Radiation Units

Slide 1 · Radiation Units

Activity, absorbed dose and biologic dose

An applied pathology and medical-imaging lecture

Activity

Ci / Bq

Absorbed dose

Gy / cGy

Biologic weighting

Sv / mSv

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · What the units must tell us

Identify whether a number describes radionuclide activity, deposited energy or radiation-weighted dose.

Convert correctly among Ci/Bq, Gy/cGy/rad and Sv/mSv/rem.

Explain why a reported absorbed dose cannot always be read as an effective dose.

Interpret common medical, environmental and radiation-protection examples.

Slide 3 · Radiation measurement answers three different questions

SOURCE

How frequently do nuclear transformations occur?

Curie (Ci), becquerel (Bq)

TARGET

How much energy is absorbed per unit mass?

Gray (Gy), centigray (cGy)

BIOLOGIC WEIGHTING

How is radiation type and tissue distribution accounted for?

Sievert (Sv), millisievert (mSv)

Slide 4 · A single word—“dose”—can conceal the quantity

Ambiguous

“The radiation dose was 5.”

No unit, target tissue, radiation type or measurement context.

Informative

“The organ absorbed dose was 5 mGy.”

Quantity and unit identify deposited energy per mass.

Slide 5 · Ionizing radiation removes bound electrons

Incident radiation

Energy transfer

Atom or molecule

Electron displaced

Ionization

Reactive chemical changes

Biologic consequences

Depend on tissue and exposure

Slide 6 · Not every type of radiation is ionizing

Nonionizing

Insufficient photon/particle energy to eject tightly bound electrons.

Examples: Infrared, microwave and ultraviolet radiation.

Ionizing

Sufficient energy to remove electrons and create ionization.

Examples: X-rays, gamma rays, alpha, beta and neutrons.

Slide 7 · Radiation may be photons or particles

Category

Examples

Relevant distinction

Electromagnetic

X-rays; gamma rays

Penetrating photons; radiation weighting factor 1

Charged particles

Alpha; beta

Different ionization densities and penetration

Uncharged particles

Neutrons

Radiation weighting depends on neutron energy

Slide 8 · The same source can generate different tissue doses

Radionuclide activity

Transformations each second

Emission and transport

Particle energy and geometry

Tissue energy deposition

Absorbed dose depends on mass

Biologic response

Radiation and tissue matter

Slide 9 · Curie measures radionuclide activity

1 Ci

3.7 × 10¹⁰ nuclear disintegrations per second

Activity is a property of the radioactive source—not a patient dose.

Slide 10 · Activity is a rate, not a stored energy

A radionuclide has a probability of nuclear transformation over time.

Activity counts transformations per second in the source.

Decay may release radiation that does not reach or deposit energy in tissue.

Activity alone does not specify radiation energy, geometry or exposure duration.

Slide 11 · Becquerel is the SI unit of activity

1 Bq = 1 nuclear transformation per second

1 Ci = 3.7 × 10¹⁰ Bq

The SI definition expresses rate directly in reciprocal seconds.

Slide 12 · Use prefixes to scale activity

Notation

Equivalent activity

Interpretation

1 kBq

10³ Bq

One thousand transformations per second

1 MBq

10⁶ Bq

One million transformations per second

1 GBq

10⁹ Bq

One billion transformations per second

Slide 13 · Activity conversion: 0.20 Ci

GIVEN / SETUP

0.20 Ci × 3.7 × 10¹⁰ Bq/Ci

RESULT

7.4 × 10⁹ Bq = 7.4 GBq

This is an activity conversion; It does not calculate absorbed dose.

Slide 14 · Disintegrations do not equal absorbed joules

What activity supplies

How many nuclear transformations occur per unit time.

Bq and Ci

What dose requires

Energy deposited in a specified mass of tissue.

Gy and cGy

Slide 15 · A dose calculation needs exposure context

Radionuclide / beam

Radiation type and emission energies

1

Transport

Distance, shielding and anatomy

2

Deposition

Energy reaching target tissue

3

Target mass

Energy divided by tissue mass

4

Slide 16 · Two patients beside the same source

The source activity is unchanged. One patient is closer and remains exposed longer.

May we infer identical absorbed doses?

No. Dose also depends on exposure geometry, time, attenuation and absorbed energy.

Slide 17 · Gray quantifies energy absorbed per unit mass

1 Gy

1 joule absorbed per kilogram of material

Gy is a physical absorbed-dose quantity; It does not itself encode radiation quality.

Slide 18 · Absorbed dose is energy divided by mass

D = ε / m

[J] / [kg] = Gy

The symbol ε denotes the energy imparted; D denotes mean absorbed dose.

Slide 19 · Why the denominator matters

1 joule in 1 kilogram

D = 1 J / 1 kg = 1 Gy

Reference case

1 joule in 0.1 kilogram

D = 1 J / 0.1 kg = 10 Gy

Same deposited energy; Tenfold smaller mass

Slide 20 · Robbins expresses gray using ergs

1 Gy = 10⁴ erg per gram

1 erg = 10⁻⁷ J

The energy unit changes; The underlying physical quantity does not.

Slide 21 · Centigray is one hundredth of a gray

1 cGy

0.01 Gy = 100 erg/g

The centi- prefix means 10⁻², as it does in other SI units.

Slide 22 · Practice: Centigray to gray

GIVEN / SETUP

250 cGy ÷ 100

RESULT

2.50 Gy

Convert units without changing the physical absorbed dose.

Slide 23 · The historical absorbed-dose unit is the rad

1 rad = 0.01 Gy

100 rad = 1 Gy = 100 cGy

The rad and gray quantify the same physical quantity with different unit scales.

Slide 24 · Important correction to the extracted textbook text

The supplied Robbins Markdown states “1 cGy ... Equivalent to 100 Rad.”

The correct identity is 1 cGy = 1 rad; 100 rad = 1 Gy.

This follows from the textbook’s own 1 Gy = 10⁴ erg/g and 1 cGy = 100 erg/g.

Slide 25 · Do not confuse rad with roentgen

Rad

Absorbed dose in matter.

1 rad = 0.01 Gy; Normally lower-case unit symbol rad.

R (roentgen)

Historical exposure in air.

Not the abbreviation for absorbed-dose rad.

Slide 26 · Legacy to SI unit map

Physical quantity

Legacy unit

SI unit

Unit relation

Source activity

curie (Ci)

becquerel (Bq)

1 Ci = 3.7 × 10¹⁰ Bq

Absorbed dose

rad

gray (Gy)

100 rad = 1 Gy

Equivalent dose

rem

sievert (Sv)

100 rem = 1 Sv

Exposure in air

roentgen (R)

C/kg

1 R = 2.58 × 10⁻⁴ C/kg

Slide 27 · Absorbed dose in clinical radiotherapy

Prescribed physical dose is ordinarily expressed in Gy or cGy.

Dose is assigned to a target volume and the relevant normal tissues.

Tissue volume and dose distribution matter, not simply the largest displayed number.

Do not treat a radiotherapy Gy value as a whole-body effective dose in Sv.

Slide 28 · A dose value must identify its target

PATIENT / TISSUE

Which anatomic volume absorbed the energy?

Organ, marrow, tumor or whole body

SPATIAL PATTERN

Uniform or highly localized deposition?

Organ average versus dose-volume distribution

DELIVERY

How rapidly and in how many exposures?

Dose rate and fractionation

Slide 29 · Gray does not alone predict tissue injury

Radiation quality modifies damage at an equivalent absorbed dose.

Cell proliferative activity alters susceptibility to DNA damage.

Field size, dose rate, oxygenation and repair influence outcome.

Gy is essential but insufficient for full biologic interpretation.

Slide 30 · Dose rate changes the opportunity for repair

Same total physical dose

Gy held constant

Different delivery rates

Exposure concentrated or spread

Intervening repair

Recovery differs

Different tissue effect

Dose history matters

Slide 31 · Fractionation is not merely arithmetic

Single exposure

All prescribed physical dose delivered without planned recovery interval.

Limited time for inter-fraction repair.

Fractionated exposure

Physical dose delivered in separated fractions.

Normal-cell recovery can reduce cumulative injury.

Slide 32 · Field size changes clinical consequences

Localized field

A relatively high local absorbed dose may be tolerated with shielding.

Limited volume exposed.

Large field / total body

Even a lower widespread dose can injure multiple vulnerable systems.

Widespread tissue exposure.

Slide 33 · Rapidly dividing tissues are more vulnerable

DNA injury

Ionization / free radicals

Replication

Damage tested during division

Cell loss

Marrow and mucosa especially vulnerable

Clinical effect

Depends on field and time

Slide 34 · Radiation can damage DNA indirectly

Ionizing radiation

Energy transfer

Water radiolysis

Free radical generation

Reactive oxygen species

Chemically attack DNA

Repair or injury

Outcome depends on response

Slide 35 · High LET tracks concentrate ionization

Photons

More sparsely ionizing interactions along their tracks.

Radiation weighting factor = 1.

Alpha particles

Dense local energy deposition along short tracks.

Radiation weighting factor = 20.

Slide 36 · Sievert is a radiation-protection dose unit

1 Sv

A radiation-weighted quantity for biologic detriment

The name Sv is used for equivalent dose and effective dose; The quantities differ.

Slide 37 · Equivalent dose weights radiation type

Organ absorbed dose

D in Gy

Radiation weighting

Wᵣ depends on radiation type

Organ equivalent dose

Hₜ in Sv

Slide 38 · Radiation weighting factors are dimensionless

Radiation type

Radiation weighting factor (wᵣ)

Scope

X-rays / gamma rays

1

Photon radiation

Electrons / beta particles

1

Standard external electron radiation

Alpha particles

20

High-LET particles

Neutrons

Energy dependent

No single universal neutron factor

Slide 39 · Equivalent dose: The formal definition

Hₜ = Σᵣ (wᵣ × Dₜ,ᵣ)

Absorbed dose (Gy) × radiation weighting = equivalent dose (Sv)

For mixed radiation fields, sum each radiation-type contribution.

Slide 40 · Worked example: X-ray dose to one organ

GIVEN / SETUP

2 mGy × wᵣ 1

RESULT

2 mSv equivalent dose to that organ

This does not establish a patient’s whole-body effective dose.

Slide 41 · Worked example: Alpha dose to one organ

GIVEN / SETUP

2 mGy × wᵣ 20

RESULT

40 mSv equivalent dose to that organ

The physical absorbed dose remains 2 mGy in both examples.

Slide 42 · RBE and protection weighting are not synonyms

Relative biologic effectiveness (RBE)

Experimental ratio tied to a specified biological endpoint.

Can depend on radiation, endpoint, dose and context.

Radiation weighting factor (wᵣ)

Standardized factor adopted for radiological protection.

Based on RBE evidence; Not an individual tissue response measurement.

Slide 43 · Use mSv and mGy with the correct qualifier

MGy

Physical absorbed dose in a specified tissue.

A measured or modeled energy-per-mass quantity.

MSv

Equivalent dose to an organ OR effective dose.

The surrounding words identify which quantity is reported.

Slide 44 · Rem is the historical equivalent-dose unit

1 rem = 0.01 Sv

1 Sv = 100 rem

Never convert rem to rad without specifying radiation weighting.

Slide 45 · Effective dose additionally weights tissues

Organ absorbed doses

Dₜ,ᵣ in Gy

Radiation factors

Wᵣ

Organ equivalent doses

Hₜ in Sv

Tissue factors

Wₜ → effective dose E in Sv

Slide 46 · A tissue factor represents detriment weighting

Different organs contribute differently to aggregate stochastic radiation detriment.

ICRP assigns tissue weighting factors (wₜ) in a reference-person framework.

Weighting is applied after radiation type has been accounted for.

These factors are not direct predictions for a particular patient.

Slide 47 · Effective dose is a weighted sum

E = Σₜ (wₜ × Hₜ)

Equivalent dose by organ × tissue factor → whole-body detriment-related metric

E is principally a radiological-protection quantity; It is not an observed lesion.

Slide 48 · ICRP tissue weighting is not uniform

Illustrative tissue

ICRP 103 wₜ

Interpretation

Red bone marrow

0.12

Larger contribution to reference stochastic detriment

Thyroid

0.04

Smaller contribution than marrow

Skin

0.01

Smaller contribution than marrow

Slide 49 · Example: Localized thyroid photon exposure

GIVEN / SETUP

10 mGy thyroid × wᵣ 1 × wₜ 0.04

RESULT

0.4 mSv contribution to effective dose

Hypothetical simplified case: All other organ contributions set to zero.

Slide 50 · Equivalent and effective dose are distinct

Equivalent dose, Hₜ

Refers to a specified tissue or organ.

Weights absorbed dose by radiation type.

Effective dose, E

Reference-person aggregate across tissues.

Also weights tissue contributions to stochastic detriment.

Slide 51 · Medical-imaging reports often quote effective dose

Diagnostic radiography and CT summaries commonly use mSv for effective dose.

This is not interchangeable with absorbed dose to skin, marrow or an imaged organ.

Imaging geometry and organ exposure distribution affect effective-dose estimates.

Effective dose is useful for broad procedure comparisons, not individual cancer-risk prediction.

Slide 52 · The “1 mSv = 1 mGy” shortcut has a limit

For photons, wᵣ = 1: 1 mGy organ absorbed dose corresponds numerically to 1 mSv organ equivalent dose.

This does NOT imply 1 mGy to one organ equals 1 mSv whole-body effective dose.

The mSv unit alone does not tell us whether Hₜ or E is intended.

Slide 53 · Local dose and population metric are not substitutes

Local absorbed dose (Gy)

Useful for assessing energy deposited in tumor and normal tissue.

Depends on spatial distribution and tissue reactions.

Effective dose (Sv)

Useful for broad radiation-protection comparisons.

Reference model of stochastic detriment; Not a treatment-planning prescription.

Slide 54 · The measurement chain has distinct transitions

Activity (Bq)

Nuclear transformations

Absorbed dose (Gy)

Energy per tissue mass

Equivalent dose (Sv)

Radiation-type weighting

Effective dose (Sv)

Tissue weighting

Slide 55 · Choose a unit by asking what is quantified

Source

Transformations per second → Bq / Ci

Tissue energy

Deposited joules per kilogram → Gy / cGy

Radiation quality

Organ weighted dose → Sv / mSv

Overall detriment

Tissue-weighted effective dose → Sv / mSv

Slide 56 · Consolidated radiation-unit comparison

Quantity

SI unit

What is being quantified

Activity

Bq

Radionuclide transformations per second

Absorbed dose

Gy

Deposited energy per kilogram

Equivalent dose

Sv

Absorbed dose weighted by radiation type

Effective dose

Sv

Equivalent doses weighted across tissues

Slide 57 · Case: A radioisotope is labeled “5 MBq”

The isotope vial bears an activity label of 5 MBq.

Can that number tell you the organ dose in mGy?

No. It specifies transformations per second; Patient dosimetry also needs the nuclide, uptake, time and energy deposition.

Slide 58 · Case: A treatment field records 250 cGy

The treatment sheet gives a local absorbed dose of 250 cGy.

Express this dose in Gy and rad.

250 cGy = 2.5 Gy = 250 rad; All express the same physical dose.

Slide 59 · Case: A legacy chart records 175 rad

An older radiotherapy record specifies 175 rad.

What absorbed dose is represented in SI units?

175 rad × 0.01 Gy/rad = 1.75 Gy = 175 cGy.

Slide 60 · Case: A photon organ dose is 2 mGy

Photons deposit 2 mGy in a defined organ, with radiation factor wᵣ = 1.

State the equivalent dose and whether this equals the effective dose.

Organ Hₜ = 2 mSv. E cannot be concluded without organ distribution and tissue weighting.

Slide 61 · Case: Alpha and photon doses in one organ

GIVEN / SETUP

(1 mGy alpha × 20) + (2 mGy photons × 1)

RESULT

22 mSv organ equivalent dose

The absorbed-dose total is 3 mGy, which is not 22 mGy.

Slide 62 · Case: Same effective dose, different organ doses

Two different imaging examinations are summarized by the same effective dose in mSv.

Do they necessarily give identical absorbed dose to each organ?

No. A common weighted aggregate can arise from different organ-dose distributions.

Slide 63 · Dose magnitude and time cannot be conflated

Dose

An accumulated physical quantity.

Examples: 2 Gy, 5 mSv effective dose.

Dose rate

How quickly dose accumulates.

Examples: Gy/h, mSv/h; The rate needs time integration.

Slide 64 · Five recurrent errors in radiation-dose reading

Calling activity (Ci or Bq) the patient absorbed dose.

Confusing rad with roentgen (R).

Reading Gy as though it included radiation or tissue weighting.

Interchanging organ equivalent dose and effective dose because both use Sv.

Applying the photon mGy↔mSv numerical shortcut to all imaging situations.

Slide 65 · Unit literacy supports defensible clinical interpretation

START WITH THE QUANTITY

Activity, absorbed, equivalent or effective?

Never infer meaning from a number alone.

PRESERVE THE TARGET

Source, organ, field or reference person?

Specify anatomy and exposure geometry.

STATE THE LIMIT

What the metric does not establish

A single unit never describes the full biologic response.

Slide 66 · Selected sources for further reading

*Robbins, Cotran & Kumar: Pathologic Basis of Disease*, 11th ed., Chapter 9, “Radiation Units” and adjacent ionizing-radiation sections.

International Commission on Radiological Protection. ICRP Publication 103: The 2007 Recommendations.

ICRP Publication 92: RBE, Quality Factor, and Radiation Weighting Factor; ICRP Publication 147: Use of Dose Quantities.

International Atomic Energy Agency: radiation-dose and unit conversion guidance (including Gy, rad, Sv, rem, Ci, Bq).