Radiation Exposure, Dose, and Units: Medical Physics Fundamentals

Exposure (X) — what does it measure?

Ionization (electrical charge) produced per unit mass of dry air by x-rays or gamma rays.

Exposure (X) — SI unit

Coulomb per kilogram (C/kg).

Exposure (X) — traditional unit

Roentgen (R).

1 roentgen (R) equals how many C/kg?

2.58 × 10^-4 C/kg.

Exposure quantity applies to what radiation/material?

X-rays or gamma rays in air only; it does not directly measure absorbed energy in tissue.

Air kerma — meaning

Kinetic energy released per unit mass; describes energy transferred from photons to charged particles in air.

Air kerma — unit

Gy_a (gray in air); 1 Gy = 1 J/kg.

Gy_a vs Gy_t

Gy_a = gray in air; Gy_t = gray in tissue.

DAP formula

DAP = air kerma × irradiated area.

DAP unit

mGy·cm².

What increases DAP?

Higher air kerma, larger field size, or both.

Effect of tighter collimation on DAP

Decreases DAP because the irradiated area is smaller.

DAP example: 20 mGy over 100 cm²

20 × 100 = 2,000 mGy·cm².

Absorbed dose (D) — definition

Energy absorbed per unit mass of irradiated material.

Absorbed dose formula

D = energy absorbed ÷ mass.

Absorbed dose — SI unit

Gray (Gy).

1 gray (Gy) equals what in J/kg?

1 Gy = 1 J/kg.

What affects absorbed dose?

Tissue atomic number, tissue mass density, incident photon energy, and radiation interaction/type.

1 Gy = ? cGy

100 cGy.

1 Gy = ? mGy

1,000 mGy.

1 Gy = ? microGy

1,000,000 microGy.

Gy to mGy conversion

Multiply Gy by 1,000.

mGy to Gy conversion

Divide mGy by 1,000.

Gy to cGy conversion

Multiply Gy by 100.

cGy to Gy conversion

Divide cGy by 100.

1 Gy = ? rad

100 rad.

LET — definition

Linear energy transfer: average energy transferred by radiation per unit length of its track through matter.

LET — unit

keV/micrometer (keV/µm).

Low-LET radiation examples

X-rays, gamma rays, and beta particles/electrons.

High-LET radiation example

Alpha particles.

High LET vs low LET biologic damage

At the same absorbed dose, high-LET radiation generally produces more biologic damage because energy is deposited more densely.

Radiation weighting factor (W_R) accounts for what?

Radiation type and energy, not the organ exposed.

W_R for x-rays

1.

W_R for gamma rays

1.

W_R for electrons

1.

W_R for alpha particles

20.

W_R for protons

2.

W_R for neutrons

5.

W_R for neutrons 10-100 keV

10.

W_R for neutrons >100 keV-2 MeV

20.

W_R for neutrons >2-20 MeV

10.

W_R for neutrons >20 MeV

5.

Equivalent dose (EqD) formula

EqD = D × W_R.

Equivalent dose (EqD) — unit

Sievert (Sv).

Equivalent dose accounts for what?

Absorbed dose adjusted for the biologic effectiveness of the radiation type/energy.

EqD example: 0.010 Gy of x-rays

0.010 Gy × 1 = 0.010 Sv = 10 mSv.

EqD example: 0.010 Gy of alpha radiation

0.010 Gy × 20 = 0.20 Sv = 200 mSv.

Effective dose (EfD) formula

EfD = D × W_R × W_T.

Effective dose alternate formula

EfD = EqD × W_T.

Effective dose (EfD) — unit

Sievert (Sv).

Effective dose accounts for what?

Absorbed dose + radiation type/energy + radiosensitivity of the irradiated tissue.

W_R mnemonic

R = Radiation: W_R tells you what radiation type/energy.

W_T mnemonic

T = Tissue: W_T tells you the relative radiosensitivity/detriment of the tissue.

Course Table 4.3 — W_T for gonads

0.08.

Course Table 4.3 — W_T for active marrow

0.12.

Course Table 4.3 — W_T for colon

0.12.

Course Table 4.3 — W_T for lungs

0.12.

Course Table 4.3 — W_T for stomach

0.12.

Course Table 4.3 — W_T for bladder

0.05.

Course Table 4.3 — W_T for breast

0.05.

Course Table 4.3 — W_T for liver

0.04.

Course Table 4.3 — W_T for esophageal wall

0.04.

Course Table 4.3 — W_T for thyroid

0.04.

Course Table 4.3 — W_T for skin

0.01.

Course Table 4.3 — W_T for bone surface

0.01.

Course Table 4.3 — W_T for urinary bladder wall

0.04.

Course Table 4.3 — W_T for brain

0.01.

Course Table 4.3 — W_T for remainder tissues

0.12.

Highest tissue weighting factor in Course Table 4.3

0.12.

Tissues at W_T = 0.12 in Course Table 4.3

Active marrow, colon, lungs, stomach, and remainder tissues.

Lowest tissue weighting factor in Course Table 4.3

0.01.

Tissues at W_T = 0.01 in Course Table 4.3

Skin, bone surface, and brain.

EfD example: D = 0.5 Gy, W_R = 20, W_T = 0.12

EfD = 0.5 × 20 × 0.12 = 1.2 Sv.

1 Sv = ? mSv

1,000 mSv.

1 Sv = ? microsievert

1,000,000 microsievert (µSv).

Sv to mSv conversion

Multiply Sv by 1,000.

mSv to Sv conversion

Divide mSv by 1,000.

1 Sv = ? rem

100 rem.

Collective effective dose formula

Collective effective dose = average individual EfD × number of people exposed.

Collective effective dose — unit

Person-sievert (person-Sv).

Collective effective dose applies to whom?

A population or group, not one individual.

Collective dose example: 200 people × 0.25 Sv

50 person-Sv.

CEDE stands for

Committed Effective Dose Equivalent.

CEDE — what does it represent?

Internal radioactive material that continues irradiating tissue until it is eliminated or physically decays.

TEDE stands for

Total Effective Dose Equivalent.

TEDE concept/formula

TEDE = external occupational dose component + CEDE from internal exposure.

Traditional annual TEDE limit — occupational worker

0.05 Sv/year = 50 mSv/year.

Traditional annual TEDE limit — general public

0.001 Sv/year = 1 mSv/year.

NCRP Report No. 116 — Chapter 4 association

Radiation protection; the course emphasizes its definition of effective dose as the sum of weighted equivalent doses for irradiated tissues/organs.

1895 — key Chapter 4 event

Wilhelm Conrad Roentgen discovered x-rays.

1925 — key Chapter 4 event

ICRU (International Commission on Radiation Units and Measurements) was formed.

1928 — key Chapter 4 event

The roentgen (R) was accepted as a unit of exposure.

1980 — key Chapter 4 event

ICRU adopted SI units for use with ionizing radiation.

Quick unit match: Exposure

C/kg.

Quick unit match: Air kerma

Gy_a.

Quick unit match: Absorbed dose

Gy.

Quick unit match: Equivalent dose

Sv.

Quick unit match: Effective dose

Sv.

Quick unit match: DAP

mGy·cm².

Quick unit match: LET

keV/µm.