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Radiation Oncology/Physics/Radiation Protection

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Radiation Protection


Effective Dose

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  • Dose equivalent
    • H = Absorbed dose (D) * WR * N
    • Accounts for different impact of different RT modalities on the same issue (e.g. high LET has higher biological impact than low LET)
    • WR, previously known as Q, is the quality factor
    • N is geometry factor
    • Unit in Sievert (Sv)
    • X-ray, Gamma-ray, electrons: 1
    • Neutrons <10 keV: 5
    • Neutrons 10 keV - 100 keV: 10
    • Neutrons >100 keV to 20 MeV:10
    • Neutrons >20 MeV:5
    • Protons >2 MeV: 2-5
    • Alpha, heavy nuclei: 20
  • Effective dose equivalent
    • Total effective dose to a given organ, across all types of radiation received by that organ. This may be useful in estimating the effective organ dose from nuclear explosion, etc.
    • HT: ΣR (D * WR)
  • Effective dose
    • Whole body effective dose, across multiple tissues and multiple types of radiation
    • E: ΣT (HT * WT)
    • Gonads: 0.20
    • Bone marrow, colon, stomach, lung: 0.12
    • Breast, bladder, esophagus, liver, thyroid: 0.05
    • Skin, brain, kidneys, salivary gland: 0.01
    • 14 remaining tissues composite: 0.1
  • Committed dose: integral over 50 years of dose-equivalent

Sources of Exposure

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  • Natural background (~3 mSv)
    • Radon: 2 mSv/year (56% total). EPA action level 148 Bq/m3 (4 pCi/l)
    • Internal: 0.4 mSv/year (11%)
    • Terrestrial nonradon: 0.3 mSv/year
    • Cosmic: 0.3 mSv/year
    • By location
      • USA Average: 0.8 mSv/year
      • Atlantic Coast: 0.3 mSv/year
      • Ramsar, Iran: 390 mSv/year
      • Cross-country U.S. flight: 0.05 mSv/flight
  • Man-made background
    • Diagnostic x-rays: 0.4 mSv/year (11%) - historical, may be as high as 3 mSv per year with CT scan use
      • Chest x-ray: 0.08 mSv/study
      • Dental x-ray: 0.1 mSv/study
      • Body CT: 11 mSv/study
    • Nuclear medicine studies: 0.1 mSv/year

Exposure Limits

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  • Anual exposure limits (NCRP #91/116)
    • Occupational exposure
      • Radiation worker: 50 mSv/year (actual average dose received ~2 mSv)
      • Lens: 150 mSv/year
      • Individual organs (e.g. hands): 500 mSv/year
      • Emergency permissible dose: 0.5 Sv (=50 R)
    • Public exposure
      • Infrequent: 5 mSv/year
      • Continuous: 1 mSv/year
      • Lens: 15 mSv/year
      • Individual organs: 5 mSv/year
      • By Age: 10 mSv x years
      • Negligible individual dose: 0.01 mSv/year
    • Children
      • Child <18: 1 mSv/year
      • Embryo/fetus: 5 mSv/year
      • Embryo/fetus (after declared pregnancy): 0.5 mSv/month
  • Area limits
    • Controlled area: 1 mSv/week = 50 mSv/year
    • Uncontrolled area: 0.1 mSv/week = 5 mSv/year
  • Dose rate signs
    • Controlled area sign: >0.02 mSv/h
    • Radiation area sign: >0.05 mSv/h
  • Shipping
    • Transportation Index (TI) is dose rate at 1 m from surface of package
    • Container must be DOT approved
    • Radioactive source warning label must be on the container
    • Activity and radionuclide must be on the label
  • White label: No dose rate
    • Yellow II: Dose rate 0-0.01 mSv/h
    • Yellow III: Dose rate 0.01 - 0.1 mSv/h
  • Genetically significant dose (GSD)
    • Equivalent dose to gonads, weighted for age of exposed individuals, over entire population. It is the dose that, if received by every member of the population, would result in the same genetic injury to the entire population
    • Natural background: 0.25 mSv
    • Medical diagnostic: 0.2-0.3 mSv
    • Nuclear medicine: 0.02 mSv
  • Doubling dose
    • The doubling dose is estimated as a ratio of the average rates of spontaneous and induced mutations in a defined set of genes
    • Human spontaneous mutation rate ~3 x 10-6 per gene
    • Mouse induced mutation rate ~3 x 10-6 per gene per Gy
    • Resultant dose to double risk of mutation over spontaneous (doubling dose) ~1 Gy
    • Human data from Hiroshima/Nagasaki suggest somewhat higher average doubling dose of 1.6 Sv
  • Risk of radiation induced cancers
    • Dose and dose rate effectiveness factor (DDREF): Dose >0.2 Gy or dose rate >0.1 Gy/hr double rate of radiation carcinogenesis
    • Whole population fatal cancer: 5% / Sv (leukemia is 20% = 1% / Sv)
      • High dose or high dose rate risk: 10% / Sv
    • Working population fatal cancer: 4% / Sv (excludes children who have higher risk)
      • High dose or high dose rate risk: 8% / Sv
    • Hereditary disorders for reproductive population: 0.2% / Sv
    • Hereditary disorders for working population: 0.1% / Sv
    • Excess risk for population = Population (persons) * excess risk (risk per Sv) * exposure (Sv)
  • Risk of mental retardation during fetal exposure
    • Risk is 40% per 1 Sv
    • Largest risk is between 8-15 weeks; at 16-25 weeks, this risk is 4 times lower.
    • IQ drops 30 points per 1 Sv
    • Threshhold is no larger than 0.2 Gy

Internal Sources

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  • Types of internal radiation
    • Locally absorbed: within 1 cm (typically beta and low energy characteristic)
    • Penetrating: >=1cm (internal gamma)
  • Maximum permissible body burden (MPBB) per 40 hour week
    • I-131: Thyroid 0.7 μCi
    • P-32: Bone 6.0 μCi
  • Effective half-life
    • Accounts for physical half-life and for biologic half-life, always less than either
    • teff,uptake = (tbiol, uptake * tphys) / (tbiol, uptake + tphys)
    • teff,elim = (tbiol, elim * tphys) / (tbiol, elim + tphys)
  • Contaminated articles must be stored until decay (usually 5 half-lives)
  • Patient cannot be released until dose rate at 1 m falls below
    • I-125: 0.01 mSv/h
    • Pd-103: 0.03 mSv/h
    • Ir-192: 0.008 mSv/h
    • I-131: 0.07 mSv/h
  • Total annual dose to individual members of public cannot exceed 1 mSv
  • Visitors to inpatients with radioactive material can visit if
    • Total dose <5 mSv
    • Authorized user approves visit

Medical Events (NRC 2002)

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  • Dose differs from prescribed dose by
    • At least (and Relative below)
      • 50 mSv effective dose
      • 500 mSv organ dose
      • 500 mSv skin
    • Relative (given At least above)
      • Total dose 20%
      • Fraction dose 50%
      • Wrong radioactive drug
      • Wrong route of administration
      • Wrong individual
      • Wrong mode of treatment
      • Leaked sealed source
  • Any event that will result in unintended permanent functional damage
  • NRC must be notified by phone by next calendar day
  • Written report must be submitted within 15 days
  • Referring physician must be notified
  • Patient shall be notified within 24 hours, unless referring physician feels that telling the patient would be harmful


Shielding

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  • Barriers
    • Primary: attenuate direct beam
    • Secondary: attenuate scattered radiation and leakage radiation
  • Shielding equation: D = B * W * U * T / d2
    • Where D is dose
    • B is barrier transmission factor
    • W is workload: beam-on time, given in Gy at 1 m, usually 200-500 Gy/wk
    • U is use factor: fraction of time during which RT is striking the barrier; floory typically 1, walls typically 1/4, ceiling typically 1/2. With IMRT/Cyberknife, these numbers are higher
    • T is occupancy factor: fraction of time during which area behind barrier is occupied; work areas 1, corridors 1/4, stairways 1/8
    • d is distance: typically in meters from isocenter
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