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Dosimetry > Dosimetry > Flashcards

Dosimetry Flashcards

1
Q

Dosimetry

A
  • The science of measuring/calculating dose levels
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2
Q

Radiation

A
  • the movement and transfer of energy through space/time/matter
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3
Q

Dose

A
  • The amount or quantity of radiation absorbed by matter
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4
Q

Radiation Quantity : Air Dose (exposure) (X)

A
  • Definition: Ionizing radiation transfers its energy to air
  • How is it measured?
    • Victoreen ‘R’ meters (ionization chambers)
  • Current uses:
    • used in the measurement of skin dose (ESD)
    • check calibration of xray equipment
  • Exposure facts:
    • applies to x-ray/gamma radiation only, no particulates of energies up to 3 MeV
  • Units of Measurement:
    • Traditional - (R) Roentgen
    • S.I. - c/Kg
    • 1 R = 2.58 x 10^-4 c/Kg
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5
Q

What parameters influence the “X” value amount in Air Dose (exposure)?

A
  • ↑ kVp = ↑ R value
  • ↑ mAs = ↑ R value
  • ↑ SID = ↓ R value
  • ↑ collimation = ↓ R value
  • ↑ filtration = ↓ R value.
  • intensity = quality
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6
Q

Radiation Quantity : Absorbed Dose (D)

A
  • Definition: The quantity of radiation deposited (transfered to) in an object (liquid or solid) per unit of mass
  • Unit of measurement:
    • Traditional - Rad
    • S.I. - Gray
    • 1 Gray = 100 Rads
    • 1 Rad = 100 ergs/g
    • 1 Rad = 1 j/Kg
  • Facts:
    • energy is transfered to the molecules of the absorbing material
    • applies to all types of ionizing radiation at all enery levels
  • What influences absorbed does amount?
    • Techincal factors:
    • kVp
    • mAs
    • collimation
    • filtration
    • Material factors:
    • Z#
    • mass density
    • thickness
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7
Q

Radiation Quantity : Biological Dose (D)

A
  • Currently two terms are used to define Biological dose:
    • Equivalent dose (HT) - dose specific to organ
    • Effective dose (E) - whole body dose
  • Unit of measurement:
    • Rem or Sievert (Sv)
    • 1 Sv = 100 Rem
    • 1 Rem = 10mSv
    • 1 mSv = 100 mRem
    • 1 Gy = 100 Rad
    • 1 Rad = 10 mGy
    • 1 mGy = 100 mRad
  • How are HT and E the same?:
  • Both measurements of biological damage take into account the following:
    • (D) = actual amount of energy absorbed by the tissue(s)
    • the type of radiation the person was exposed to
    • the actual tissues that were exposed to (D)
    • both are ‘bio doses’ measured in rem/Sv
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8
Q

Relative Biological Effectiveness (RBE)

A
  • Definition: It is the measure of the ability of a specific type of radiation to cause a biological effect to a living tissue or cells.
  • Facts:
    • Laboratory experiments typically comparing 2 types of radiation and their ability to cause cellular damage (typically death)
    • For given dose, some radiations do more damage than others
  • Equation: D= dose in Rads (Gy)
    • Dref / D test
    • Dref = 250 kVp of x rays
    • D test = dose of ionizing radition being “compared” with the xray dose in terms of how many cells were killed
  • Summary :
    • Not all radiations are created equal in the amount of cellular death they create, even if given in equal doses
    • From RBE studies, a concept called Linear Energy Transfer was formulized
  • Unit of measurement:
    • Traditional - Rem
    • S.I. - Sievent
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9
Q

Timeline of Radiation

A
  • 1895 - 1930 - dosimetry was essentially non existent. No such thing as J.O.D.
  • up until the early 1930’s, an overdose of radiation was “measured” when your skin turned red.
    • This was called skin erythema dose.
    • This was considered the first dose limit
  • 1934 Tolerance dose introduced
    • 0.2 R/day
    • 1.4 R/week
  • 1936 (redefined Tolerance dose)
    • 0.1 R/day
    • 0.7 R/week
  • 1937 - Roentgen (R) was defined and used as the unit of measurement for the Tolerance Dose.

Radiation quantities were not discovered at the same time, here is the order:

  1. Air dose (exposure)
  2. Absorbed dose
  3. Biological dose
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10
Q

Bragg & Gray Experiments

A
  • Demonstrated that air exposed to x rays, ionizes
  • The more x rays, the more ionizations
  • Based on their experiments, the Roentgen became the unit for “air exposure”
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11
Q

There are three definitions for radiation does Quanities:

A
  1. Air Dose(Exposure) (X) - measuring number of air molecules from ionization
  2. Absorbed Dose (D) - number of molecules going into a mass
  3. Biological Dose (H) -
    1. Equivalent dose (HT)
    2. Effective dose (E)
  • Units of measurement for each:
  1. Air Exposure (X):
    1. Traditional - R
    2. S.I. - c/Kg
  2. Absorbed Dose (D):
    1. Traditional - Rad
    2. S.I. - Gray
  3. Biological dose (H):
    1. Traditional - Rem
    2. S.I. Sievent
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12
Q

Linear Energy Transfer (LET)

A
  • Definition : The rate at which a specific type of ionizing radiation loses its energy (transfers its energy) as it traverses through a substance in a linear path
  • How does radiation lose it’s energy?
    • removing electrons from its orbit
  • Unit of measurement:
    • KeV/µm of absorbing material
  • Facts:
    • Not all ionizing radiation loses their energy at the same rate
    • High LET - loses it quickly
    • Low LET - loses it slowly
  • LET value is related to RBE value of a specific type of radiation
    • Alpha = ↑ RBE & ↑ LET
    • x rays = ↓ RBE & ↓ LET
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13
Q

High LET

A
  • creates a lot of ionizations (damage) in a short distance of travel
  • energy is expended quickly
  • not much penetration but high amounts of damage (bio effects) in a short distance of travel
  • Example:
    • Alpha particles
    • Neutrons
    • Protons
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14
Q

Low LET

A
  • create much less ionizations (damage) over a given distance of travel.
  • They are not absorbed/scattered at a high rate, rather a low rate of ionizations
    • less interactions
    • more penetrating
  • Examples:
    • electromagnetic ionizing radiations
    • beta particles
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15
Q

Weighting Factors (W)

A
  • Two types of Weighting factors:
    • Radiation Weighting Factor (WR)
    • Tissue Weighting Factor (WT)
  • Why do we even need them?
    1. Depending on their LET value, some radiations produce more bio damage than others.
    • 1 Rad of Alpha particles will produce a lot more bio damage than 1 Rad of x rays would
    1. Different types of tissues are more easily damaged (radiosensitive) than others.
  • Three factors needed to do calculation:
  1. Absorbed Dose (D) - Rads/Gy
  2. Type(s) of radiation exposed to LET
  3. What tissue(s) were exposed on the person
  • What is WR?
  • Radiation weighting factor is a numerical value (1-20) given to each type of known radiation based on LET value
    • Alpha - 20
    • x ray - 1
    • gamma - 1
  • What is WT?
  • Tissue weighting factor. Numerical values based on radiosensitivity of specific organ
  • WT values are based on an arbitrary number system. The higher the number, the greater the sensitivity
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16
Q

Practice question for Weighting Factors

A
  • There is a Nuclear explosion and different types of radiation are released. These include gamma, neutrons and protons. Let’s see how HT (Equivalent dose) would be calculated:
  • Step 1 - need to know the WR for each type of radiation:
    • Protons = 2
    • Gamma = 1
    • Neutrons = 10
  • Step 2 - the values of D (absorbed dose) would have to be known for each type of radiation… let’s say 2 Rads
  • Step 3 - use the equation
  • HT = Σ D x WR or…
  • HT = (2x2) protons + (2x1) gamma + (2x10) neutrons
  • HT = 4 + 2 +20
  • HT = 26 Rems

This is the biological dose, or risk of potential biological damage to an organ

17
Q

Equivalent Dose (HT)

A
  • Definition: The amount of radiation absorbed by a specific tissue or organ
  • Formula:
    • HT = ΣDT x WR
    • HT - equivalent dose
    • Σ - sum of
    • D - Rad (or Gy) value
    • WR - radiation weighting factor
18
Q

Effective Dose (E)

A
  • Effective dose (E) - whole body dose
  • Defined by ICRP in 1991
  • Represents the end of the time-line for radiation quantities.
    • 1930’s - Exposure (X)
    • 1950’s - Absorbed dose (D)
    • 1960’s/present - Biological dose
      • HT and E
  • Equation:
  • E = Σ HT x WT
  • HT - equivalent dose
  • WT - Tissue weighting factor
  • E - Effective dose
  • Units = Rems

Effective Dose is technically a whole body dose, even if the whole body was not irradiated. Whether one organ is exposed or multiple organs they each have an effect on the body as a whole.

19
Q

Collective Effective Dose (ColEfD)

A
  • ColEfD has been designated for use in the description of population or group exposure from low doses of different sources of ionizing radiation.
  • it is determined as the product of the average EfD for an individual belonging to the exposed population or group and the number of persons exposed.
  • Unit of measurement:
    • the person-sievert is the unit of choice
  • Example:
  • If 1000 people are exposed to low doses of different sources of ionizing radiation and recieve an average EfD of 0.5 mSv, the ColEfD is 500 person-mSv. Which equals 0.5 person-Sv.
20
Q

Practice question for Effective Dose

A
  • E = ΣHT x WT
  • HT = 100 mRems Lung, WT = .12
  • HT = 50 mRems thyroid, WT = .05
  • HT = 100 mRems bone mar, WT = .12

So, E = (100)(.12) + (50)(.05) + (100)(.12)

E = 12 + 2.5 + 12

E = 25.5 mRems (very low dose)

21
Q

Organ tissue weighting factors (WT)

A
  • Gonads - 0.20
  • Red Bone Marrow - 0.12
  • Colon - 0.12
  • Lung - 0.12
  • Stomach - 0.12
  • Bladder - 0.05
  • Breast - 0.05
  • Esophagus - 0.05
  • Thyroid - 0.5
  • Skin - 0.01
  • Bone surface - 0.01
  • Other - 0.05
22
Q

Somatic damage

A
  • “body” damage
23
Q

Short term somatic effects

A
  • Nausea
  • fatigue
  • redness of the skin
  • loss of hair
  • intestinal disorders
  • fever
  • blood disorders
  • shedding of the outer layer of skin
24
Q

Long term somatic effects

A
  • cancer
  • birth defects
  • formation of cataracts
25
Q
A

Dosimetry (1 decks)

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