Radiation Protection for X-Ray Technologists
Linacs, radioactive substances and
all the rest
John Saunderson
Consultant Physicist / Radiation Protection Adviser
1
Beam energy - kV or MV?
mid = 9%
mid = 77%
2
• Kilovoltage X-ray
• good for imaging
• good for radiotherapy near skin surface
• Megavoltage X-ray
• deep therapy
• imaging poor, but possible for verification
3
Electron production in the X-ray tube
kV
Applied voltage chosen to give
correct velocity to the electrons
mA
-
+
Filament
Target
(heats up on prep.)
Voltage to arc 1 m air at STP = 3.4 MV
4
How to get megavoltage energy
photons?
• Normal X-ray tube? - insulation needed too
thick, arcing, etc.
• Van de Graaff generator
2-MeV Van de
Graaff accelerator
5
How to get megavoltage energy
photons?
• Normal X-ray tube? - insulation needed too
thick, arcing, etc.
• Van de Graaff generator
• Radioactive sources - Co-60 (1.2 MeV gamma rays)
6
Linear Accelerator
(linac)
electrons
microwaves
Klystron or
magnetron
7
8
CL6
9
Typical dose rates
• Fluoroscopy
• entrance dose rate < 50 mGy/min
• > 40 minutes to erythema threshold
• > 3 min to annual hand dose limit
• Linac
• entrance dose rate > 2.4 Gy/min
• < 1 minute to erythema threshold
• < 4 seconds to annual hand dose limit
10
MV – electrons / X-ray photons
11
12
MV – electrons / X-ray
photons
Photon or
electron energy
6 MeV
Electron range
3 g/cm2
= 2.6 mm Pb or
X-ray
transmission
10% → 55 mm Pb
1% → 110 mm Pb
= 30 mm H2O
15 MeV
10 g/cm2
= 8.8 mm Pb or
10% → 57 mm Pb
1% → 114 mm Pb
= 74 mm H2O
13
CL3 6MeV Electron Depth Dose Variation with Field Size
120
6x6cm
15x15cm
25x25cm
100
CHH ELECTRON COMMISSIONING
Depth Dose Commissioning Dataset
PDD
80
Machine: CL3
Detector: NACP-02
Energy: 6 MeV
TSD: 100 cm
Ionisation data converted to dose using
OmniPro Accept AAPM TG51 algorithm
60
40
20
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Depth (cm )
14
Hull & East Yorks. CL1 & CL2
15
16
Tenth Value Layers / mm
Material
Density
Zeff
Concrete
2.4
12.5 (?) 17.4 mm 432 mm
Water
1.0
7.42
78 mm
620 mm
Lead
11.4
82
0.2 mm
56 mm
Iron
80kVp x 15MVx
i.e.
7.9
26
1.0 mm 108 mm
• @ 80 kVp, 1 mm lead  90 mm concrete
Tungsten
19.3 1 mm lead
74  8 mm concrete
• @ 15 MVx,
• (lead 5 x18.95
density of concrete)
Uranium
92
17
Neutron Production
• Binding energy (BE) of
neutrons 7-20 MeV (mostly)
18
B10
Pb207
19
Neutron Production
• Binding energy (BE) of
neutrons 7-20 MeV (mostly)
• Probability increases with (EX
– BE) up to (2 x BE)
• Very little neutron production
below 10MVx
MVx
Gyn / GyX
10
0.003%
15-18
0.010%
20-25
0.030%
20
Neutron Shielding
• TVL neutrons
• ~ 400 cm lead (thermal)
• < 31cm concrete for medical
linacs
• < 10 cm polythene
• (TVL for 15MVx is 43.2 cm)
21
Hull & East Yorks. CL1 & CL2
22
Neutron Activation
23
Neutron Activation Products
Port
1m
24
Neutron Activation Products
Half lives
•
•
•
•
(2)
Aluminium ≈ 2.5 minutes
Bed end ≈ 9.2 minutes
Linac head ≈ 8.4 minutes
Plasterboard, water, Perspex – no activity
detected
25
26
Why 7.5 Sv/h ?
• Originally
• 7.5 Sv/h x 8 h/d x 5 d/wk x 50 wk/y = 15mSv
• Pre 1/1/2000, 15 mSv was unclassified person
effective dose “limit”
• Today
• Post 2000, 6 mSv is
unclassified person effective
dose “limit”
• 6mS/y / (7.5 Sv/h x 7.5h/d) =
100 d/y
27
28
Why 75 Sv/h ?
• 75 Sv/h x 8 h/d x 5 d/wk x 50 wk/y = 150mSv/y
• Eye dose limit = 150 mSv/y
• Skin dose limit = 500 mSv/y
• Remember
• Dose limit not like a speed limit
• Optimisation
• Keep doses As Low As Reasonably Achievable
• Follow local rules.
29
• Half-life no more than 81/2 minutes
• So in ½ hour, dose rate will be 10 times or
more lower
30
Radioactive materials
• Iridium-192
• Used for brachytherapy in wire form
• 0.1-0.7 MeV beta particles absorbed by
platinum coating
• 0.2-1.06 MeV gamma rays emitted (effective
energy 0.4MeV)
• TVL = 12 mm Pb, 185 mm concrete
31
Smoke detector vs Flexitron
32
Smoke detector
Americium-241
37 kBq (kilobecquerels)
@10 cm
• 0.005 mSv/h @ 1 cm
• Erythema threshold in 45 years
Direct skin contact
• 0.2 mSv/h
• Erythema threshold in > 1 year
• Hand dose limit in 1 month
33
Flexisource
400 GBq (gigabecquerels)
• 400 trillion gamma rays per second
44 mGy/hour @ 100 cm
44 x 1002/12 = 440,000 mGy/h @ 1 cm
122 mGy/second @ 1 cm
erythema threshold in 16 secs
hand dose limit in 1.2 secs
(direct skin contact 1.2 - 72 Gy/s)
34
“Radioactive Patients”
Patients may be radioactive if
• they’ve been injected with
or swallowed radioactive
pharmaceuticals
• they have solid radioactive
sources surgically implanted
• they have been involved in
an accident with radioactive
materials .
35
Radioactive Decay - half life
Beginning








After 2 half lives
     
     
     
      
     
      
     
     
After 4 half lives
     
     
     
     
     
     
     




 
 
 
 

 
 

After 1 half life
   
   
    
    
   
    
   
   
After 3 half lives
     
    
     
     
    
     
    
    

 
 
 

 
 
 
After 5 half lives
     
     
     
     
     
     
     














36
Nuclear Medicine Scan
• Patient injected with or swallows a radioactive
pharmaceutical
• Gamma camera traces where that
37
radiopharmaceutical is concentrated .
38
Thyroid treatments with radio-iodine
•Hyperthyroidism / thyrotoxicosis
•overactive thyroid - 400 MBq
•Thyroid cancer
•must destroy all tumour - 3000 MBq
•c.f. thyroid scan - 0.2 MBq.
39
Other unsealed source therapies
• Phosphorus-32 for polycythemia (too many red
blood cells)
• Yttrium-90 colloid for arthritic conditions
• Strontium-90 for bone metastases.
40
Brachytherapy
(radioactive implants)
•
•
•
•
Intracavity afterloading
Iridium wire afterloading
Iridium pins
Iodine-125 seeds .
41
HDR-microSelectron
42
Iridium Implant
43
Iodine-125 seeds in Prostate
44
New HDR Flexitron
• Iridium-192
• Used for brachytherapy in wire form
• 0.1-0.7MeV beta particles absorbed by platinum coating
• 0.2-1.06MeV gamma rays emitted (effective energy 0.4MeV)
• TVL = 12mm Pb, 185mm concrete
45
Non-Ionising Radiations (briefly)
e.g.
• lasers
• ultraviolet
• MRI scanners
46
Laser Device Classes & Hazards
•
•
•
•
•
•
•
Class 1
Class 1M
Class 2
Class 2M
Class 3R
Class 3B
Class 4
• Applies to device as a whole.
47
• Class 1
• no risk to eyes (including using optical viewing
instruments)
• no risk to skin
• (either low power device or totally encased)
48
• Class 1M
• no risk to the naked eye
• no risk to skin
49
• Class 2
• no risk to eyes for short term exposure
(including using optical viewing instruments)
• no risk to skin
• (visible, so blink response protects)
• (may cause dazzle or flash blindness)
50
• Class 2M
• no risk to naked eye for short time exposure
• no risk to skin
51
• Class 3R
•
•
•
•
low risk to eyes
no risk to skin
(risk for intentional intrabeam viewing only)
(may be a dazzle hazard)
52
• Class 3B
• medium to high risk to eyes
• low risk to skin
• (aversion response protects skin, or must be
focussed to such a small spot that pin-prick
effect only)
53
• Class 4
•
•
•
•
high risk to eyes and skin
low risk to skin
(diffuse reflection may be hazardous)
(possible fire hazard)
54
DIR 690 - 1290 L4
1
/10000th of 690-1290 nm laser light
transmitted
Laser
Wavelength
Suitable?
CO2
10600 nm

Ho:YAG
2100 nm

Nd:YAG
1060 nm

LaserTripter
504 nm

Argon
477 or 515 nm

Risk Assessments
56
HEYH Trust CP137
Health & Safety at Work Policy
- Lasers -
• Includes safety of class 3B and class 4 lasers
57
UV
• Eye hazard, skin hazard
• Dermatology
• TL01 and PUVA
58
UV treatment of psoriasis
Erythema
clearance of psoriasis
10
Relative effect
1
UVC
UVA
0.1
UVB
0.01
0.001
0.0001
250
275
300
325
350
375
400
Wavelength / nm
59
Relative Spectral Power of UV
Therapy lamps
1.2
1.0
0.8
UVB
UVA
UV6 lamp
0.6
0.4
0.2
0.0
250
PUVA
lamps
Physio'
UVB bed
300
nm
350
400
60
61
MRI
62
63
30/11/08
64
fin
65