Week 1 Radiation Protection Flashcards
Ionization
Process by which a neutral atom acquires a positive or a negative charge
Ionizing Radiation
Produces positively and negatively charged particles as it passes through matter
International Commission on Radiological Protection (ICRP)
set the first rad protection standards
National Council on Radiation Protection and Measurement (NCRP)
was established in the US and is the primary standard setter
Nuclear Regulatory Commission (NRC)
US agency that regulates the safe use of reactor-produced radioactive materials in medical and nonmedical practices and naturally occurring radioactive material such as radium and radon
Agreement States
Agreement in which the federal government could relinquish authority to the states portions of its regulatory authority (agreement between the governor and the commission). These programs are reviewed by the NRC.
States
regulates x-ray machines (including Linacs)
Dose equivalent
Biological effects depend on dose & type of radiation
Same dose of radiation can have different effect based on the type of radiation
𝐻=𝐷 ∙ Q
H = dose equivalent D = absorbed dose Q = quality factor
Quality Factor (Q)
Takes into account the biologic effectiveness
of the radiation type.
X-ray, Gamma Ray Quality Factor
1
Neutrons <10KeV Quality Factor/Thermal Neutrons
5
Protons >2Mev QF.
2
Alpha particles, fast neutrons QF
20
SI Unit of Dose Equivalent
Sievert! SV
1Sv=1J/kg
1Sv=1Gy
1rem=10^-2 Sv
Effective Dose Equivalent
Definition: sum of the weighted dose equivalents for irradiation tissues or organs
Effective dose
quantity that account for the differences
Associated with the same probability of cancer and genetic effects for a non-uniform irradiation as a uniform irradiation.
Total effective equivalent from background
3.0 mSv/yr (300 mrem/yr)
<10 cGy
Can produce: genetic effects (gene mutations), neoplastic diseases (leukemia), effect on growth and development, effect on life span, cataracts.
Stochastic effects
the probability of occurrence increases with absorbed dose, but severity does not depend on dose
Ex: cancer or genetic effects
No threshold dose
All or none effect
Nonstochastic effects
Increases in severity with increasing dose
Ex: organ atrophy, fibrosis, cataracts, blood changes, decreases in sperm count
Has threshold dose
Annual occupational exposure limit:
50mSV (5 rem) per year
General Public limit: Infrequent exposure
5mSv (0.5rem) per year
General Public limit Frequent exposure
1mSV (0.1 rem) per year (also students <18yo)
Embryo Fetus Exposure limit:
total is 5 mSv (0.5rem) and per month 0.5 mSv (0.05 rem)
Lifetime total effective dose
< 10 times x age (years)
Maximum permissible dose–Controlled
0.1 mSv/wk (5 mSv/yr
Maximum permissible dose–Noncontrolled
0.02 mSv/wk (1 mSv/yr
Primary Barrier-
any barrier that can be struck by the primary beam
Secondary Barrier-
a barrier that can be struck only by secondary/scatter radiation
Shielding is measured in
half-value layers (HVL)
HVL- The amount of material needed to reduce radiation exposure in half
Workload (W)
For MV machines- usually stated in terms of weekly dose delivered at 1 m from the source
Multiply the number of patients treated per week with the dose per patient at 1 m. (dose/wk at 1m)
Use Factor (U)
Fraction of the operating time during which the beam is directed towards a particular barrier
0=31%, 90/270=21.3%, 180=26.3%
Occupancy Factor (T)
Fraction of the operating time during which the area of interest is occupied by the individual
1=full time occupancy
Adjacent rooms=1/2
Corridors=1/5
Distance (d)
Distance in meters from the radiation source to the area to be protected. Inverse square law is assumed for both the primary and stray radiation.
Primary Radiation Barrier Calculation
𝑇𝑟𝑎𝑛𝑠𝑚𝑖𝑠𝑠𝑖𝑜𝑛 𝐹𝑎𝑐𝑡𝑜𝑟=(𝑀𝑎𝑥𝑖𝑚𝑢𝑚 𝑃𝑒𝑟𝑚𝑖𝑠𝑠𝑖𝑏𝑙𝑒 𝐷𝑜𝑠𝑒 𝑥 𝐷𝑖𝑠𝑡𝑎𝑛𝑐𝑒2)/(𝑊𝑜𝑟𝑘𝑙𝑜𝑎𝑑 𝑥 𝑈𝑠𝑒 𝐹𝑎𝑐𝑡𝑜𝑟 𝑥 𝑂𝑐𝑐𝑢𝑝𝑎𝑛𝑐𝑦 𝐹𝑎𝑐𝑡𝑜𝑟)
Barrier Material
Concrete: Cheap, walls and roof barriers are usually constructed out of concrete
Lead & Steel: Can be used where space is at a premium
comparing TVL (tenth value layers) for the given beam energy
For megavoltage and gamma radiation, the equivalent thickness of various materials can be calculated
Secondary Barriers
A barrier designed for primary radiation provide adequate protection against leakage and scattered radiation
Leakage Radiation
For MV installations, the leakage barrier usually far exceeds that required for the scattered radiation, because the leakage radiation is more penetrating than the scattered radiation
LINAC is designed to have at 1 meter < 0.1% of useful beam
Scatter Radiation
Scatter comes from the patient, so phantom should be used when doing survey measurements.
Linac Doors No maze
Door is extremely heavy, requires a motor to drive the door (manual operation in emergency)
Linac Doors Maze entryway
Drastically reduces the shielding for the door by preventing direct incidence of radiation at the door
Door is exposed mainly exposed to radiation that has scattered multiple times
Compton scatter at 90 degrees will reduce the energy to 500 keV or less
High energy x-ray beams over 10 MV are contaminated with neutrons
Contamination increases rapidly between 10-20 MV and then remains constant above
Ex: 16-25MV: 0.5% of the x-ray dose is neutrons (0.1% outside the field)
Produced by high-energy photons and electrons incident on various materials: target, flattening filter, collimators and other shielding components
polyethylene
but also need lead or steel to protect against scattered x-rays
Maze should be >5m to reduce neutron fluence at the door
Brachytherapy Storage
Lead-lined safes with lead-filled drawers
Storage area should be ventilated by a direct filtered exhaust to the outdoors, because of the possibility of radon leaks
Sinks should have a filter or trap to prevent loss of source
Door must be secure and be kept secure under lock and key
Brachytherapy Source Preparation
Source preparation bench should be provided close to the safe
Should have a suitable barrier to shield the operator adequately
“L-block” usually constructed of lead
Sources should never be touched with bare hands
Long forceps should be used
Operator must be aware of “time, distance and shielding”
Source Transportation
Can be transported in lead containers or leaded carts
Thickness depends on the type of source and amount of radioactive material
Leak Testing
Radium source can be checked for radon leaks by placing it in a small test tube with some activated carbon or a ball of cotton
After 24 hours, the carbon or cotton ball can be counted in a scintillation-well counter
Periodic leak testing of radium is specified by state regulations.
A source is considered leaking if a presence of 0.0005 uCi or more of removable contamination is measured
Dosimeter
device used to detect and measure exposure to radiation
Field Survey
Portable radiation devices that are designed to detect the presence of radiation and to make a rough measurement of the radiation level, often in terms of mR/h. Three basic types: Ionization chamber* Geiger-Mueller detector* Neutron Detectors
Ionization Chamber
Used for low-level x-ray measurements and has a large volume to obtain high sensitivity
A direct-current voltage is applied between the outer shell and the central electrode to collect ionization charge produced by radiation in the internal air volume when the chamber is exposed.
This creates an ion current flow, which is measured.
Output is directly proportional to the exposure rate.
Detect x-rays, gamma rays, and high energy beta radiation
Do not detect alpha radiation or low levels of radiation
Geiger-Mueller Counter
Also determines the amount of radiation by collecting ions in a chamber filled with gas
Thin window that allows for detection of alpha, beta and gamma radiation
Much more sensitive than ionization chamber
Better for detection, rather than measurement, of low-level radiation sources and contamination
Used in a nuclear medicine lab where sources are stored
Neutron detectors
Common to have 2 detectors outside the treatment room.
One for x-rays and one for neutrons
Personnel Monitoring
Optically Stimulated Luminescence
Film badges
Thermoluminescent dosimeters
Pocket ionization Chamber
OSL Dosimeters
Can detect x-rays, gamma rays in range of 5 MeV to 40 MeV
Advantage: Unaffected by heat, moisture, and pressure
Film Badges
2 pieces of film, gets darker in response to the amount and energy of the radiation to which it’s exposed
Advantage: high spatial resolution, permanent record
Disadvantage: no immediate reading, sensitive to temperature and humidity changes, limited accuracy
Thermoluminescent Dosimeter
The crystals are heated and the trapped energy is released in the form of a light photon, which is collected and analyzed
Advantage: reusable, more sensitive than film & more accurate
Disadvantage: no permanent record, expensive
(ring)
Pocket Dosimeters
Advantage: Immediate reading
Disadvantage: Subject to false readings, no permanent record, must be calibrated, mechanical trauma can change the reading
NRC
Reactor-produced materials
Co-60, brachytherapy, γ-knife radiosurgery, and therapeutic nuclear medicine
Must have a license to use these materials and meet administrative requirements.
Radiation Safety Program
Should have members form each department (Surgery, Radiation Oncology, Diagnostic Radiology, Nuclear Medicine)
Personal Monitoring Program
Mandatory for any individual working with radiation receives or is likely to receive 10% of the applicable radiation dose limit
Medical Events
Majority of events reported include wrong dose of radiation wrong patient wrong location wrong side wrong setup
Reporting Errors External Beam
Results in the total dose delivered differing from the prescribed dose by 20% or more
Results in any single delivered fraction of a fractionated treatment exceeding the prescribed dose by 50% or more; or
Involves the wrong patient, wrong treatment modality, or wrong treatment site
Reporting Errors Brachytherapy
Results in the total dose delivered differing from the prescribed dose by 20% or more
Results in any single delivered fraction of a fractionated treatment exceeding the prescribed dose by 50% or more; or
Involves the wrong patient, wrong source, wrong route of administration, leaking sealed source or wrong treatment site.
Dose to the skin or an organ other than the treatment site that exceeds by 0.5 Sv (50 from) or more of the dose expected from the administration.
Radiation Units
Roentgen (C/kg): measure exposure in air and used to measure the “intensity” of the radiation
Rads (Gy): measure absorbed energy and used to talk about “dose” from radiation
Rems (Sv): measure biological effects
Exposure in Air
Roentgen (R): Measure of the ionization (number of electrons knocked off atoms) in air produced by x-radiation and gamma
Coulombs per kilogram (C/kg): the charge (number of electrons) moving though a conductor by a steady current of 1 ampere in 1 second
Absorbed dose
Also referred as kerma: kinetic energy released in matter
Rad (radiation absorbed dose)
f-factor: ratio between the number roentgens (exposure) and the number of rads (energy transfer)
Used for any type of radiation
Dose Equivalent (occupational)
units of biological effect
Rems are determined by multiplying the absorbed dose (rad) times a quality factor (measure of the biological impact)
Most common use of rems is for personnel radiation monitoring
The quantity of radiation that has same biologic effectiveness of 1 rad of x-ray
Sievert= 100 rem
Radioactivity
Curie (Ci) and Becquerel (Bq): measure rate of nuclear disintegration (decay) of a material
Used for radioactive material
Coverting to SI unites
1R x2.58x10^-4=C/kg
1 rad x.01=Gy
1 rem x.01=Sv
1 Ci x3.7 x10^10=Bq
