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
0
10
20
30
millimetres tissue
40
50
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
– 226Ra, 60Co, 137Cs, 192Ir, 90Sr/90Y, 106Ru, 103Pd, 125I,
198Au, (169Yb, 170Tm), electronic (X-rays)
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
–
226Ra, 60Co, 137Cs,
169
170
192Ir, 90Sr/90Y, 106Ru, 103Pd, 125I, 198Au,
Iridium-192
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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
– 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)
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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=
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2
(P.d )
/ (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 Xrays 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 counterterrorism standard
• CCTV to observe patient
• Warning lights
Other Safety Features
• Door interlocked to afterloader unit
– source will not be deployed if door not closed
– deployed source will retract if door opened
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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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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
– 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
– @ 1 metre
2-67 uSv/h
0 - 1.6 uSv/h
• Dose rate from seed
– On contact 100 Sv/h
– @ 1 cm
5 mSv/h
–@1m
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 0.1100 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)