Radiation Therapy Physics
Chapter 17
Exposure:
- Oldest unit to describe the interactions of radiation
- Only applicable to photons (energies below 3 MeV)
- Defined as the amount of ionization produced in photons in air per unit mass of air
- 1 Roentgen = 2.48 x 10^-4 C/kg
Absorbed dose:
- Amount absorbed per mass of any material while radiation interacts in the material
- Measured in gray (Gy)
- 1 rad = 1 cGy
- 1 Gy = 1 J/kg
Equivalent dose:
- An attempt to account for biologic effects of different types of radiation as they interact
in tissue
- Radiations such as alpha particles and neutrons deposit more energy per unit path length
(LET) than photos
- Absorbed dose is multiplied by a radiation weighting (Wr) factor
- 1 Sv = 100 rem
Activity:
- Number of radioactive disintegrations per unit time
- Originally based on disintegrations measured from 1g radium in an equilibrium
- Now defined by milligrams of radium
- 1 Bq = 1 d/s
- 1 Ci = 3.7 x 10^10 Bq
The largest contributor to internal exposure is from potassium 40 (40K) which amount to 0.2mSv
per year
Radon = 2mSv per year
Natural background radiation per year in the U.S = 3mSv
“Time, distance, and shielding” are the cornerstones of radiation protection
The choice of shielding material depends on the energy of radiation
Photoelectric = kV = lead shielding
Compton = MV = concrete and steel (space and cost)
Radiation weighting factors:
Photons = 1
Chapter 17
Exposure:
- Oldest unit to describe the interactions of radiation
- Only applicable to photons (energies below 3 MeV)
- Defined as the amount of ionization produced in photons in air per unit mass of air
- 1 Roentgen = 2.48 x 10^-4 C/kg
Absorbed dose:
- Amount absorbed per mass of any material while radiation interacts in the material
- Measured in gray (Gy)
- 1 rad = 1 cGy
- 1 Gy = 1 J/kg
Equivalent dose:
- An attempt to account for biologic effects of different types of radiation as they interact
in tissue
- Radiations such as alpha particles and neutrons deposit more energy per unit path length
(LET) than photos
- Absorbed dose is multiplied by a radiation weighting (Wr) factor
- 1 Sv = 100 rem
Activity:
- Number of radioactive disintegrations per unit time
- Originally based on disintegrations measured from 1g radium in an equilibrium
- Now defined by milligrams of radium
- 1 Bq = 1 d/s
- 1 Ci = 3.7 x 10^10 Bq
The largest contributor to internal exposure is from potassium 40 (40K) which amount to 0.2mSv
per year
Radon = 2mSv per year
Natural background radiation per year in the U.S = 3mSv
“Time, distance, and shielding” are the cornerstones of radiation protection
The choice of shielding material depends on the energy of radiation
Photoelectric = kV = lead shielding
Compton = MV = concrete and steel (space and cost)
Radiation weighting factors:
Photons = 1
