IMRT requires increased monitor units per cGy at
  isocenter
• Typical IMRT ratio is 5 MU per cGy, as high as 10
  for some systems
• Percent workload with IMRT impacts shielding
• 50 typically assumed 100 if vault is dedicated
  to IMRT
• Account for IMRT by multiplying x-ray leakage by
  IMRT factor
• IMRT Factor IMRT x IMRT ratio (1 -
  IMRT)
• 3 is typical IMRT factor (50 workload with IMRT
  ratio of 5)
• IMRT factor lower for neutrons if machine is dual
  energy
• 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)

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IMRT above 10 MV Significantly Increases Patient
Photonuclear Dose

• Neutrons dominate patient total body dose for
  high energy linacs
• Neutron dose equivalent as high as ten times
  photon dose
• Potentially 1 of workload vs 0.1 photon leakage
• 0.05 required absorbed neutron dose x 20 quality
  factor
• Typical neutron dose equivalent is lower than
  requirement
• 0.1 to 0.2 of workload
• IMRT factor of 5 increases patient incidental
  dose 5X
• Results in typical neutron total body exposure of
  0.5 to 1.0 of WL
• Significantly increases risk of secondary cancer

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 for lateral
  barriers
• Generally a good assumption
• 90 degree scatter has low energy
• Scatter is significant adjacent to primary
  barrier
• Calculations indicate comparable to leakage
• Slant thickness through barrier compensates for
  the increase in unshielded dose due to scatter
• Barrier thickness comparable to lateral is
  adequate for same P/T

22
Patient Scatter Fraction for 400 cm2 Field

• Based on recent simulation work by Taylor et.al.
• Scatter fraction increases as angle decreases
• Scatter fraction vs MV may increase or decrease
• Tends to increase with MV at small scatter angles
• Decreases with increasing MV at large scatter
  angles

23
Patient Scatter Energy

• Mean Scatter Energy
• No standardized scatter Tenth-Value Layer
• Primary MV rating based on peak MV in spectrum,
  not mean energy
• Primary TVL at slightly higher MV (e.g, 50)
  appears reasonable
• increase little more than wild guess more
  research is needed

Ambiguity remains as to TVL to use for scatter
24
Maze Calculation Likely Revised in Upcoming NCRP
Report

• New method identifies and evaluates specific
  mechanisms
• Patient Scatter, Wall Scatter, Leakage scatter
• Direct leakage
• Neutrons, capture gammas
• Mechanisms calculated at most stressing
  orientation
• Scatter calculations multiplied by 2/3 to
  compensate for this
• Scatter energy relatively low at maze door
• Primary 0.3 MV TVLs used for patient and wall
  scatter (2 bounces)
• Primary 0.5 MV TVLs used for leakage scatter (1
  bounce)
• Scatter is significant typically only for low
  energy linacs

Goal More-precise calculation avoiding over or
under-shielding
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Maze Patient Scatter

• Unshielded dose
• where
• a0.5 is 0.5 MV scatter fraction
• Second bounce fraction
• 0.02 per m2 typically used
• Other constants as before, e.g.,
• a patient scatter fraction
• F field size in cm2
• h room height

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Maze Wall Scatter

• Unshielded dose
• where
• f patient transmission
• a1 first reflection coefficient
• 0.005 per m2 for 6 MV
• 0.004 per m2 for ? 10 MV
• A1 beam area (m2) at wall
• AM Maze cross section (m2)
• dM x room height

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Maze Leakage Scatter

• Unshielded dose
• where
• Constants as previously defined

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Maze Direct Leakage

• Unshielded dose
• Same as standard secondary photon leakage
  calculation
• Standard neutron leakage not typically used
• Use only if it exceeds the maze neutron
  calculation
• e.g., if maze wall not sufficiently thick

29
Maze Neutron Calculation Based on Modified Kersey
Method

• Unshielded dose equivalent
• where
• Ln is neutron leakage fraction
• Same as used for secondary neutron leakage
  calculation
• Modification to Kersey is assuming first
  tenth-value distance is 3 m instead of 5 m

Upcoming NCRP report may recommend a more-complex
approach than this
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Maze Neutron Shielding

• Modeled as 50 thermal neutrons and 50 fast
  neutrons
• 1 inch borated poly effectively eliminates all
  thermal neutrons
• Fast neutron TVL is 2.4 inches for the first 4
  inches
• Fast neutron TVL is 3.6 inches beyond 4 inches
  thickness

31
Maze Capture Gammas from Concrete

• Gamma rays generated by neutron capture in the
  maze
• Very significant for high energy linacs
• 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)
• Capture gammas have moderate energy (3.6 MeV)
• TVL of 61 mm for lead
• Limited attenuation also provided by polyethylene
  (278 mm TVL)

Dominates X-Ray dose at maze entrance for high
energy linacs
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Direct-Shielded Door

• Neutron Door is simply a secondary barrier
• Typically more layers and different materials
  than a wall
• Lead to attenuate leakage photons
• Borated polyethylene to attenuate leakage
  neutrons
• Typically sandwiched between layers of lead
• Steel covers
• Specialized shielding procedure adjacent to door
• Compensates for relatively small slant thickness
  in this location
• Vault entry toward isocenter similar to maze
• Vault entry away from isocenter is secondary
  barrier
• But with specialized geometry

33
Direct-Shielded Door Far Side of Entrance

• Extra material added to corner
• Lead to entrance wall
• Borated polyethylene or concrete beyond wall
• Uses standard secondary barrier calculation
• Goal provide same protection as wall or door for
  path through corner

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Direct-Shielded Door Near Side of Entrance

• Geometry similar to short maze
• Maze calculation can be used but is likely
  pessimistic
• Requires less material than far side of entrance
• Lower unshielded dose
• Lower energy

35
Shielding for Heating, Ventilation, and Air
Conditioning (HVAC) Ducts

• HVAC penetration is located at ceiling level in
  the vault
• For vaults with maze, typically located
  immediately above door
• 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 entrance
• Material thickness 1/2 to 1/3 that required of
  the door
• 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 location) vs. at the HVAC
  duct opening
• NCRP 49 recommends that shielding extend at least
  a factor of three times the width of the HVAC
  penetration

36
Photon Skyshine

• Unshielded dose
• where
• W (steradians) 0.122
• for 40 x 40 cm beam
• Multiplying by additional factor of two is
  recommended
• Primary TVLs used to calculate attenuation

New construction seldom shields solely for
skyshine due to vigilance required to prevent
unauthorized roof access
37
Neutron Skyshine

• Unshielded dose
• where
• W 2.71 (steradians) typical (target above
  isocenter)
• Hpri is neutron dose-eq in beam (0.00013, 0.002,
  0.0039, 0.0043, and 0.014 times W for 10, 15, 18,
  20, and 24 MV, respectively)
• Use factor is not applied since neutrons in all
  orientations
• Multiplying by additional factor of two is
  recommended

38
Primary Goal of Upcoming NCRP Report is Improved
Shielding Calculation Accuracy

• Very little impact for low energy accelerators
• Primary and secondary barrier calculation method
  unchanged
• Very little impact to calculated shielding for
  given protection limit
• Improved accuracy for high-energy accelerators
• Avoids extra cost of over design due to
  pessimistic calculations
• Avoid extra cost of retrofitting if inaccurate
  calculations underestimate required shielding

39
References

• Biggs, Peter J. Obliquity factors for 60Co 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. Central axis depth dose data for use in
  radiotherapy, 1972.
• Chibani, Omar and C.C. Ma. Photonuclear dose
  calculations for high-energy beams from Siemens
  and Varian linacs, Medical Physics, Vol 30, No.
  81990-2000, August 2003.
• Kleck, J. Radiation therapy facility shielding
  design. 1998 AAPM Annual Meeting

40
References (Continued)

• McGinley, P.H. Shielding Techniques for Radiation
  Oncology Facilities, 2nd ed. Madison, WI Medical
  Physics Publishing, 2002.
• National Council on Radiation Protection and
  Measurements. Structural shielding design and
  evaluation for medical use of x-ray and gamma
  rays of energies up to 10 MeV. Washington, DC
  NCRP, NCRP Report 49, 1976.
• National Council on Radiation Protection and
  Measurements. Radiation protection design
  guidelines for 0.1-100 MeV particle accelerator
  facilities. Washington, DC NCRP, NCRP Report 51,
  1977.

41
References (Continued)

• National Council on Radiation Protection and
  Measurements. Neutron Contamination from Medical
  Accelerators. Bethesda, MD NCRP, NCRP Report 79,
  1984.
• 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
  Annual Meeting, 2001.
• 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 from linear accelerators with
  x-ray energies from 6 to 24 MV," Medical Physics,
  Vol. 26, No. 8, 1442-46, 1999.