1. 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

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

3. 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)

4. 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)

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

6. 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 30 cm = 670 mGy/h

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

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

10. DESIGN CONSTRAINTS Instantaneous Dose Rate
Assuming > 2 metres from source to point of interest for 15 Ci source
Kair = 15 2 m
(en/)tissue / (en/)air x (1-g) = 1.10
Equivalent dose rate, H = m
For < m
transmission,B < 4.5 x 10-4
For < m
transmission, B < 1.5 x 10-4

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

12. 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 = 1 metre
U = use factor for barrier
T = occupancy factor

13. 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”.

14. 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 = 1 m in a year = ?

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

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

17. Workload 10 Gray per fraction to 2 cm from source
Maximum 25 patients per week
250 2 cm
250,000 mGy/wk x (22/1002) x 52wk/y
W = 5,200 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

18. Shielding

19. Handbook of Radiological Protection Part 1, 1971
Manual door opening

20. 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)

21. 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

22. Entrance
Door
Maze

23. Maze & position of treatment
Adapted from IPEM Report 75

24. 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

25. Former orthovoltage room with extra shielding
From IPEM Report 75

26. Motor driven lead door

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

29. 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

30. 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

31. HDR - alternative sources
Cobalt-60
Ytterbium-169

32. 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)

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

34. 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

35. 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??

36. 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

37. Permanent Seeds - Dose rates
Dose rate from implanted patient (Leeds 125I)
patient surface uSv/h
@ 1 metre 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

38. LDR Iridium Implant

39. Example LDR Ir-192 treatment
8 wires, 100 mm each, 20 hours
0.4 1 m
for m
30 cm concrete 2 m) or
2.5 cm lead 2 m)
Local shielding may be an option

40. 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
IPEM Report 75 The Design of Radiotherapy Treatment Room Facilities, 2002 (currently being revised)
NCRP Report No 51, Radiation Protection Design Guidelines for 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)