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Brachytherapy Rooms Mr John Saunderson Consultant Physicist / Radiation Protection Adviser IPEM Radiation Shielding in Medical Imaging and Radionuclide Therapy The Geological Society, London, 19/01/2012 11/01/2012

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External Beam Radiotherapy (linac etc) vs Brachytherapy EBRT: 6 MV X-rays at 100 cm to skin Brachytherapy: Ir-192 source

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Brachytherapy Conditions treated –Cancer, artery restenosis, choroidal neovascularization Placement –Intracavity, intraluminal, interstitial, intravascular, surface, intraocular Dose rates –High Dose Rate (HDR), Pulsed Dose Rate (PDR), MDR, LDR, permanent implant Sources – 226 Ra, 60 Co, 137 Cs, 192 Ir, 90 Sr/ 90 Y, 106 Ru, 103 Pd, 125 I, 198 Au, ( 169 Yb, 170 Tm), electronic (X-rays)

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Brachytherapy Conditions treated –Cancer, artery restenosis, choroidal neovascularization Placement –Intracavity, intraluminal, interstitial, intravascular, surface, intraocular Dose rates –High Dose Rate (HDR), Pulsed Dose Rate (PDR), MDR, LDR, permanent implant Sources – 226 Ra, 60 Co, 137 Cs, 192 Ir, 90 Sr/ 90 Y, 106 Ru, 103 Pd, 125 I, 198 Au, ( 169 Yb, 170 Tm), electronic (X-rays)

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Iridium-192 Beta decays to platinum-192 Half life = 74 days 0.079 - 0.672 MeV beta particles 0.2 - 1.06 MeV photons Effective photon energy 0.4 MeV HVL = 4.5 mm Pb (c.f. 120 kVp HVL <0.1mm Pb)

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Typically –Activity = 370 GBq (10 Ci) source, but up to 15 Ci –Active core = 3.5 mm long x 0.6 mm diameter –Encased in stainless steel attached to cable 192 Ir HDR source 15 Ci @ 30 cm = 670 mGy/h

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HDR afterloader Source stored in built-in tungsten safe Dose rate at 5 cm < 0.1 mSv/h (IEC 60601-2-17 Particular requirements for the safety of automatically-controlled brachytherapy afterloading equipment) Hull’s Flexitron <= 0.005 mSv/h

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Iridium-192 HDR source Air kerma rate constant = 0.113 mGy/h per GBq @ 1 metre For 15 Ci source (555 GBq) –60 mGy/h @ 1 m (7.5 microSv/h @ 90 m) 10 Gy/minute @ 1 cm 2 Gy/second contact Only patient being treated inside controlled area when source is out

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Assuming > 2 metres from source to point of interest for 15 Ci source –K air = 15 mGy/h @ 2 m –( en/ ) tissue / ( en/ ) air x (1-g) = 1.10 –Equivalent dose rate, H = 16.5 mSv/h @ 2 m DESIGN CONSTRAINTS Instantaneous Dose Rate For < 7.5 Sv/h @ 2 m –transmission,B < 4.5 x 10 -4 For < 2.5 Sv/h @ 2 m –transmission, B < 1.5 x 10 -4

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DESIGN CONSTRAINTS Annual Dose Source only out for a few minutes per patient Only a few patients per day

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NCRP approach (Report 151 etc) B = (P.d 2 ) / (W.U.T) B = barrier transmission factor P = dose constraint d = distance to point of interest W = workload @ 1 metre U = use factor for barrier T = occupancy factor

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P - Shielding design goals NCRP Reports 147 (diagnostic) & 151 (RT) –Controlled area: 5 mSv/y (0.1 mSv/wk) –Uncontrolled area: 1 mSv/y (0.02mSv/wk) HSE Guidance L121 –Members of the public: 0.3 mSv/y (NRPB) –Occupational: separate dose constraint “not appropriate” for “most radiotherapy”.

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B = (P.d 2 ) / (W.U.T) P = 0.3 mSv a year (= 0.0058 mSv/wk) d = varies, but typically 2 metres or more U = 1 (all barriers receive primary and scatter) T = 1 (staff always present when source out) W = mSv @ 1 m in a year = ?

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Workload - Dose per treatment Gynaecological cancers –Vaginal vault: 4 Gy x 3 fractions @ 1.5, 1.75, 2 or 2.25 cm + EBRT, or 4.7 Gy x 5 fractions @ 1.5, 1.75, 2 or 2.25 cm –intra-uterine 7 Gy @ 2 cm x 3 fractions + EBRT Prostate: –8.5 Gy x 2 fractions @ prostate border +EBRT Bronchus: 8 Gy x 2 fractions @ 1 cm Oesophagus: 5 Gy x 3 fractions @ 0.5 cm

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Workload - Dose per treatment Gynaecological cancers –Vaginal vault: 4 Gy x 3 fractions @ 1.5, 1.75, 2 or 2.25 cm + EBRT, or 4.7 Gy x 5 fractions @ 1.5, 1.75, 2 or 2.25 cm –intra-uterine 7 Gy @ 2 cm x 3 fractions + EBRT Prostate: –8.5 Gy x 2 fractions @ prostate border +EBRT Bronchus: 8 Gy x 2 fractions @ 1 cm Oesophagus: 5 Gy x 3 fractions @ 0.5 cm Reasonably safe to assume < 10 Gy @2cm

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Workload 10 Gray per fraction to 2 cm from source Maximum 25 patients per week –250 Gy/wk @ 2 cm –250,000 mGy/wk x (2 2 /100 2 ) x 52wk/y –W = 5,200 mGy/y @ 1 m Barrier transmission acceptable –B = (P.d 2 ) / (W.U.T) = 0.3 x 2 2 / (5200 x 1 x 1) –B = 2.3 x 10 -4 c.f. IDR –< 7.5 uSv/h, B < 4.5 x 10 -4 –< 2.5 uSv/h, B < 1.5 x 10 -4

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Shielding

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Handbook of Radiological Protection Part 1, 1971 Manual door opening

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Shielding ( 192 Ir) IPEM Report 75 The Design of Radiotherapy Treatment Room Facilities –Concrete TVL = 113 mm (Pb 15mm) –For B = 10 -4, 452 mm concrete (Pb 60mm) Handbook of Radiological Protection 1971 –For B = 10 -4, 605 mm concrete (Pb 65mm) NCRP Report 49 Structural design and evaluation for medical use of X- rays and gamma rays of energies up to 10 MV –For B = 10 -4, 600 mm concrete (Pb 60mm) Lymperopoulou et al 2006 (Monte Carlo simulation) –For B = 10 -4, 595 mm concrete (Pb 64mm)

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Note Distance to 7.5 uSv/h isodose for 15 Ci = 90 metres Shielding must be full height + ceiling/floor Beware ducts –ICRP 97: Prevention of High-dose-rate Brachytherapy Accidents 2005 –Up to 470 uSv/h in unrestricted public area above HDR room –contractor has not installed shield over ceiling vent

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Entrance Door Maze

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Maze & position of treatment Adapted from IPEM Report 75

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Maze Scatter Calculation (Described in IPEM Report 75 / NCRP Report 51) D I,ro =D Io.a x.A/d i 2 Tricky and approximate!! Reflected dose rate at 1 m from wall = dose rate at 1 m from source x reflection coefficient (< 3 x 10 -2 for 0.4 MeV) x area of wall irradiated / (distance to wall) 2 This will generally overestimate maze scatter considerably

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From IPEM Report 75 Former orthovoltage room with extra shielding

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Motor driven lead door

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Initially closed door did not overlap wall sufficiently, leading to > 100 microsieverts/hour by door opening button Some adjustment reduced dose rate significantly

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Other Safety Features Door interlocked to afterloader unit –source will not be deployed if door not closed –deployed source will retract if door opened Must be able to manually open door (so can’t be security measure) HASS source so security measures to counter- terrorism standard CCTV to observe patient Warning lights

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Other Safety Features Door interlocked to afterloader unit –source will not be deployed if door not closed –deployed source will retract if door opened Must be able to manually open door (so can’t be security measure) HASS source so security measures to counter-terrorism standard CCTV to observe patient Warning lights Portable monitor Independent dose rate meter (battery back up) Emergency off Manual retraction Emergency container and cutters

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HDR - alternative sources Cobalt-60 Ytterbium-169

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Cobalt-60 HDR Longer half life than Ir-192 –Ir-192 = 71 days, exchange 3 monthly –Co-60 = 5.25 year, exchange 5 yearly Higher energy photons (1.17 MeV & 1.33 MeV) –more shielding and/or lower activity sources For B = 10 -4 – 192 Ir, concrete = 600 mm (Pb = 65 mm) – 60 Co, concrete = 920 mm (Pb = 165 mm)

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Ytterbium-169 HDR Proposed as an alternative to 192 Ir 50-300 keV photons For B = 10 -4 – 192 Ir, concrete = 600 mm (Pb = 65 mm) – 169 Yb, concrete = 450 mm (Pb = 17 mm) Half life 32 days

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Systems of treatment Permanent implant –e.g. I-125 seeds for prostate cancer LDR - low dose rate –e.g. 20 hour Cs-137 afterloading for gynae cancer PDR - pulsed dose rate –similar overall time to LDR with pulses of high dose rate HDR - high dose rate –e.g. few minute Ir-192 afterloading for gynae cancer

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Pulsed Dose Rate - PDR 192 Ir, 1 Ci source (a tenth of HDR source) Simulates LDR e.g. cervical cancer –10 minute “pulse” per hour for 12 hours Shielding –IDR PDR = 1 / 10 th IDR HDR –Fewer patients per week –LDR room likely to need more shielding Question of overnight treatments??

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Permanent Seed Implants For prostate cancer Low energy photons – 125 I, 28 keV photons, 59 day half-life – 103 Pd, 21 keV photons, 17 day half-life

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Permanent Seeds - Dose rates Dose rate from implanted patient (Leeds 125 I) –patient surface 2-67 uSv/h –@ 1 metre 0 - 1.6 uSv/h Dose rate from seed –On contact100 Sv/h –@ 1 cm5 mSv/h –@ 1 m 5 uSv/h Therefore, for protection –local shielding (lead pig) & distance –no room shielding required

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LDR Iridium Implant

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Example LDR Ir-192 treatment 8 wires, 100 mm each, 20 hours –0.4 mSv/h @ 1 m for 7.5 uSv/h @ 1 m –30 cm concrete (20cm @ 2 m) or –2.5 cm lead (1.5cm @ 2 m) Local shielding may be an option

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References D Granero et al, A dosimetric study on the Ir-192 high dose rate Flexisource, Med. Phys. 33 (12) 2006, 4578 G Lymperopoulou at al, Comparison of radiation shielding requirements for HDR brachytherapy using 169 Y and 192 Ir sources, Med. Phys. 33 (7), July 2006 2541-2547 IPEM Report 75 The Design of Radiotherapy Treatment Room Facilities, 2002 (currently being revised) NCRP Report No 51, Radiation Protection Design Guidelines for 0.1- 100 MeV Particle Accelerator Facilities, 1977 NCRP Report No 49, Structural Shielding Design and Evaluation for Medical Use of X-Rays and Gamma Rays of Energies Up to 10 MeV Particle Accelerator Facilities, 1976 Handbook of Radiological Protection - Part 1: Data, HMSO, 1971 (out of print)

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