Peter J. Biggs Ph.D., Massachusetts General lhospital, Harvard Medical School, Boston, MA 02114

Transcription

1 National Council on Radiation Protection Report #151 Structural Shielding Design and Evaluation for Megavoltage X- and Gamma-Ray Radiotherapy Facilities Peter J. Biggs Ph.D., Massachusetts General lhospital, Harvard Medical School, Boston, MA 02114

2 December 2005

3 Why Update NCRP 49? - NRCP 49 (1976)was a medical physics protection guideline - ~30 yrs between publication of NCRP 49 and NCRP 51 added additional high energy data (1977) however, this report was primarily for particle accelerators rather than medical linacs - NCRP 79 added neutron methodology and data (1984)

4 Why Update NCRP 49? - NCRP 51 was updated in 2003 (NCRP 144) this initiated a request to produce a medical accelerator only document. - The AAPM formed TG 57 (J. Deye, chair; R. Wu, cochair) in around 1997 to address this problem and this was later subsumed into NCRP Scientific committee Primarily, it was realized that existing reports did not reflect common practice in the field nor provide adequate methodology thdl and up-to-date dt data dt

5 Rationale for Update (NCRP # 151) 1. Introduction of dual energy machines 2. Upgrading facilities with laminated shielding 3. New modalities and special procedures es 4. Improved calculational methodology 5. Additional and improved data 6. Time-averaged dose rate considerations 7. Special considerations

6 1. Dual Energy Machines - 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 for shielding (3D CRT), but with popularity of IMRT at 6MV, that has changed (W l >>W p, W s ).

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

8 2. Why Laminated Shielding? - A simple and, perhaps sole, solution to upgrading a vault holding a 60 Co 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 photoneutron production and subsequent capture gamma rays arises and this is a complex issue

9 - Different modalities include: 3. New Modalities 1. Robotic arm machines (Cyberknife ) - no fixed isocenter - all barriers except ceiling are primary barriers - uses only 6 MV 2. Tomotherapy (helical) - radiotherapy CT - also uses only 6 MV - uses extra shielding so the 0.1% rule does not apply - uses a beam stopper 3. Tomotherapy (serial) - device attached to conventional linac (MIMIC) - uses table indexer to simulate helical Rx - not in much use now

10 3. Special Procedures (1) - We have come a long way from the 4 field box treatment t t arrangement, using many different procedures, including: 1. IMRT - usually only at 6 MV (Verhay et. al.) - Leakage workload >>primary, scatter workload - Serial tomotherapy has highest relative leakage workload - For helical l tomotherapy, th 100% of workload - for conventional linacs, can be 70% or more of the workload - use factors may also be different

11 3. Special Procedures (2) 2. Stereotactic t ti 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 R x 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 mobile lead barrier. Neutrons have been source of discussion i recently, but appear not to be problematic

12 4. Calculational Methodology - While much of the methodology for low energy photons 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) 3. Refined calculations for capture gamma rays at the end of a maze (McGinley) 4. Direct shielded doors for high energy linacs

13 5. Additional Data - Updated occupancy factors (in conjunction with NCRP #147) - Primary TVLs continue TVL 1 and TVL e practice from NCRP #51 but values are slightly different - Leakage TVLs (90 ) also now uses the same convention - Scatter fractions: 6 MV corrected and higher energies added - Scatter TVLs for energies other than 6 MV, plus lead. - Tabulated albedo factors for concrete as well as iron and lead

14 6. 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 instantaneous t 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) - The weekly exposure limit is conventionally taken to be 1/50 x the annual limit, namely msv/wk

15 6. 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?) - Specifically the use of a measured instantaneous dose rate at maximum x-ray output does not represent the radiation environment of the facility - This 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 the IDR when evaluating a barrier

16 7. Special considerations - Skyshine: - this was included in NCRP #51, but no experimental verification, for photons or neutrons, had been provided until now - side scattered photon radiation - Groundshine Gou radiation - Activation - Ozone production

17 Examples 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 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 on calculations for a robotic arm machine

18 Summary - NCRP #151 provides a significant improvement in the methodology and data needed by the shielding designer for a modern radiotherapy department - It covers all aspects of the calculational methodology and provides sufficient data for these calculations - However, there are still some calculations that cannot easily be solved using a spreadsheet, that would benefit from further insight

19 Thank you for your attention! Muito obrigado pela sua atenção!