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

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

Brachytherapy Conditions treated Placement Dose rates Sources 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 226Ra, 60Co, 137Cs, 192Ir, 90Sr/90Y, 106Ru, 103Pd, 125I, 198Au, (169Yb, 170Tm), electronic (X-rays)

Brachytherapy Cancer, artery restenosis, choroidal neovascularization 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 226Ra, 60Co, 137Cs, 192Ir, 90Sr/90Y, 106Ru, 103Pd, 125I, 198Au, (169Yb, 170Tm), electronic (X-rays)

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)

192Ir HDR source Typically 15 Ci @ 30 cm = 670 mGy/h 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 15 Ci @ 30 cm = 670 mGy/h

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

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

DESIGN CONSTRAINTS Instantaneous Dose Rate Assuming > 2 metres from source to point of interest for 15 Ci source Kair = 15 mGy/h @ 2 m (en/)tissue / (en/)air x (1-g) = 1.10 Equivalent dose rate, H = 16.5 mSv/h @ 2 m 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

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

NCRP approach (Report 151 etc) B = (P.d2) / (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

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”.

B = (P.d2) / (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 = ?

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

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

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 (22/1002) x 52wk/y W = 5,200 mGy/y @ 1 m Barrier transmission acceptable B = (P.d2) / (W.U.T) = 0.3 x 22 / (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

Shielding

Handbook of Radiological Protection Part 1, 1971 Manual door opening

Shielding (192Ir) 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)

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

Entrance Door Maze

Maze & position of treatment Adapted from IPEM Report 75

Maze Scatter Calculation (Described in IPEM Report 75 / NCRP Report 51) DI,ro=DIo.ax.A/di2 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

Former orthovoltage room with extra shielding From IPEM Report 75

Motor driven lead door

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

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

Other Safety Features Portable monitor 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

HDR - alternative sources Cobalt-60 Ytterbium-169

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 192Ir, concrete = 600 mm (Pb = 65 mm) 60Co, concrete = 920 mm (Pb = 165 mm)

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

Systems of treatment Permanent implant LDR - low dose rate 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

Pulsed Dose Rate - PDR 192Ir, 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 = 1/10th IDR HDR Fewer patients per week LDR room likely to need more shielding Question of overnight treatments??

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

Permanent Seeds - Dose rates Dose rate from implanted patient (Leeds 125I) patient surface 2-67 uSv/h @ 1 metre 0 - 1.6 uSv/h Dose rate from seed On contact 100 Sv/h @ 1 cm 5 mSv/h @ 1 m 5 uSv/h Therefore, for protection local shielding (lead pig) & distance no room shielding required

LDR Iridium Implant

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

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 169Y and 192Ir 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)