Dosimetry

Question 1

Dosimetry

Answer 1

• The science of measuring/calculating dose levels

Question 2

Radiation

Answer 2

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

Question 3

Dose

Answer 3

• The amount or quantity of radiation absorbed by matter

Question 4

Radiation Quantity : Air Dose (exposure) (X)

Answer 4

• 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

Question 5

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

Answer 5

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

Question 6

Radiation Quantity : Absorbed Dose (D)

Answer 6

• 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

Question 7

Radiation Quantity : Biological Dose (D)

Answer 7

• Currently two terms are used to define Biological dose:
  
  • Equivalent dose (H_T) - 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 H_T 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

Question 8

Relative Biological Effectiveness (RBE)

Answer 8

• 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)
  
  • D_ref / D _test
  • D_ref = 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

Question 9

Timeline of Radiation

Answer 9

• 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

Question 10

Bragg & Gray Experiments

Answer 10

• 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”

Question 11

There are three definitions for radiation does Quanities:

Answer 11

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 (H_T)
   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

Question 12

Linear Energy Transfer (LET)

Answer 12

• 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

Question 13

High LET

Answer 13

• 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

Question 14

Low LET

Answer 14

• 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

Question 15

Weighting Factors (W)

Answer 15

• Two types of Weighting factors:
  
  • Radiation Weighting Factor (W_R)
  • Tissue Weighting Factor (W_T)
• 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 W_R?
• 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

Question 16

Practice question for Weighting Factors

Answer 16

• There is a Nuclear explosion and different types of radiation are released. These include gamma, neutrons and protons. Let’s see how H_T (Equivalent dose) would be calculated:
• Step 1 - need to know the W_R 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
• H_T = Σ D x W_R or…
• H_T = (2x2) protons + (2x1) gamma + (2x10) neutrons
• H_T = 4 + 2 +20
• H_T = 26 Rems

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

Question 17

Equivalent Dose (H_T)

Answer 17

• Definition: The amount of radiation absorbed by a specific tissue or organ
• Formula:
  
  • H_T = ΣD_T x W_R
  • H_T - equivalent dose
  • Σ - sum of
  • D - Rad (or Gy) value
  • W_R - radiation weighting factor

Question 18

Effective Dose (E)

Answer 18

• 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
    
    • H_T and E
• Equation:
• E = Σ H_T x W_T
• H_T - equivalent dose
• W_T - 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.

Question 19

Collective Effective Dose (ColEfD)

Answer 19

• 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.

Question 20

Practice question for Effective Dose

Answer 20

• E = ΣH_T x W_T
• H_T = 100 mRems Lung, W_T = .12
• H_T = 50 mRems thyroid, W_T = .05
• H_T = 100 mRems bone mar, W_T = .12

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

E = 12 + 2.5 + 12

E = 25.5 mRems (very low dose)

Question 21

Organ tissue weighting factors (W_T)

Answer 21

• 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

Question 22

Somatic damage

Answer 22

• “body” damage

Question 23

Short term somatic effects

Answer 23

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

Question 24

Long term somatic effects

Answer 24

• cancer
• birth defects
• formation of cataracts