Shielding Calculation for LINAC

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<p>Therapy Shielding CalculationsMelissa C. Martin, M.S., FACR, FACMP American College of Medical Physics 21st Annual Meeting &amp; Workshops Scottsdale, AZ June 13, 2004</p> <p>Therapy Shielding Design Traditionally Relies on NCRP Reportss</p> <p>NCRP Report 49 Primary and secondary barrier calculation methodology Applicable up to 60Cobalt and linacs up to 10 MV</p> <p>s</p> <p>NCRP Report 51 Extended NCRP 49 methodology up to 100 MV Empirical shielding requirements for maze doors</p> <p>s</p> <p>NCRP Report 79 Improved neutron shielding methodology</p> <p>s</p> <p>NCRP Report 144 Update of NCRP 51 primarily aimed at non-medical facilities</p> <p>Reports reflect progress in linac design and shielding research Reports reflect progress in linac design and shielding research</p> <p>Revised NCRP Report in Drafting Stage by AAPM Task Group 57, NCRP SC 46-13s</p> <p>Design of Facilities for Medical Radiation Therapy 4 MV - 50 MV (including 60Co)</p> <p>s s s s</p> <p>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</p> <p>s</p> <p>Laminated barriers for high energy x-rays Photoneutron generation due to metal in primary barrier</p> <p>Goal: Improved accuracy Goal: Improved accuracy</p> <p>Linear Accelerator Energy and Workloads</p> <p>BJR #11 megavoltage (MV) definition used here British Journal of Radiology (BJR) Supplement No. 11</p> <p>s</p> <p>Comparison of BJR #11 and BJR #17 MV definitions4 4 6 6 10 10 15 16 18 23 20 25 24 30</p> <p>BJR #11 MV BJR #17 MVs</p> <p>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</p> <p>Radiation Protection Limits for Peoples</p> <p>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</p> <p>s</p> <p>NCRP 116 design dose limit (P) 0.10 mSv/week for occupational exposure 0.02 mSv/week for the general public</p> <p>s</p> <p>Typical international design dose limits 0.12 mSv/week for controlled areas 0.004 mSv/week for uncontrolled areas</p> <p>NCRP 116 dose limit is a factor of 5 lower than NCRP 49 value NCRP 116 dose limit is a factor of 5 lower than NCRP 49 value</p> <p>Radiation Protection Limits for Locationss s</p> <p>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</p> <p>s</p> <p>S max =</p> <p>P T</p> <p> Assuming occupancy factor T for protected location</p> <p>Maximum shielded dose is traditionally referred to simply as P/T Maximum shielded dose is traditionally referred to simply as P/T</p> <p>Occupancy Values from NCRP 49s s</p> <p>Full occupancy for controlled areas by convention (T=1) Full occupancy uncontrolled areas (T=1) Offices, laboratories, shops, wards, nurses stations, living quarters, childrens play areas, and occupied space in nearby buildings</p> <p>s</p> <p>Partial occupancy for uncontrolled areas (T=1/4) Corridors, rest rooms, elevators with operators, unattended parking lots</p> <p>s</p> <p>Occasional for uncontrolled areas (T=1/16) Waiting rooms, toilets, stairways, unattended elevators, janitors closets, outside areas used only for pedestrian or vehicular traffic</p> <p>Hourly Limit for Uncontrolled Areass s s</p> <p>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)</p> <p>s</p> <p>Not applied to low occupancy locations with no public access e.g., unoccupied roof, machinery room</p> <p>T = 1/10 rather than 1/16 typically used for exterior walls T = 1/10 rather than 1/16 typically used for exterior walls</p> <p>NCRP 134 Impact on Linac Shieldings</p> <p>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</p> <p>s</p> <p>Occupancy assumptions proposed for general public T=1/40 for occasional occupancy</p> <p>s</p> <p>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.</p> <p>Impact increases if higher occupancy than T=1/40 adopted Impact increases if higher occupancy than T=1/40 adopted</p> <p>Basic Primary Barrier Calculation Unchanged from NCRP 49s</p> <p>Unshielded dose calculation</p> <p>S pris</p> <p>=</p> <p>WU 2 d pri</p> <p>Door T a rg e t R o ta tio n a l P la n e D'</p> <p>A</p> <p>A'</p> <p>Attenuation in tenth-value layers</p> <p>n =s</p> <p> S pri log10 P / T TVL1 + (n 1) TVLe</p> <p>D</p> <p>M aze</p> <p>*tC C</p> <p>T arget Is o c e n te r</p> <p>B</p> <p>d</p> <p>p ri</p> <p>Barrier thickness (tc) calculation</p> <p>C'</p> <p>tC =</p> <p>1 ft</p> <p>Margin in primary barrier thickness is recommended to Margin in primary barrier thickness is recommended to compensate for potential concrete density variation compensate for potential concrete density variation</p> <p>Primary Barrier Photon Tenth-Value Layers (mm) Come from a Variety of SourcesMV 0.2 0.25 0.3 0.4 0.5 1 2 4 6 10 15 18 20 24 Lead TVL1 TVLe 1.7 1.7 2.9 2.9 4.8 4.8 8.3 8.3 11.9 11.9 26 26 42 42 53 53 56 56 56 56 56 56 56 56 56 56 56 56 NCRP 51 Concrete TVL1 TVLe 84 84 94 94 104 104 109 109 117 117 147 147 210 210 292 292 367 323 410 377 445 416 462 432 470 442 483 457 Steel TVL1 TVLe 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 Earth TVL1 TVLe 135 135 151 151 167 167 175 175 188 188 236 236 336 336 468 468 572 572 648 648 720 720 740 740 752 752 773 773 Borated Poly TVL1 TVLe 84 84 94 94 104 104 109 109 117 117 147 147 210 210 292 292 343 343 379 379 379 379 379 379 390 390 401 401</p> <p>NCRP 49</p> <p>Nelson &amp; LaRiviere</p> <p>McGinley</p> <p>Estimated from Concrete</p> <p>Anticipate upcoming NCRP report to review and update TVL data Anticipate upcoming NCRP report to review and update TVL data</p> <p>Primary Barrier Widths</p> <p>0.3 meter margin on each side of beam rotated 45 degrees Barrier width required assuming 40 cm x 40 cm field size</p> <p>wC =s</p> <p>0.4 2 d C ' + 1.0 ft</p> <p>Field typically not perfectly square (corners are clipped) 35 cm x 35 cm field size typically used to account for this</p> <p>T a r g e t to N a r r o w P o in t D is t a n c e ( d C ')</p> <p>*w CC</p> <p>T a rg e t Is o c e n te rT a r g e t to N a r r o w P o in t D is t a n c e ( d C ')</p> <p>*w CC</p> <p>T a rg e t Is o c e n te rT a r g e t to N a r r o w P o in t D is t a n c e ( d C ')</p> <p>*</p> <p>T a rg e t Is o c e n te r</p> <p>1 ft</p> <p>1 ft</p> <p>1 ftM e ta l</p> <p>1 ft</p> <p>C'1 ft</p> <p>C'1 ft</p> <p>w</p> <p>C</p> <p>Slant Factor and Obliquity Factors</p> <p>Slant Factor Path from target to protected location diagonally through barrier Incident angle of line with respect to perpendicular Required barrier thickness reduced by cos( ) Same total distance through barrier to protected location</p> <p>s</p> <p>Scatter causes slant factor to underestimate exit dose Multiplying thickness by obliquity factor compensates for thisAngle 0 30 45 60 70 4 MV 1.00 1.03 1.07 1.21 1.44 Lead 10 MV 1.00 1.02 1.07 1.21 1.47 18 MV 1.00 1.03 1.10 1.22 1.52 Concrete 4 MV 10 MV 1.00 1.00 1.02 1.00 1.07 1.04 1.20 1.14 1.47 1.28 18 MV 1.00 1.00 1.04 1.08 1.22 4 MV 1.00 1.02 1.07 1.20 1.48 Steel 10 MV 1.00 1.02 1.07 1.17 1.42 18 MV 1.00 1.04 1.08 1.20 1.45</p> <p>Photoneutron Generation Due to Metal in Primary Barrier (Linacs 10 MV)s</p> <p>Dose-equivalent 0.3 m beyond barrier (McGinley) WU NF t / TVL t / TVL 1 P = SN 10 10 3 N t2 + t + 0.305 3 2 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)s</p> <p>Recent safety survey indicated somewhat higher 3.8 x 10-4 value for steel at 18 MV is appropriate</p> <p> 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)</p> <p>Patient Photonuclear Dose Due to Metal in Primary Barrier for MV &gt; 10s</p> <p>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</p> <p>s</p> <p>Concrete or borated polyethylene inside metal in primary barrier is recommended if MV &gt;10 Each inch of borated poly decreases patient dose from metal barrier photoneutron by approximately factor of 2</p> <p>s</p> <p>Impact of IMRT on patient photonuclear dose is addressed later Avoid metal as inside layer of primary barrier if MV &gt; 10 Avoid metal as inside layer of primary barrier if MV &gt; 10</p> <p>Secondary Barriers</p> <p>Patient scatter unshielded dose</p> <p>Sp</p> <p>=</p> <p>a W ( F / 400) 2 2 d sca d sec</p> <p>Door T a rg e t R o ta tio n a l D' P la n e</p> <p>A</p> <p>A'</p> <p> F is field size in cm2 typically 1600 </p> <p>a</p> <p>= scatter fraction for 20 x 20 cm beam</p> <p>D</p> <p>M aze</p> <p>d</p> <p>sca</p> <p>*C</p> <p>T a rg e t Is o c e n te r</p> <p>B</p> <p>d</p> <p>sec</p> <p>s</p> <p>Leakage unshielded dose Assumes 0.1% leakage fraction</p> <p>tB 1 ft C'</p> <p>SL</p> <p>=</p> <p>W 10 2 d sec</p> <p>3</p> <p>Leakage Photon Tenth-Value Layers (mm) Also Come from a Variety of Sources</p> <p>MV 4 6 10 15 18 20 24</p> <p>Lead TVL1 TVLe 53 53 56 56 56 56 56 56 56 56 56 56 56 56</p> <p>Concrete TVL1 TVLe 292 292 341 284 351 320 361 338 363 343 366 345 371 351</p> <p>Steel TVL1 TVLe 91 91 96 96 96 96 96 96 96 96 96 96 96 96</p> <p>Earth TVL1 TVLe 468 468 546 455 562 512 578 541 581 549 586 552 594 562</p> <p>Borated Poly TVL1 TVLe 292 292 341 284 351 320 361 338 363 343 366 345 371 351</p> <p>NCRP 49</p> <p>Nelson &amp; LaRiviere</p> <p>Kleck &amp; Varian Average</p> <p>Estimated from Concrete</p> <p>Neutron Leakages s</p> <p>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</p> <p>s</p> <p>Shielded dose equivalent based on leakage neutron TVLs 211 mm for concrete 96 mm for borated polyethylene</p> <p>Intensity Modulated Radiation Therapy (IMRT)s</p> <p>IMRT requires increased monitor units per cGy at isocenter Typical IMRT ratio is 5 MU per cGy, as high as 10 for some systems</p> <p>s</p> <p>Percent workload with IMRT impacts shielding 50% typically assumed; 100% if vault is dedicated to IMRT</p> <p>s</p> <p>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)</p> <p>s</p> <p>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)</p> <p>IMRT above 10 MV Significantly Increases Patient Photonuclear Doses</p> <p>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 leakages</p> <p>0.05% required absorbed neutron dose x 20 quality factor</p> <p> Typical neutron dose equivalent is lower than requirement 0.1 to 0.2% of workloads</p> <p>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</p> <p>Most IMRT is performed at 6 MV to mitigate increased secondary Most IMRT is performed at 6 MV to mitigate increased secondary cancer risk from photoneutrons cancer risk from photoneutrons</p> <p>Patient Scatter Significant Adjacent to Primary Barriers</p> <p>Scatter traditionally neglected for lateral barriers Generally a good assumption 90 degree scatter has low energyDoor T a rg e t R o ta tio n a l P la n e D'</p> <p>A</p> <p>A'</p> <p>s</p> <p>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</p> <p>D</p> <p>M aze</p> <p>d</p> <p>sca</p> <p>*C</p> <p>T a rg e t Is o c e n te rS c a tte r A n g le</p> <p>B</p> <p>S la n t t h ic k n e s s u s e d t o c a lc u l a t e a tte n u a tio n</p> <p>d</p> <p>sec</p> <p>C'A c tu a l b a r r ie r t h ic k n e s s</p> <p>1 ft</p> <p>Patient Scatter Fraction for 400 cm2 Fields s s</p> <p>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 anglesMV 4 6 10 15 18 20 24 10 1.04E-02 1.04E-02 1.66E-02 1.51E-02 1.42E-02 1.52E-02 1.73E-02 20 6.73E-03 6.73E-03 5.79E-03 5.54E-03 5.39E-03 5.66E-03 6.19E-03 30 2.77E-03 2.77E-03 3.18E-03 2.77E-03 2.53E-03 2.59E-03 2.71E-03 Angle (degrees) 45 60 2.09E-03 1.24E-03 1.39E-03 8.24E-04 1.35E-03 7.46E-04 1.05E-03 5.45E-04 8.64E-04 4.24E-04 8.54E-04 4.13E-04 8.35E-04 3.91E-04 90 6.39E-04 4.26E-04 3.81E-04 2.61E-04 1.89E-04 1.85E-04 1.76E-04 135 4.50E-04 3.00E-04 3.02E-04 1.91E-04 1.24E-04 1.23E-04 1.21E-04 150 4.31E-04 2.87E-04 2.74E-04 1.78E-04 1.20E-04 1.18E-04 1.14E-04</p> <p>Patient Scatter Energys</p> <p>Mean Scatter EnergyMV 6 10 18 24 0 1.7 2.8 5.0 5.7 Scatter Angle (degrees) 20 45 1.2 0.6 1.4 0.6 2.2 0.7 2.7 0.9 90 0.25 0.25 0.3 0.3</p> <p>s</p> <p>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</p> <p>Ambiguity remains as to TVL to use for scatter Ambiguity remains as to TVL to use for scatter</p> <p>Maze Calculation Likely Revised in Upcoming NCRP Reports</p> <p>New method identifies and evaluates specific mechanisms Patient Scatter, Wall Scatter, Leakage scatter Direct leakage Neutrons, capture gammas</p> <p>s</p> <p>Mechanisms calculated at most stressing orientation Scatter calculations multiplied by 2/3 to compensate for this</p> <p>s</p> <p>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</p> <p>Goal: More-precise calculation avoiding over or under-shielding Goal: More-precise calculation avoiding over or under-shielding</p> <p>Maze: Patient Scatters</p> <p>Unshielded dose</p> <p>Sp =s</p> <p>a W ( F / 400) 0.5 AC 2 2 2 d P1 d P 2 d P 3D</p> <p>Door T a rg e t R o ta tio n a l P la n e D'</p> <p>A</p> <p>A'</p> <p>where 0.5 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</p> <p>d</p> <p>dP3</p> <p>P1</p> <p>*C</p> <p>T arg et Is o c e n te r</p> <p>B</p>...