Transcript Linacs, radioactive substances, etc - comp

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
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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
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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
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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)
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Linear Accelerator
(linac)
electrons
microwaves
Klystron or
magnetron
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8
CL6
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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
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MV – electrons / X-ray photons
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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
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CL3 6MeV Electron Depth Dose Variation with Field Size
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6x6cm
15x15cm
25x25cm
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CHH ELECTRON COMMISSIONING
Depth Dose Commissioning Dataset
PDD
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Machine: CL3
Detector: NACP-02
Energy: 6 MeV
TSD: 100 cm
Ionisation data converted to dose using
OmniPro Accept AAPM TG51 algorithm
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40
20
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Depth (cm )
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Hull & East Yorks. CL1 & CL2
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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
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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
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Neutron Production
• Binding energy (BE) of
neutrons 7-20 MeV (mostly)
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B10
Pb207
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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%
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Neutron Shielding
• TVL neutrons
• ~ 400 cm lead (thermal)
• < 31cm concrete for medical
linacs
• < 10 cm polythene
• (TVL for 15MVx is 43.2 cm)
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Hull & East Yorks. CL1 & CL2
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Neutron Activation
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Neutron Activation Products
Port
1m
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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
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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
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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.
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• Half-life no more than 81/2 minutes
• So in ½ hour, dose rate will be 10 times or
more lower
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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
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Smoke detector vs Flexitron
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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
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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)
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“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 .
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Radioactive Decay - half life
Beginning

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After 2 half lives
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After 4 half lives
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
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
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After 1 half life
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   
    
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After 3 half lives
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    
     
     
    
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After 5 half lives
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     

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

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Nuclear Medicine Scan
• Patient injected with or swallows a radioactive
pharmaceutical
• Gamma camera traces where that
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radiopharmaceutical is concentrated .
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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.
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Other unsealed source therapies
• Phosphorus-32 for polycythemia (too many red
blood cells)
• Yttrium-90 colloid for arthritic conditions
• Strontium-90 for bone metastases.
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Brachytherapy
(radioactive implants)
•
•
•
•
Intracavity afterloading
Iridium wire afterloading
Iridium pins
Iodine-125 seeds .
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HDR-microSelectron
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Iridium Implant
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Iodine-125 seeds in Prostate
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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
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Non-Ionising Radiations (briefly)
e.g.
• lasers
• ultraviolet
• MRI scanners
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Laser Device Classes & Hazards
•
•
•
•
•
•
•
Class 1
Class 1M
Class 2
Class 2M
Class 3R
Class 3B
Class 4
• Applies to device as a whole.
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• Class 1
• no risk to eyes (including using optical viewing
instruments)
• no risk to skin
• (either low power device or totally encased)
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• Class 1M
• no risk to the naked eye
• no risk to skin
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• 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)
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• Class 2M
• no risk to naked eye for short time exposure
• no risk to skin
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• Class 3R
•
•
•
•
low risk to eyes
no risk to skin
(risk for intentional intrabeam viewing only)
(may be a dazzle hazard)
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• 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)
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• Class 4
•
•
•
•
high risk to eyes and skin
low risk to skin
(diffuse reflection may be hazardous)
(possible fire hazard)
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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
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HEYH Trust CP137
Health & Safety at Work Policy
- Lasers -
• Includes safety of class 3B and class 4 lasers
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UV
• Eye hazard, skin hazard
• Dermatology
• TL01 and PUVA
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UV treatment of psoriasis
Erythema
clearance of psoriasis
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Relative effect
1
UVC
UVA
0.1
UVB
0.01
0.001
0.0001
250
275
300
325
350
375
400
Wavelength / nm
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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
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MRI
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30/11/08
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fin
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