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Therapy Shielding Calculations. Melissa C. Martin, M.S., FACR, FACMP American College of Medical Physics 21st Annual Meeting & Workshops Scottsdale, AZ June 13, 2004. Therapy Shielding Design Traditionally Relies on NCRP Reports. NCRP Report 49

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Therapy shielding calculations

Therapy Shielding Calculations

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

American College of Medical Physics

21st Annual Meeting & Workshops

Scottsdale, AZ

June 13, 2004


Therapy shielding design traditionally relies on ncrp reports

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


Revised ncrp report in drafting stage by aapm task group 57 ncrp sc 46 13

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


Linear accelerator energy and workload

BJR #11 MV

4

6

10

15

18

20

24

BJR #17 MV

4

6

10

16

23

25

30

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 > 10: W = 500 Gy/wk

      • Based on NCRP 51 Appendix B Table 5


Radiation protection limits for people

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


Radiation protection limits for locations

P

S

=

max

T

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


Occupancy values from ncrp 49

Occupancy Values from NCRP 49

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

  • Full occupancy uncontrolled areas (T=1)

    • 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)

    • Corridors, rest rooms, elevators with operators, unattended parking lots

  • Occasional for uncontrolled areas (T=1/16)

    • Waiting rooms, toilets, stairways, unattended elevators, janitor’s closets, outside areas used only for pedestrian or vehicular traffic


Hourly limit for uncontrolled areas

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


Ncrp 134 impact on linac shielding

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

    • T=1/40 for occasional occupancy

  • Equivalent to T=1/10 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

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

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

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


Basic primary barrier calculation unchanged from ncrp 49

W

U

=

S

pri

2

d

pri

=

+

-

t

TVL

(

n

1

)

TVL

1

C

e

é

ù

S

pri

=

n

log

ê

ú

10

P

/

T

ë

û

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


Primary barrier photon tenth value layers mm come from a variety of sources

1.7

1.7

84

84

15

15

135

135

84

84

2.9

2.9

94

94

19

19

151

151

94

94

4.8

4.8

104

104

22

22

167

167

104

104

8.3

8.3

109

109

29

29

175

175

109

109

11.9

11.9

117

117

33

33

188

188

117

117

26

26

147

147

54

51

236

236

147

147

42

42

210

210

76

69

336

336

210

210

53

53

292

292

91

91

468

468

292

292

56

56

367

323

100

100

572

572

343

343

56

56

410

377

104

104

648

648

379

379

56

56

445

416

108

108

720

720

379

379

56

56

462

432

109

109

740

740

379

379

56

56

470

442

110

110

752

752

390

390

56

56

483

457

110

110

773

773

401

401

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

McGinley

Estimated from Concrete

Anticipate upcoming NCRP report to review and update TVL data


Primary barrier width

=

+

w

0

.

4

2

d

1

.

0

ft

'

C

C

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


Slant factor and obliquity factor

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

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


Photoneutron generation due to metal in primary barrier linacs 10 mv

W

U

N

F

-

-

=

t

/

TVL

t

/

TVL

S

10

10

3

N

1

P

N

t

+

+

2

t

0

.

305

3

2

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)


Patient photonuclear dose due to metal in primary barrier for mv 10

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

  • Metal in primary barrier can increase patient total body dose if MV > 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 >10

    • 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 >10


Secondary barrier

-

3

a

W

(

F

/

400

)

W

10

=

=

S

S

p

L

2

2

2

d

d

d

sec

sca

sec

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


Leakage photon tenth value layers mm also come from a variety of sources

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

Kleck & Varian

Average

Estimated

from Concrete

NCRP 49

Nelson & LaRiviere


Neutron leakage

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


Intensity modulated radiation therapy imrt

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)


Imrt above 10 mv significantly increases patient photonuclear dose

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


Patient scatter significant adjacent to primary barrier

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


Patient scatter fraction for 400 cm 2 field

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

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


Patient scatter energy

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

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


Maze calculation likely revised in upcoming ncrp report

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


Maze patient scatter

a

a

W

(

F

/

400

)

A

=

0

.

5

C

S

p

2

2

2

d

d

d

P

1

P

2

P

3

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 cm^2

      • h = room height


Maze wall scatter

a

a

f

W

A

A

=

1

1

0

.

5

M

S

S

2

2

2

d

d

d

S

1

S

2

S

3

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


Maze leakage scatter

-

a

3

W

10

A

=

1

C

S

LS

2

2

d

d

L

1

L

2

Maze: Leakage Scatter

  • Unshielded dose

    where

    • Constants as previously defined


Maze direct leakage

-

-

/

t

TVL

3

W

10

10

D

'

=

S

L

2

d

L

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


Maze neutron calculation based on modified kersey method

W

L

=

n

H

NT

+

-

[

1

(

d

3

)

/

5

]

2

d

10

N

2

N

1

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


Maze neutron shielding

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


Maze capture gammas from concrete

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


Direct shielded door

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


Direct shielded door far side of entrance

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


Direct shielded door near side of entrance

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


Shielding for heating ventilation and air conditioning hvac ducts

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


Photon skyshine

W

1

.

3

0

.

0249

W

U

=

S

sky

2

2

d

d

Y

1

Y

2

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


Neutron skyshine

-

´

W

4

5

.

4

10

H

pri

=

H

sky

p

2

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


Primary goal of upcoming ncrp report is improved shielding calculation accuracy

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


References

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. 8:1990-2000, August 2003.

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


References continued

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.


References continued1

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.


References continued2

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.


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