Page 2 Key Messages in This Presentation Shielding Design Methods for Linear Accelerators Each linear accelerator vault is unique Challenges in generating a shielding report – Identifying the locations around the vault that require a calculation – Appropriately calculating the shielded dose rate for these Melissa Martin, MS, FACR, FAAPM, FACMP locations – Communicating the calculation implications to the architect and Therapy Physics Inc., Gardena, CA 90248 contractor [email protected] Do not expect to generate a report simply by filling numbers in a spreadsheet AAPM 51st Annual Meeting – Assumptions implicit in spreadsheet may not match vault AAPM 51st Annual Meeting – Especially true if you do not understand the calculations in the spreadsheet Anaheim, CA July 27, 2009 » Including how to adapt the calculations to the vault Page 3 Page 4 Required Information for Shielding Therapy Shielding Calculations Are Designs Primarily Based on NCRP Report No. 151 Architectural drawings of equipment layout Report Title: “Structural Shielding in room Design and Evaluation for Megavoltage X- and Gamma-Ray Architectural drawings of surrounding areas Radiotherapy Facilities” indicating usage of these areas - offices, – Released December 31, 2005 restrooms, corridor, exterior, etc. Calculations here illustrate the NCRP 151 recommendations Elevation view of room or construction of Elevation view of room or construction of Previous NCRP reports are also floor and ceiling and distance between cited in some cases floors – e.g., NCRP 51 and NCRP 79 NCRPNCRP 151151 recommendationsrecommendations areare addressedaddressed throughoutthroughout thisthis presentationpresentation Page 5 Page 6 Topics Linear Accelerator Energy Primary and Secondary Barriers – Simple primary barrier calculations, including required width BJR #11 megavoltage (MV) definition used here – Secondary barrier calculations – British Journal of Radiology (BJR) Supplement No. 11 » Photon leakage, neutron leakage, scatter, and IMRT impact » Photon leakage, neutron leakage, scatter, and IMRT impact Comparison of BJR #11 and BJR #17 MV definitions – Laminated primary barrier calculations (i.e., barrier with metal) – Tapered ceilings BJR #11 MV 4 6 10 15 18 20 24 – Lightly shielded wall for vault below ground level BJR #17 MV 4 6 10 16 23 25 30 Vault entrances – Mazes (five examples with five different layouts) – Direct-shielded doors Skyshine – Photon and neutron skyshine for lightly-shielded ceiling – Generally not recommended for new construction – May be appropriate for cost-effective retrofit to existing vault Page 7 Page 8 NCRP 151 Recommended Workload [1 of 2] NCRP 151 Recommended Workload [2 of 2] 30 patients treated per day is default assumption Workload (W) – NCRP 151 default recommendation for busy facility – “Time integral of the absorbed-dose rate determined at the depth of the maximum absorbed dose, 1 m from the source” – Can also base on a conservative estimate influenced by factors such as historical workload and demographics 450 Gy/wk maximum weekly workload cited in NCRP 151 » e.g. lower patient workload for facility in small town – Kleck (1994) 3 Gy absorbed dose per patient treatment default » Maximum 350 Gy/wk for 6 MV – Assumption used in NCRP 151 Section 7 examples » Maximum 250 Gy/wk at high MV for dual energy – Consistent with 450 Gy/wk with 30 patients treated per day – Mechalakos (2004) » 450 Gy/wk = 5 treatments/wk/patient x 3 Gy/treatment x 30 patients » Maximum 450 Gy/wk for 6 MV single-energy – Equivalent to 219 cGy treatment fraction (0.73 tissue maximum ratio) » Maximum 400 Gy/wk for dual energy » Maximum 400 Gy/wk for dual energy » Intentionally somewhat conservative (compared to ~200 cGy fraction) – NCRP 151 Section 7 examples assume 450 Gy/wk at high MV since no specific allowance for quality or maintenance workload – Can be based on direct knowledge of accelerator use instead » But preferable to stick with the NCRP 151 default 450450 GyGy/ / wkwk absorbedabsorbed dosedose isis thethe defaultdefault weeklyweekly workload workload 450450 Gy Gy/wk/wk is is consistent consistent with with 30 30 patients patients & & 3 3 Gy Gy/treatment/treatment Page 9 Page 10 Workload Assumptions for Dual Energy Radiation Protection Limits Linear Accelerators Shielding Design Goal (P) Preferable to assume full 450 Gy/wk workload is at the higher energy – Level of dose equivalent (H) used in the design calculations – Applies to barriers designed to limit exposure to people – Simpler, more conservative calculation » Limiting exposure to unoccupied locations is not the goal – Appropriate for new construction – Stated in terms of mSv at the point of nearest occupancy For existing construction, dual-energy calculation may be appropriate Recommended values for shielding design goal – If modifications to existing vault are difficult and size constrained – 0.10 mSv/week for controlled areas – Split 30 patient workload to ensure at least 250 Gy/wk at higher MV – 0.02 mSv/week for uncontrolled areas » With 17 patients, 255 Gy/wk at higher MV Typical international shielding design goals W Mode Gy/wk/patient Patients/day – 0.12 mSv/week for controlled areas (Gy/wk) Single x-ray mode 15 30 450 – 0.004 mSv/week for uncontrolled areas Dual x-ray mode 15 30 450 At least 250 High-X mode 15 17 255 Gy/wk at high Low-X mode 15 13 195 MV mode Page 11 Page 12 Controlled Areas Uncontrolled Areas Limited-access area in which the occupational exposure of personnel to radiation or radioactive material is under All other areas in the hospital or clinic and the the supervision of an individual in charge of radiation surrounding environs protection Trained radiation oncology personnel and other trained Access, occupancy and working conditions are workers, as well as members of the public, frequent controlled for radiation protection purposes many areas near controlled areas such as examination rooms or restrooms Areas are usually in the immediate areas where radiation – Choice of appropriate occupancy factors ensures the protection is used, such as treatment rooms and control booths, or of both those who are occupationally exposed as well as others other areas that require control of access, occupancy, who might be exposed in these areas and working conditions for radiation protection purposes The workers in these areas are those individuals who are specifically trained in the use of ionizing radiation and whose radiation exposure is usually individually monitored Page 13 Page 14 Radiation Protection Limits for Locations NCRP 151 Recommended Occupancy Protected location T=1: Areas occupied full-time by an individual) e.g. – Walls: 1 ft beyond the barrier administrative or clerical offices; treatment planning areas, – Ceilings: 1.5 ft above the floor of the room above the vault treatment control rooms, nurse stations, receptionist areas, – Floors: 5.5 ft above the floor of the room below attended waiting rooms, occupied space in nearby building Permissible dose at protected location depends on T= 0.5: Adjacent treatment room, patient examination room occupancy adjacent to shielded vault T = 0.2: Corridors, employeeemployee lounges, staff rest rooms Occupancy factor (T): Fraction of time a particular location may be occupied T = 0.125: Treatment vault doors Maximum shielded dose rate at protected location: P/T T = 0.05: Public toilets, unattended vending rooms, storage areas, outdoor areas with seating, unattended waiting rooms, – Assuming occupancy factor T for protected location patient holding areas, attics, janitor’s closets T = 0.025: Outdoor areas with only transient pedestrian or MaxMax shieldedshielded dosedose raterate traditionallytraditionally referredreferred toto asas P/TP/T vehicular traffic, unattended parking Page 15 Page 16 Occupancy Factor Selection Use Factor Use Factor (U) is the fraction of the workload for which the For interior locations, T=1 and T=0.2 are most common primary beam is directed at the barrier in question – T = 1 for work locations – T = 0.2 for locations not occupied continuously Traditionally U = 0.25 for lateral barriers, ceiling, & floor For exterior locations, T = 0.05 is most common U = 0.1 for tapered portions of ceiling barrier (Example 11) T < 1 now appropriate for some controlled locations Applies to primary barrier calculations, usually not secondary – Use with T = 0.125 for vault entrance with caution: any higher occupancy location further away must also be protected NCRP 151 Table 3.1 below consistent with these values – T = 0.5 for adjacent vault appears to be reasonable assumption – TBI may require deviation from these values Select T = 0.05 for interior locations with caution 90º gantry angle intervals 45º gantry angle intervals Angle Standard – Should be very unlikely to be occupied (storage, attic, closets) U Angle Standard Interval Deviation U (percent) Interval Deviation Center (percent) (percent) T = 0.025 for exterior locations with restricted access Center (percent) 0º (down) 25.6 4.2 – NRC hourly limit is more constraining for unrestricted locations 0º (down) 31.0 3.7 45º and 315º 5.8 (each) 3.0 90º and 270º 21.3 (each) 4.7 90º and 270º 15.9 (each) 5.6 180º (up) 26.3 3.7 135º and 225º 4.0 (each) 3.3 180º (up) 23.0 4.4 Page 17 Page 18 Hourly Limit for Uncontrolled Areas Hourly Limit for Uncontrolled Areas: Recommended Approach Recommendation is based on maximum Time-Averaged Max patients / hour at highest energy: Six Dose Equivalent Rate (TADR) per hour (NCRP 151, 3.3.2) – Maximum in any one hour estimated as one each 10 minutes – TADR synonymous with shielded dose rate in this presentation – Max workload per hour (Wh) is 6 patients x 3 Gy/patient = 18 Gy Calculation adjusts weekly TADR (Rw) to hourly TADR (Rh) Max weekly P/T (mSv/wk) = 0.02 (mSv/hr) W (Gy/wk) / W (Gy/hr) R = (M / 40) R h h w – where W = 450 Gy/wk (single/dual) or W=255 Gy/wk (at high MV) – where M = ratio of maximum to average patient treatments per hour Max Gy per Max Weekly Patients W Equiv.