# Therapy Shielding Calculations

## Presentation on theme: "Therapy Shielding Calculations"— Presentation transcript:

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
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

Goal: Improved accuracy
Revised NCRP Report in Drafting Stage by AAPM 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

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

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

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

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
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 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
Unshielded dose calculation Attenuation in tenth-value layers Barrier thickness (tc) calculation 2 pri d U W S = ú û ù ê ë é = T P S n pri / log 10 e C TVL n t ) 1 ( - + = Margin in primary barrier thickness is recommended to compensate for potential concrete density variation

Anticipate upcoming NCRP report to review and update TVL data
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 1.7 84 2.9 94 4.8 104 8.3 109 11.9 117 26 147 42 210 53 292 56 15 19 22 29 33 54 51 76 69 91 100 104 108 109 110 135 84 151 94 167 104 175 109 188 117 236 147 336 210 468 292 343 740 379 752 390 773 401 0.25 0.3 0.4 0.5 1 2 4 6 367 323 410 377 445 416 462 432 470 442 483 457 572 648 720 379 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 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 ft d w C . 1 2 4 ' + =

Slant Factor and Obliquity 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 Lead Concrete Steel Angle 4 MV 10 MV 18 MV 1.00 30 1.03 1.02 1.04 45 1.07 1.10 1.08 60 1.21 1.22 1.20 1.14 1.17 70 1.44 1.47 1.52 1.28 1.48 1.42 1.45

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) N P TVL t F U W S / 3 2 1 10 305 . - + =

Avoid metal as inside layer of primary barrier if 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 ) 400 / ( d F W a S = 10 d W S =
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 2 sec ) 400 / ( d F W a S sca p = 2 sec 3 10 d W S L - =

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 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

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
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
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 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 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 1.39E-03 8.24E-04 4.26E-04 3.00E-04 2.87E-04 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 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 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 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 Scatter Angle (degrees) MV 20 45 90 6 1.7 1.2 0.6 0.25 10 2.8 1.4 18 5.0 2.2 0.7 0.3 24 5.7 2.7 0.9 Ambiguity remains as to TVL to use for scatter

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 ) 400 / ( d A F W a S = 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 2 3 1 5 . ) 400 / ( P C p d A F W a S =

Maze: Wall Scatter d A W f a = 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 2 3 1 5 . S M d A W f a =

Maze: Leakage Scatter 10 d A W S a = Unshielded dose where
Constants as previously defined 2 1 3 10 L C LS d A W S a - =

Maze: Direct Leakage 10 d W S = 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 2 / 3 ' 10 L TVL t d W S D - =

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 ] 5 / ) 3 ( 1 [ 2 10 - + = N d n NT L W H Upcoming NCRP report may recommend a more-complex approach than this

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
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 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
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
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
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 Unshielded dose where W (steradians) = for 40 x 40 cm beam Multiplying by additional factor of two is recommended Primary TVLs used to calculate attenuation 2 1 3 . 0249 Y sky d U W S = New construction seldom shields solely for skyshine due to vigilance required to prevent unauthorized roof access

Neutron Skyshine p 2 10 4 . 5 W ´ = H Unshielded dose where
W = (steradians) typical (target above isocenter) Hpri is neutron dose-eq in beam ( , 0.002, , , and 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 p 2 10 4 . 5 W = - pri sky H

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 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, , British Journal of Radiology (BJR) Supplement No 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: , August 2003. Kleck, J. “Radiation therapy facility shielding design.” AAPM Annual Meeting

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 MeV particle accelerator facilities. Washington, DC: NCRP, NCRP Report 51, 1977.

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: , 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 (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, , 1999.