Basic Principles of Radiation Therapy Shielding Design - …

1 AAPM Physics Review July 19, 2014 Basic Principles of Radiation Therapy Shielding DesignPeter J. Biggs ,Massachusetts General Hospital,Harvard Medical School,Boston, MA 02114 AAPM Physics Review July 19, 2014 Outline1 Basic Principles and definitions2 Calculations3 Workload, use and occupancy factors4 Shielding materials5 Tenth value layers (TVL)6 Room layout & features, construction details7 Neutrons & laminated barriers8 Mazes and doors9 DuctsAAPM Physics Review July 19, 2014 DefinitionsP:Weekly Design dose limit (Sv/wk)d:Distance from target to measurement pointW:Workload (Gy/wk)U:Use FactorT:Occupancy factora:Scatter fraction; ( , E)dsec:Distance from scatterer to measurement pointdsca:Distance from target to scattererdl:Distance from the target to measurement pointF:Area of the beam in the plane of the scatterer (cm2)B:Barrier transmission factorAAPM Physics Review Aug 3, 2013 Also IPEMR eport #75 AAPM Physics Review July 19, 2014 Basic PrinciplesThe purpose of Radiation Shielding is to reduce theeffective equivalent dose from a linear accelerator to apoint outside the room to a level that is determined byindividual or uncontrolled area - mSv per week- mSv in any one hourControlled area - 1 mSv/wk (in practice mSv/wk)AAPM Physics Review July 19, 2014 Basic Shielding Equations: NCRP #151 MethodologyPrimary:Scatter:Leakage.

2 WUTPdB2p F400ddaWTPB2sca2secs WTPd*1000B2ll Bxis the barriertransmissionfactorThe factor 1000 isdue to the requirementIinIoutoutxinIBI AAPM Physics Review July 19, 2014 Number of Tenth Value Layers (Primary & Leakage)The number, n, of tenth value layers (TVLs)required to reduce the dose to the value P isgiven by:and the thickness for primary and leakageradiation is given by: Bn10log elplpTVLnTVLT,1,)1( NCRP report No. 151 Table Physics Review July 19, 2014 Number of Tenth Value Layers for Scattered RadiationScatter TVLs are dependent only on energyand scattering angle, so:NCRP report No. 151, Table sssTVLnT AAPM Physics Review July 19, 2014 Tenth Value LayersPrimary TVLs (cm)*Energy (MV)4610151820 Concrete35(30) 37(33) 41(37) 44(41) 45(43) 46(44) TVLs at 90 * (cm)Energy (MV)4610151820 Concrete33(28) 34(29) 35(31) 36(33) 36(34) 36(34)Steel** ** * First term is 1st TVL and term in brackets is for all other TVLs.

3 Data from NCRP #151** Data from McGinley AAPM Physics Review July 19, 2014 Obliquity Factor - IPAAPM Physics Review July 19, 2014 Obliquity factor - IIRule of thumb:-for >45 - and B< Add ~2 HVL for low-energy photons- And ~1 HVL for high-energy photonsAAPM Physics Review July 19, 2014 Obliquity factor - III- Experimental data by Kirn et. al. (1954)- verified by Biggs (1996) using the Monte Carlo for a large range of clinical energies- Monte Carlo results are in good agreement with previous recommendations at low energy, but more detail is given- Caveat: beware of applying obliquity factor corrections for large angles!!AAPM Physics Review July 19, 2014 Conventional Workload-Number of patients treated per week multipliedby the average dose (MU)/patient at isocenter-Include calibration, service, if indicated-Recommended values: 500 or 1000 Gy wk-1-Survey shows values from 250 to 450 Gy wk-1-Balance between high energy and low energyuse?

4 Using high energy is conservative approach-Electron-only (IORT) machines?AAPM Physics Review July 19, 2014 Workload TBI-Workload for TBI >> workload forconventional Therapy due to the extendeddistances:-Leakage workload is also higher, butpatient- and wall-scattered workload is usually directed at one barrier2 TBITBITBIdDW AAPM Physics Review July 19, 2014 Workload IMRT (1)-Workload for IMRT is complicated by the factthat the #MU is much larger than forconventional Therapy . The IMRT factor isdefined as:and varies between 2 and 10 or more-This only affects leakage Radiation , not primaryor scattered AAPM Physics Review July 19, 2014 Workload IMRT (2)-Helical tomotherapy has the highest C values, > based IMRT at MGH gives an average C valueof about 5-Impact of IMAT on MUs (VMAT, Rapid Arc) reduction of MUs-Anecdote: For a 6/18 MV machine the energy use priorto IMRT was 20%/80% (MU).

5 With 28% IMRT patientload, the use was 70%/30%. Hence need to considerbalancing the workload for two energiesAAPM Physics Review July 19, 2014 Conventional Use Factors90 interval:-0 (31%); 90 and 270 ( % each); 180 ( %)-previously, all barriers were assigned interval:-0 ( %); 45 and 315 ( % each); 90 and 270 ( % each); 135 and 225 (4 % each); 180 (23 %)AAPM Physics Review July 19, 2014 Use Factor for Conventional TherapyAAPM Physics Review July 19, 2014 Use Factors Special Cases-A significant TBI load will require onewall to have an increased use factor- IMRT may also require a change in thevalues assigned to the use factor for theprimary barrierAAPM Physics Review July 19, 2014 Use Factor Dedicated Machines-One should pay attention to the types ofprocedures to be carried out in new room-A room dedicated to SRS and SRT requiresdifferent considerations from a general therapyroom-A room dedicated to breast treatments will usespecific ranges of primary beam anglesAAPM Physics Review July 19, 2014 Example.

6 Use Factor for Stereotactic RadiosurgeryAAPM Physics Review July 19, 2014 Workload - Summation (1)Primary:Patient scatter:(note that the TBI contribution is calculatedseparately source not at isocenter)..)( )(, QAIMRT convisoscatterWWWWAAPM Physics Review July 19, 2014 Workload - Summation (2)Leakage:..)( QAQAIMRTITBI convLWCWCWWWAAPM Physics Review July 19, 2014 Occupancy Factors-Occupancy factors down to 1/40 are now allowed-This is the limiting factor to permit mSv in any onehour using an MPD of mSv/wk-Area beyond vault door can now have an occupancy of 1/8-Caveat: be sure to check not only the occupancy of theimmediately adjacent area, but also the area beyond. Forexample, an office beyond a corridor becomes plans Physics Review July 19, 2014If the Dose Rate is High, Should I Freak Out?

7 Note that even for a weekly limit of mSv/wk,the dose rate outside a barrier could be quite for a primary barrier, an occupancy of 1/40would imply a weekly dose of mSv and for abeam on time of hr, the instantaneous doserate is mSv/hr!!Moral: Choose factors wisely!!AAPM Physics Review July 19, 2014 Instantaneous Dose Rate (IDR)- NCRP recommends use of 1 hr as the minimum period for measuring IDR- Compares with many places ( , Europe) where the instantaneous dose rate at the highest dose delivery rate is used- Not an issue for occupancy of 1 for uniformly spaced treatments- Only an issue when treatments are not given uniformly throughout the 40 hr week or occupancy is <1 AAPM Physics Review July 19, 2014 Shielding MaterialsMaterialsDensityComments(g/cm2) ; High density concrete is very expensiveConcrete ; ; Lack structural integrity of Great for photons; bad for neutrons.

8 Needs structural Not as efficient as lead for photons, but better for Cheap! Therefore, build - ;~ Used to shield against neutrons in doors, ducts polyethyleneAAPM Physics Review July 19, 2014 Width of Primary Barrier - I- Allow 1 foot ( m) either side of the primary beam- allow for fixed primary collimator diameter ( m)- Allow for greater width at the top of the ' AAPM Physics Review Aug 3, 2013 ACDBW idth evaluated in elevatedplaneAAPM Physics Review July 19, 2014 Contouring the Primary Shielding in the CeilingAAPM Physics Review July 19, 2014 Lead-Only Room: Groundshine McGinley effectAAPM Physics Review July 19, 2014A - wallB - ceilingC - ceilingfor primary barriers, laser support shouldbe attached to the steel channelAAPM Physics Review July 19, 2014 Neutron Spectra from Medical Linacs-Neutrons produced in the head of the linac are first moderated by the x-ray Shielding -Neutrons are further moderated by scattering off the concrete walls of the Therapy room-Walls with only regular density concrete that are sufficient for x-ray protection will be adequate for Physics Review July 19, 2014 Neutron Spectra from Medical Linacs- The total neutron fluence therefore consists of direct ( fast )

9 Neutrons, scattered neutrons and thermal neutrons- Fast neutrons obey the inverse law, but scattered and thermal neutrons are isotropically distributed; hence the neutron fluence drops off less fast than inverse square totaldirscath=+ +AAPM Physics Review July 19, 2014 Shielding Thickness Required to Halve the Average Neutron Energymost probableenergyE (MeV)Half Energy Layer (cm)AAPM Physics Review July 19, 2014 Production of Neutrons by Primary Beam in a Laminated Barrier1t2tmtFor high energyx-ray beams,the x-ray dose is enhanced by a factor of tobe conservativeIncomingphotonneutronCapture rayAAPM Physics Review July 19, 2014 Production of Neutrons by Primary Beam in a Laminated BarriernxTVLtTVLtmttFRDH//2max02110*10*) where H is the neutron dose equiv.( Sv wk-1)D0is the x-ray dose at isocenter (cGy wk-1)R is the neutron prodn.

10 Rate ( Sv cGy-1m-2)andFmaxis the max. beam area at isocenter (m2)AAPM Physics Review July 19, 2014 Total Dose Behind a Laminated BarrierHtot= Hn+ Hph= Hn+ *HtrThe factor accounts for the production of capturegamma rays by the photo-produced neutrons. This is anconservative, empirical figure given by McGinleyAAPM Physics Review July 19, 2014 Contributions to the Dose at the End of a Maze1. Primary scatter2. Patient scatter3. Leakage along maze4. Leakage through maze wallAAPM Physics Review July 19, 2014 Secondary Radiation at Door due to Wall Scatter of Primary Beam 2212211rriBsdddAAUWS AAPM Physics Review July 19, 2014 Secondary Radiation at Door due to Patient Scatter 2sec11400/sscaBpdddAFUaWS AAPM Physics Review July 19, 2014 Secondary Radiation at Door due to Leakage 21110ddAUWLLsB AAPM Physics Review July 19, 2014 Secondary Radiation at Door due to Leakage Through Maze Wall 2)/(010dlTVLtBddUWLL Note that this Radiation is more energetic thanradiation along themazeAAPM Physics Review July 19, 2014 MazesThe total dose when the beam is directed towards the wall is the sum of the four components:Dt= f*Ss+ Sp+ L + LdNote that the Sscomponent is reduced by a factor f to account for patient attenuation.