Radiation Protection Chapter 23

Biological Effects of Radiations  high doses  lethal with whole body exposure (LD-50 ~ 5 Sv)  partial body exposure effects are related to dose and area exposed  2 Sv  hair loss ~ 3 Sv  intermediate doses  risk of cancer increases proportionality with dose  diagnostic dose range  no demonstrated effects

Radiation Dosimetry Terms   Exposure   Air kerma   Entrance skin exposure (ESE)   Entrance air kerma   Radiation absorbed dose   Dose equivalent (H)   Effective dose equivalent (H E or EDE)

 Radiation exposure is measured in Roentgens (R) - convenient, or Coulomb/kg,  Not what we want to know  Radiation dose relates to patient risk but cannot be measured - dose must be calculated  Dose is calculated based on: entrance exposure, x-ray beam energy and tissue thickness  Dose is calculated based on: entrance exposure, x-ray beam energy and tissue thickness and type What is radiation exposure and dose?

Radiation exposure and dose The absorption of x-ray photons results in the deposition of energy in the tissue. The energy deposited per unit mass of tissue is the radiation absorbed dose. The traditional unit of measurement of absorbed dose is the rad ( r adiation a bsorbed d ose). Defined as 100 erg/g. SI units: 100 rads = 1 Gray = 1 Joule / kg.

Radiation exposure and dose Equivalent dose takes into acount the biological effectiveness of the radiation. It is expressed in rems ( r ad e quivalent m an) and is equal to the dose in rads times a radiation weighting factor w r. For x-rays, w r = 1. Therefore equivalent dose in rems is exactly equal to the dose in rads, i.e. 1 rad = 1 rem. SI units: 100 rem = 1 Sievert (Sv)

Radiation exposure and dose Effective dose equivalent is the sum of equivalent doses in all tissues or organs irradiated, each multiplied by the appropriate tissue weighting factor. H = Σ w T x H T Unit is the rem. SI units: 100 rem = 1 Sievert (Sv)

Radiation exposure and dose If a radiation exposure involves less than the entire body (such as in mammography, chest x-ray, etc), the effective dose is defined as the equivalent dose that, if delivered to the whole body, would produce the same risk to the patient as the given exposure. For example if ½ of the body receives 100 mrem the effective dose would be 50 mrem.

Radiation exposure and dose some w T values: gonads = 0.2 red bone marrow, lung, colon, stomach = 0.12 breast, bladder, liver, esophagus,thyroid = 0.05 skin = 0.01

Quantities and Radiation Units  Exposure - Roentgen  measurement of ionization in air  exposure  Absorbed Dose – rad, Gray (1 Gray = 100 rad)  1 rad = 1 cGy  amount of energy deposited per gram  Equivalent Dose – rem, Sievert (1 Sv = 100 rem)  1 mrem = 10 microSv  radiation protection  Effective Dose – rem, Sievert

Sources of Exposure to US Population  Annual average total effective dose:  3.6 mSv or ~10 microSv/day  360 mrem or ~ 1 mrem/day  Most of dose from  Radon  Internal emitters  Terrestrial radioactivity  Cosmic rays

Sources of Exposure to US Population  Radon  2 mSv or 200 mrem/yr  radioactive noble gas  daughter of radium-226  alpha emitter  4 day half-life  Daughters of radon  also radioactive  attach to dust  irradiate bronchial mucosa  % of lung cancers

Sources of Exposure to US Population  Internal emitters  0.4 mSv or 40 mrem/yr  K-40 primary source  present in muscle  male dose > female dose

Sources of Exposure to US Population  Terrestrial emitters  0.28 mSv or 28 mrem/yr  long-lived natural emitters  uranium, radium, thorium, C-14

Sources of Exposure to US Population  Cosmic rays  0.27 mSv or 27 mrem/yr  high energy protons, x-rays, etc  >.05% reach sea level  air attenuation equivalent to ~34 feet of water  Altitude  dose doubles per 1500 m 40,000 feet ~ 0.5 mrem/hr  5 hour flight at 40,000’ results in dose mSv or 2.5 mrem or 25 microSv

Sources of Exposure to US Population  Enhanced natural sources  smoking  13 mSv or 1.3 rem  medical x-rays  0.39 mSv or 39 mrem  nuclear medicine  0.14 mSv or 14 mrem  fallout  0.01 mSv or 1 mrem  nuclear power  <<0.01 mSv or < 0.05

Average Annual Occupational Effective Dose Equivalent

Concepts to Remember  Collective Effective Dose Equivalent  product of average effective dose equivalent and size of exposed population (person-Sv)  Genetic Significant Dose (GSD)  index of potential genetic damage  the equivalent dose that, if received by every member of the population, would be expected to produce the same genetic injury to the population as do the actual doses receive by the exposed population  GSD from medical x-rays is 0.2 mSv or 20 mrem  GSD from nuclear medicine is 0.02 mSv or 2 mrem

Personnel Dosimetry  Film badge  metal filters (lead, copper, aluminum) used to identify photon energy  open window used to measure betas  range 100 microGy (10 mrad) to 15 Gy (1500 rad)  provide deep and shallow (skin) dose  Optically Stimulated Luminescent (OSL)  trapped electrons, which are proportional to dose, read out using laser  Thermoluminescent Dosimeter (TDL)  trapped electrons, which are proportional to dose, read out using heat  small size

Exposure Control  Time  Distance  inverse square law  Shielding  lead apron of 0.5 mm lead equivalent attenuates ~ 90% of incident radiation

X-ray Room Shielding

X-ray Room Shielding Terms to Remember  Workload, W (mA-minutes/week)  amount of x-rays produced per week  Use Factor, U  fraction of time radiation is directed at a particular barrier (wall)  0 to 1  Occupancy Factor, T  fraction of time per week an individual spends in an adjacent area,  0 to 1  T = 1 for office; T = 1/16 for hallway  Distance

X-ray Room Shielding Terms to Remember  Control Area  access controlled, individuals are monitored, trained in radiation protection  exposure to personnel limited to 50 mSv or 5 rem/year  Unrestricted Area  accessible by the general public  exposure limited to 1 mSv or 100 mrem/year

Maximum Permissible Annual Dose Limits

ALARA As Low As Reasonably Achievable  dose limits to workers and public are regarded as upper limits rather than a acceptable doses  radiation safety programs in hospitals operate such that doses are one tenth of annual dose limits  personnel doses reviewed quarterly

Advisory Bodies  National Council on Radiation Protection (NCRP)  publishes monographs containing recommendations on radiation health issues  monographs serve as reference documents from which regulations are developed  International Commission on Radiation Protection (ICRP)  international body similar to NCRP

Regulatory Agencies  US Nuclear Regulatory Agency (USNRC)  regulates byproduct materials, and accelerator produced materials, etc  specifies dose limits  agreement states (~37) assume regulatory authority of the NRC  Food and Drug Agency (FDA)  regulates pharmaceuticals, all x-ray equipment, certifies mammography facilities  Department of Transportation DOT  regulates transportation of radioactive materials

Sum of Internal and External Doses  Internal doses  continues after ingestion until decay or excretion  committed dose equivalent is dose to tissue or organ over 50 years  committed effective dose equivalent (CEDE) is sum of dose equivalent to organ times a tissue weighting factor  External doses  deep dose as indicated by personnel dosimeter  Total effective dose equivalent (TEDE)  sum of internal and external doses

1. Define the following quantities and give the units of each: a.Absorbed Dose b. Dose Equivalent c.Effective Dose Equivalent 2.What is the annual, average per capita total effective dose equivalent to the US population and what are the sources of this exposure? 3.What is the total effective dose for a 5 hour transcontinental flight? 4.What is the average, annual occupational exposure for radiologists and x-ray technologists? 5.If the distance from a point source of radiation is doubled, how much does the exposure rate change? 6.What fraction of a diagnostic beam incident on a patient is scattered at 90 degrees one meter from the patient? 7.What sources of radiation must be considered when designing the shielding for a diagnostic x-ray room? 8.Define workload, use factor, occupancy factor, primary barrier and secondary barrier. 9.The principle that radiation doses should be kept to the lowest level consistent with cost and other factors is known as: a.de minimus dose b.ALARA c.dose threshold d.MPD (maximum permissible dose) e.EDE (Effective Dose Equivalent)

10.The advisory group that recommends standards for radiation protection in the United States is the: a.NRC (Nuclear Regulatory Commission) b.DOE (Department of Energy) c.ICRP (International Council on Radiation Protection) d.NCRP (National Council on Radiation Protection) e.EPA (Environmental Protection Agency) 11.The EDE limit in mSv/year for members of the public exposed frequently or continuously to radiation is: a.100 b.10 c.1 d.0.1 e In limiting exposures to members of the public, the following sources are not included (more than one answer may be correct): a.terrestrial background b.medical x-rays c.nuclear medicine procedures c.radon in the home e.cosmic rays 13.Match the definitions a-c with the expressions 1-3. a.fraction of time beam is directed toward location 1. workload b.fraction of time people are present in location 2.use factor c.mA-min/week 3. occupancy factor 14.The walls of a room require 2mm of lead. If the workload of the x-ray unit in the room is doubled, how much lead in mm must be added to the walls? (HVL is 0.5 mm of Pb) a.none b.0.5 mm of Pb c.1 mm of Pb d.2 mm of Pb e.4 mm of Pb