National Council on Radiation Protection Report #151 ... report 151.pdf · 7/19/2009 · 2. Whyyg Laminated Shielding? - A simple and, perhaps sole, solution to upgrading a vault

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

Structural Shielding Design and EvaluationStructural Shielding Design and Evaluationfor Megavoltagefor Megavoltagefor Megavoltagefor Megavoltage

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

Peter J. Biggs Ph.D.,

M h tt G l H it lMassachusetts General Hospital,

Harvard Medical School,

Boston, MA 02114

RSMI 2009, Lisbon - July 19, 2009

,

D bDecember2005


Why Update NCRP 49?Why Update NCRP 49?- NRCP 49 (1976)was a “medical physics”

protection guideline

- ~30 yrs between publication of NCRP 49 and 151151

- NCRP 51 added additional high energy data (1977) however this report was primarily for(1977) – however, this report was primarily for particle accelerators rather than medical linacs

dd d h d l d d- NCRP 79 added neutron methodology and data (1984)


Why Update NCRP 49?Why Update NCRP 49?y py p- NCRP 51 was updated in 2003 (NCRP 144) – this initiated

a request to produce a medical accelerator only documenta request to produce a medical accelerator only document.

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

- Primarily, it was realized that existing reports did not reflect common practice in the field nor provide adequate

th d l d t d t d tmethodology and up-to-date data


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

1. Introduction of dual energy machinesgy2. Upgrading facilities with laminated shielding3. New modalities and special procedures3. New oda t es a d spec a p ocedu es4. Improved calculational methodology5 Additional and improved data5. Additional and improved data6. Time-averaged dose rate considerations7 Special considerations7. Special considerations


1. Dual Energy Machines1. Dual Energy Machinesgygy- Dual energy machines have been around for

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

- As a conservative approach, only high energy was originally considered forenergy was originally considered for shielding (3D CRT), but with popularity of IMRT at 6MV, that has changed (Wl>>Wp,IMRT at 6MV, that has changed (Wl Wp, Ws).


1. Dual Energy Machines1. Dual Energy Machines- How to split the workload between high and low

energy and still be conservative

- PJB rule of thumb: Assume 100% high energy for the primary and consider dual energies for out-of –the primary and consider dual energies for out of 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 gy p

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


2. Why Laminated Shielding?2. Why Laminated Shielding?y gy g- A simple and, perhaps sole, solution to upgrading a

vault holding a 60Co unit to a linear accelerator (notevault holding a 60Co unit to a linear accelerator (note 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

i d hi i l irays arises and this is a complex issue


3. New Modalities3. New Modalities- Different modalities include:Different modalities include:

1. Robotic arm machines (Cyberknife®)- no fixed isocenter- all barriers except ceiling are primary barriers- uses only 6 MV

2. Tomotherapy (helical)py ( )- “radiotherapy CT”- also uses only 6 MV- uses extra shielding so the 0.1% rule does not applyg pp y- uses a beam stopper

3. Tomotherapy (serial)- device attached to conventional linac (MIMIC)device attached to conventional linac (MIMIC)- uses table indexer to simulate helical Rx- not in much use now


3. Special Procedures (1)3. Special Procedures (1)W h l f th “4 fi ld b ” t t t- We have come a long way from the “4 field box” treatment arrangement, using many different procedures, including:

1 IMRT1. IMRT- usually only at 6 MV (Verhay et. al.)- Leakage workload >>primary, scatter workloadLeakage workload primary, scatter workload- Serial tomotherapy has highest relative leakage

workloadF h li l t th 100% f kl d- For helical tomotherapy, 100% of workload

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

- use factors may also be different


3. Special Procedures (2)3. Special Procedures (2)2 St t ti di / di th2. Stereotactic radiosurgery/radiotherapy- 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 barriers for retrofitting ORs- mobile linacs do not require a shielded room, except, perhaps, for a

bil l d b i N h b f di i lmobile lead barrier. Neutrons have been source of discussion recently, but appear not to be problematic


4. Calculational Methodology4. Calculational Methodology- While much of the methodology for low energy

photons has carried over albeit with improvedphotons has carried over, albeit with improved data, there has been much research on high energy processes, including:

1. Laminated primary shielding (primarily empirical)2. Refined calculations for neutron dose at the maze

door (Wu-McGinley vs Kersey)door (Wu-McGinley vs. Kersey)3. Refined calculations for capture gamma rays at the

end of a maze (McGinley)4 Direct shielded doors for high energy linacs4. Direct shielded doors for high energy linacs


5. Additional Data5. Additional Data- Updated occupancy factors (in conjunction with NCRP

#147)

P i TVL i TVL d TVL i f- Primary TVLs continue TVL1 and TVLe practice from NCRP #51 – but values are slightly different

Leakage TVLs (90°) also now uses the same convention- Leakage TVLs (90°) also now uses the same convention

- Scatter fractions: 6 MV corrected and higher energies addedadded

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

- Tabulated albedo factors for concrete as well as iron and lead


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 of i t t d t i i d f di tiinstantaneous dose rates in assessing adequacy of radiation protection

- The NCRP has never recommended dose limits for periods shorter than one month (only for the embryo-fetus in occupational situations – NCRP Report No. 116)p p )

- The weekly exposure limit is conventionally taken to be 1/50 x the annual limit namely 0 02 mSv/wk1/50 x the annual limit, namely 0.02 mSv/wk


6. Time Averaged Dose Rate: NCRP6. Time Averaged Dose Rate: NCRP- Conversion of annual limits to instantaneous dose rates

leads to linking protective measures to the time characteristics of the machine (lower the dose rate?)characteristics of the machine (lower the dose rate?)

- Specifically the use of a measured instantaneous dose rate at maximum x-ray output does not represent the radiationat maximum x ray output does not represent the radiation environment of the facility

- This problem is exacerbated by the introduction ofThis problem is exacerbated by the introduction of flattening filter-free linacs where the dose rate can ↑ x5

- Need to consider the workload and use factor together with gthe IDR when evaluating a barrier


7.7. Special considerationsSpecial considerations- Skyshine:

this was included in NCRP #51 but no- this was included in NCRP #51, but no experimental verification, for photons or neutrons, had been provided until now

- side scattered photon radiation

- Groundshine radiationG ou ds e d o

- Activation

- Ozone production


ExamplesExamples• NCRP #151 makes a point of using concrete examples to

emphasize the calculational methodology

• This was used in NCRP #49, except that there were only three calculations –P, L and S respectively

I h l i Ch 7 53• In contrast, the examples in Chapter 7 cover 53 pages out of a total of 157 pages (excluding appendices), roughly 1/3- increased complexity of the calculations

• Detailed calculations are given for 6 barriers as well as the maze door with consideration given to TADR and modifications for IMRT There is also a section onmodifications for IMRT. There is also a section on calculations for a robotic arm machine


SummarySummary- NCRP #151 provides a significant improvement in the

methodology and data needed by the shielding designer formethodology and data needed by the shielding designer for a modern radiotherapy department

- It covers all aspects of the calculational methodology and- It covers all aspects of the calculational methodology and provides sufficient data for these calculations

- However there are still some calculations that cannotHowever, there are still some calculations that cannot easily be solved using a spreadsheet, that would benefit from further insight


Thank you for your attention!Thank you for your attention!

Muito obrigadoMuito obrigadopela sua atenção!


National Council on Radiation Protection Report #151 ... report 151.pdf · 7/19/2009 · 2. Whyyg Laminated Shielding? - A simple and, perhaps sole, solution to upgrading a vault

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