biggs_ncrp report 151

7/31/2019 Biggs_NCRP Report 151

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National Council on Radiation ProtectionNational Council on Radiation ProtectionReport #151Report #151

Structural Shielding Design and EvaluationStructural Shielding Design and Evaluation

XX-- and Gammaand Gamma--Ray Radiotherapy FacilitiesRay Radiotherapy Facilities

Peter J. Biggs Ph.D.,

assac use s enera osp a ,

Harvard Medical School,

Boston MA 02114

RSMI 2009, Lisbon - July 19, 2009


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ecem er

2005



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Why Update NCRP 49?Why Update NCRP 49?

- NRCP 49 (1976)was a medical physicsprotection guideline

- ~30 yrs between publication of NCRP 49 and

- NCRP 51 added additional high energy data ,particle accelerators rather than medical linacs

- NCRP 79 a e neutron met o o ogy an ata(1984)



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Wh U date NCRP 49?Wh U date NCRP 49?- NCRP 51 was updated in 2003 (NCRP 144) this initiated

.

- The AAPM formed TG 57 (J. Deye, chair; R. Wu, co-chair) in around 1997 to address this problem and this waslater subsumed into NCRP Scientific committee 46-13

- Primarily, it was realized that existing reports did notreflect common practice in the field nor provide adequate

me o o ogy an up- o- a e a a



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Rationale for Update (NCRP # 151)Rationale for Update (NCRP # 151)

1. Introduction of dual ener machines

2. Upgrading facilities with laminated shielding3. New modalities and s ecial rocedures

4. Improved calculational methodology

.

6. Time-averaged dose rate considerations

.



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1. Dual Ener Machines1. Dual Ener Machines- Dual energy machines have been around for

a long time, but became mainstream onlywhen adopted by linear accelerators.

- As a conservative approach, only high

shielding (3D CRT), but with popularity ofIMRT at 6MV that has chan e W >>WWs).



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1. Dual Energy Machines1. Dual Energy Machines

- How to split the workload between high and lowenergy and still be conservative

- PJB rule of thumb: Assume 100% high energy for-

plane leakage. The scatter and leakage adjacent to

the primary is a toss-up

- Change in workload vs. time:

- Anecdote: For a 6/18 MV machine the energy use prior

to IMRT was 20%/80% (MU). With 28% IMRT patientload, the use was 70%/30%



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2. Wh Laminated Shieldin ?2. Wh Laminated Shieldin ?- A simple and, perhaps sole, solution to upgrading a

also, beamstopper vs. no beamstopper)

- For low energies, since only photons are involved,

calculation is straightforward.

- For high energies, however, the issue of photo-neutron production and subsequent capture gamma

rays ar ses an t s s a comp ex ssue



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3. Special Procedures (1)3. Special Procedures (1)- e ave come a ong way rom e e ox rea men

arrangement, using many different procedures, including:

.

- usually only at 6 MV (Verhay et. al.)

- Leaka e workload >> rimar scatter workload

- Serial tomotherapy has highest relative leakage

workload- or e ca omo erapy, o wor oa

- for conventional linacs, can be 70% or more of theworkload

- use factors may also be different



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3. Special Procedures (2)3. Special Procedures (2). ereo ac c ra osurgery ra o erapy

- use factors are substantially different from 3D CRT

- high dose for radiosurgery, but long set-up times

3. TBI- P, L workload is greater than Rx dose

- source of scatter radiation is not at the isocenter

4. IORT- dedicated facilities (not now in vogue) require lead/BPE barriersfor retrofitting ORs

- mobile linacs do not require a shielded room, except, perhaps, for a

mo e ea arr er. eutrons ave een source o scuss on recent y,but appear not to be problematic



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4. Calculational Methodology4. Calculational Methodology

- While much of the methodology for low energy,

data, there has been much research on highenergy processes, including:

1. Laminated primary shielding (primarily empirical)

2. Refined calculations for neutron dose at the maze- .

3. Refined calculations for capture gamma rays at theend of a maze (McGinley)

.



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5. Additional Data5. Additional Data

- Updated occupancy factors (in conjunction with NCRP#147)

- Pr mary TVLs cont nue TVL1 an TVLepract ce rom

NCRP #51 but values are slightly different

-

- Scatter fractions: 6 MV corrected and higher energies

- Scatter TVLs for energies other than 6 MV, plus lead.

- Tabulated albedo factors for concrete as well as iron andlead



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6. Time Averaged Dose Rate vs. IDR6. Time Averaged Dose Rate vs. IDR

- In response to practices in a few states in the US, in 2000,the NCRP issued a statement regarding the application ofns an aneous ose ra es n assess ng a equacy o ra a on

protection

- The NCRP has never recommended dose limits for periodsshorter than one month (only for the embryo-fetus inoccu ational situations NCRP Re ort No. 116

- The weekly exposure limit is conventionally taken to be

, .



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6. Time Averaged Dose Rate: NCRP6. Time Averaged Dose Rate: NCRP

- Conversion of annual limits to instantaneous dose ratesleads to linking protective measures to the time

- Specifically the use of a measured instantaneous dose rateat maximum x-ra out ut does not re resent the radiationenvironment of the facility

-flattening filter-free linacs where the dose rate can x5

- Need to consider the workload and use factor together with

the IDR when evaluating a barrier



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7.7. Special considerationsSpecial considerations

- Skyshine:

- ,

experimental verification, for photons orneutrons, had been provided until now

- side scattered photon radiation

- Groundshine radiation

- Activation

- Ozone production



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SummarySummary

- NCRP #151 provides a significant improvement in the

a modern radiotherapy department

-provides sufficient data for these calculations

- ,easily be solved using a spreadsheet, that would benefitfrom further insight



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