radiographers - radiation dosimetry
TRANSCRIPT
Radiation Dosimetry
John Gough, MS, CHP
Radiation Safety Officer
Swedish Medical Center
Introduction
Swedish Medical Center
• Established in 1910• 3 Hospital locations• Multiple specialty clinics • Cancer, Heart, Surgery institutes• 12 primary-care clinics• Full range nuclear med, radiation oncology • Research Hospital• Type A Medical Broad Scope License
History
History
• X-rays discovered in 1895
• Radioactivity discovered in 1896
• First document radiation fatality – 1904– Clarence Daly severely burned in
1986 by an x-ray fluoroscope
History
• First attempt at establishing a radiation dose limit – Skin Erythema Dose – 1920
• 200 mR/day dose limit established in 1931
• 25,000 mrem/year established during WWII.
Radiation Dosimeters
Radiation Dosimetry
• scientific determination of amount, rate, and distribution of radiation emitted from a source of ionizing radiation, in biological d. measuring the radiation-induced changes in a body or organism, and in physical d. measuring the levels of radiation directly with instruments.
Types of Dosimeters
• Immediate Read– Pocket Ionization Chambers, Solid state
detectors, handheld GM/Ionization detectors with dose accumulation function
• Delayed read / Personnel monitors– Film Badges, TLD (Thermo Luminescent
Dosimeters), OSL (Optically Stimulated Light-emitting Dosimeters)
Film Badges
• Most common type of radiation dosimeter used.
• Works by darkening of x-ray film in proportion to radiation absorbed.
• Cheap• Not durable• Short monitoring period per
badge (6 months or less)
TLD
• “Captures” radiation dose information in a crystal matrix
• Releases light when heated, light intensity proportional to radiation dose absorbed
• Durable• Can be expensive (reusable
chips)• Information destroyed when
processed
OSL
• “Captures” information in an Aluminum Oxide matrix
• Releases information by laser stimulation
• Can be reread after processing• Durable• Landauer Only
Pocket Ionization
• Electro-statically charged “leaf” discharges as it is exposed to ionizing radiation
• Not considered a “legal” record• Low accuracy (+/- 20%)• Physical impacts can affect
radiation dose readings
Solid State
• Provides instantaneous information regarding dose accumulation
• Simple to use
• Not a “legal” record
• Dose range device dependent
Handheld Radiation Detectors
• Ability to perform both dose rate and dose accumulation
• Good for spot checks
• Direction dependent
• Not considered a “legal” personnel dose record
Dose Limits
Occupational Dose
"Occupational dose" means the dose received by an individual in the course of employment in which the individual's assigned duties involve exposure to radiation or to radioactive material from licensed and unlicensed sources of radiation, whether in the possession of the licensee, registrant, or other person. Occupational dose does not include dose received: From background radiation, from any medical administration the individual has received, from exposure to individuals administered radioactive material and released under chapter 246-240 WAC, from voluntary participation in medical research programs, or as a member of the public.
Radiation Doses Comparison
Activity Dose (mrem)1 day of background radiation 1 mrem
1 Chest x-ray 10 mrem
Public Dose Limit 100 mrem
Nuclear Stress Test 500+ mrem
Whole Body CT 1000+ mrem
Occupational Dose LimitsNRC / WA DOH
Part of Body Annual Limit (mrem)
Whole Body (TEDE) 5,000
Organ or Extremity 50,000
Lens of the Eye 15,000
Dose to Fetus 500
Occupational Dose LimitsNRCP Recommendations
* Not to exceed 1000 mrem x Individual’s Age for Cumulative Lifetime exposure.
Part of Body Annual Limit (mrem)
Whole Body (TEDE)* 5,000*
Organ or Extremity 50,000
Lens of the Eyes 15,000
ALARA Concept
“As Low As Reasonably Achievable” means making every reasonable effort to minimize the dose received when working with sources of radiation. Essentially it means minimizing the risks while understanding that the cost vs. benefit for the dose reduction.
Typical ALARA Levels
Part of Body Quarterly Action Level
(mrem)
Level I Level II
Whole Body 125 375
Organ or Extremity 1250 3750
Lens of the Eyes 375 1125
Fetal n/a 150
Dosimetry Reports
Expected Doses from Radiography Sources
Radiography Sources
Isotope Typical Activity (Ci)
Gamma Constant (R/hr – Ci @1 meter)
Dose rate
(R/hr @ 1m)
Ir-192 100 0.48 48
Co-60 20 1.32 26.4
Se-75 100 0.2 20
X-ray n/a