Radiation Safety for Anesthesiologists

Report
Radiation Safety for
Anesthesiologists
Rachel Wang
September 2014
Physics
• X-rays are produced by accelerating electrons
through high voltage (50-150 kVp) applied to a
tungsten target in an x-ray tube
Adverse Effects
• Deterministic effects
– Cell death at tissue level
• Skin injuries, lens opacities, infertility
– Predictable when dose exceeds certain thresholds
• Stochastic effects
– Development of cancer from direct DNA ionization
or creation of hydroxyl radicals that cause DNA
damage which is not repaired in time
– Cumulative effects with long latency period
Quantification of Exposure
• Absorbed dose (unit: mGy)
– Amount of ionizing radiation energy absorbed by a
unit mass of material
• Equivalent dose (unit: mSv)
– Absorbed dose multiplied by a radiation weighting
factor (wR) specific to type of radiation (x-ray, alpha
particles, etc.)
– Accounts for biologic “harmfulness” of different types
of radiation
– For diagnostic x-rays, wR=1
Quantification of Exposure (cont.)
• Effective dose (unit: mSv)
– Weighted sum of
equivalent doses in all
specified tissues and
organs
– Calculated by multiplying
equivalent dose to each
organ by a weighting
factor (WT) determined by
the organ’s radiosensitivity
– Used to estimate overall
long-term risk
Organ/Tissue
WT
Organ/Tissue
WT
Bladder
0.04
Lung
0.12
Bone marrow
0.12
Liver
0.04
Bone surface
0.01
Salivary Glands
0.01
Brain
0.01
Skin
0.01
Breast
0.12
Stomach
0.12
Colon
0.12
Thyroid
0.04
Esophagus
0.04
Gonads
0.08
Remainder*
0.12
*adrenals, extrathoracic region, gallbladder, heart,
kidneys, lymphatic nodes, muscle, oral mucosa,
pancreas, prostate, small intestine, spleen,
thymus, uterus/cervix
Average Radiation Doses
• Natural background radiation: 1-3 mSv/yr
• For patients
–
– Fluoroscopy: 2-20 mSv/min
Dose Limits
• Based on epidemiological studies of atomic
bomb survivors and radiation workers
b Statement on Tissue Reactions (approved by the ICRP in 2011) proposed a limit of 20 mSv/yr, averaged over 5 years, not exceeding 50 mSv in any single year
Radiation and Pregnancy
• National Council on Radiation Protection and
Measurements (NCRP) recommends occupational
fetal radiation exposure <5 mSv for entire
pregnancy or <0.5 mSv/month of pregnancy
• International Commission on Radiological
Protection (ICRP) recommends <1 mSv for entire
pregnancy
• Pre-conception irradiation of either parent’s
gonads has not been shown to result in increased
risk of cancer or malformations in offspring
Radiation and Pregnancy (cont.)
Dose to conceptus (mGy)
above natural background
0
1
5
10
50
100
•
Probability of no malformation (%) Probability of no cancer
(0-19 years) (%)
97
97
97
97
97
97
99.7
99.7
99.7
99.6
99.4
99.1
Potential non-cancer health effects of prenatal radiation exposure in doses >500
mGy
– Less than 2 weeks: unknown
– 2–7 weeks: death, miscarriage, growth retardation, and neuromuscular deficiencies
– >8 weeks: miscarriage, severe mental retardation, growth retardation, brain and major
malformations
•
•
Threshold doses for these deterministic effects are well above those received by
healthcare professionals wearing appropriate protective aprons
Probability of live birth without malformation or cancer is reduced by
– 0.002% following conception exposure of 0.5 mSv
– 0.1% following exposure of 10 mSv
•
In-utero exposure to ionizing radiation at any dose is associated with increased risk
of childhood malignancy, esp. leukemia (>6% in doses above 500 mGy)
Sources of Occupational Exposure
• Of 1000 photons reaching the patient, 100-200 scattered,
20 reach image detector, rest are absorbed
• Direct exposure
• Scattered radiation from patient
– Majority of occupational exposure
– 1/1000 of entrance skin exposure rate at 1 meter from center of
field
– Larger patients generate more scatter 2/2 increased voltage and
current of X-ray beams used for better visualization
• Leakage x-rays
– Equipment regulations limit max leakage level to 1 mGy/h at 1
meter from X-ray tube for max tube potential and current
– Typically 0.001-0.01 mGy/h at operator’s position
– Equipment should be checked periodically for radiation leakage
Factors Affecting Staff Doses
• Nonmodifiable factors
– Patient related
• Complexity of procedure
• Body size
– Physician related
• Experience
• Modifiable factors
–
–
–
–
–
Duration
Distance (inverse square law)
Shielding
Education
Monitoring
Factors Affecting Staff Doses (cont.)
ANGLE DEPENDENCE
100 kV
1 mA
0.9 mGy/h
0.6 mGy/h
11x11 cm
0.3 mGy/h
1m patient distance
patient thickness 18 cm
Factors Affecting Staff Doses (cont.)
FIELD SIZE DEPENDENCE
100 kV
1 mA
11x11 cm
17x17 cm
0.8 mGy/h
1.3 mGy/h
0.6 mGy/h
1.1 mGy/h
0.3 mGy/h
0.7 mGy/h
1m patient distance
Patient
thickness 18 cm
Factors Affecting Staff Doses (cont.)
DISTANCE VARIATION
mGy/h at 0.5m mGy/h at 1m
100 kV
1 mA
11x11 cm
Factors Affecting Staff Doses (cont.)
• If possible, stand on side of image intensifier rather than X-ray source
Radiation Exposure of the Anesthesiologist in the
Neurointerventional Suite.
Anastasian, Zirka; Strozyk, Dorothea; Meyers, Philip;
Wang, Shuang; Berman, Mitchell
Anesthesiology. 114(3):512-520, March 2011.
DOI: 10.1097/ALN.0b013e31820c2b81
Fig. 3. Distribution of scatter radiation from a lateral xray source. Plan of interventional radiology procedure
room showing distribution of scatter radiation from a
lateral C-arm. There were higher concentrations of
scatter on the side with an x-ray source, for each step
increase of 0.5 m from the patient. Measurements were
taken from a cardiac catheterization laboratory. Units are
[mu]Sv radiation exposure to personnel per Gy [middle
dot] cm2 of total radiation emitted by the x-ray tube.
This figure was modified and used with permission from
page 64 of the following: Valentin J. Avoidance of
radiation injuries from medical interventional
procedures. Ann ICRP 2000; 30:7-67.
Shielding
• Structural
– Embedded into walls of radiology suite
– Mobile, transparent leaded acrylic shields
• Equipment-mounted
– Leaded drapes suspended from fluoroscopy
table/scanner
• Personal
–
–
–
–
Aprons
Thyroid shields
Eyewear
Gloves
Personal Protective Devices
• Aprons
– Traditionally made of lead-impregnated vinyl/rubber
– Newer lighter-weight materials include barium, tungsten,
tin, and antimony (alone or composite w/ lead)
– Wrap-around style typically 0.25 mm lead-equivalent 
double thickness anteriorly provides 0.5 mm lead
equivalence
– Transmission of 70–100 kVp x-rays through
• 0.5 mm lead: 0.5%–5%
• 0.5 mm lead-equivalent composite and lead-free aprons: 0.6% to
6.8%
– Should be stored on hangers with minimal folds
– Should be inspected under fluoroscopy annually to detect
deterioration and defects
Personal Protective Devices (cont.)
• Eyewear
– Scattered radiation exposure to anesthesiologist’s eye up to 3x that of
interventionist
– Radiation-induced cataracts may be stochastic rather than
deterministic effect
– Threshold dose likely lower than previously thought, may be zero
– ICRP proposed new, lower dose limit of 20 mSv/yr averaged over 5
years, not exceeding 50 mSv in any single year (versus previous 150
mSv/yr)
– Plastic prescription glasses offer inadequate protection (5% reduction
in radiation exposure)
– Prescription glasses made of optical glass offer modest protection (30–
40% reduction)
– Leaded eyeglasses generally provide 0.5 or 0.75mm lead-equivalent
protection (98% reduction, but lower w/ side exposure)
• Recommend combining multiple shielding modalities (shields,
aprons, eyewear, drapes)
Monitoring
• Machines can document peak skin dose and fluoroscopy
time
• Dosimeters
– Should be worn by all personnel working frequently in high-risk
radiation exposure areas
– Types
• Film badges: most common
– Radiation-sensitive film loaded into plastic holder containing filters that allow
the identification of type of radiation exposure
– Heat and moisture sensitive
• Thermo-luminescent dosimeters
– Composed of small chips of lithium fluoride or calcium fluoride crystals
– Small and can be loaded into plastic rings for fingers
– Heat sensitive
• Optically stimulated luminescent dosimeters
– Can be used in place of film badges
Monitoring (cont.)
• Dosimeters
– Person-specific
– Usually assigned to monitor a period up to 3
months, but if recorded exposures reach 10% of
allowable limits, then must be changed monthly
– Provides 2 values
• Hp(0.07): dose equivalent in soft tissue 0.07 mm below
body surface
• Hp(10): dose equivalent in soft tissue 10 mm below
body surface
Monitoring (cont.)
• Location
– Single dosimeter: outside shielded garment or thyroid
collar
– Two dosimeters: 1 at collar level outside lead apron or
thyroid collar; 1 at waist level under lead apron
• Estimating dose
– Effective dose (estimate) = 0.5 Hp(10) at waist (shielded) +
0.025 Hp(10) at neck (unshielded)
– Hp(0.07) from collar dosimeter used to estimate dose
delivered to unshielded skin and lens of eye
– Shielded waist dosimeter used to estimate fetal dose
(overestimates actual fetal dose, does not account for
attenuation by mother’s tissues)
Dose Limits Reviewed
• US: state-specific dose limits; most use NCRP
recommendations
– 50 mSv/yr, lifetime limit of 10 mSv x age (yrs)
• EU: 20 mSv/yr averaged over 5 consecutive yrs,
50 mSv in any given yr
– Germany: 400-mSv lifetime dose limit
• WHO recommends investigation when monthly
exposure reaches 0.5 mSv for effective dose, 5
mSv to lens of eye, or 15 mSv to hands or
extremities
References
• Anastasian ZH, Strozyk D, Meyers PM, et al. Radiation exposure of
the anesthesiologist in the neurointerventional suite.
Anesthesiology 2011; 114:512–520.
• Dagal A. Radiation safety for anesthesiologists. Curr Opin
Anesthesiol 2011; 24:445–450.
• IAEA. Diagnostic and interventional radiology. [cited 5 Sept 2014].
http://rpop.iaea.org/RPOP/RPoP/Content/AdditionalResources/Trai
ning/1_TrainingMaterial/Radiology.htm.
• Miller DL, Vano E, Bartal G, et al. Occupational radiation protection
in interventional radiology: a joint guideline of the Cardiovascular
and Interventional Radiology Society of Europe and the Society of
Interventional Radiology. J Vasc Interv Radiol 2010; 21:607–615.
• Mitchell EL, Furey P. Prevention of radiation injury from medical
imaging. J Vasc Surg 2011; 53 (1 Suppl.):22S–27S.

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