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Radiation dose units and ALARA

Absorbed dose is measured in Gy, equivalent dose adds the radiation type, and effective dose in Sv adds organ sensitivity — and every examination needs justification, optimisation and the lowest dose that still answers the question (ALARA).

לקריאה בעברית

Why it matters in the exam

Radiology physics questions repeat the same chain in almost every sitting: exposure is the ionisation produced in air and is measured in C/kg; absorbed dose is energy per kilogram of tissue, measured in Gy; equivalent dose is absorbed dose multiplied by the radiation weighting factor (wR), measured in Sv; and effective dose sums the organ equivalent doses multiplied by tissue weighting factors (wT), also in Sv. The unit that expresses biological harm is the Sv, not the Gy.

A typical question lists several people exposed to different radiation types and asks which harm is greatest. For X-rays, gamma rays and electrons wR is 1, for protons 2 and for alpha particles 20; so 40 mGy of alpha radiation equals 800 mSv and is more harmful than 200 mGy of X-rays, which equals 200 mSv. Effective dose is used to compare examinations and populations; it is not an accurate measure of an individual patient's risk.

The second theme is the type of harm. Deterministic effects (tissue reactions) occur only above a threshold and their severity rises with dose — skin erythema above about 2 Gy and cataract above about 0.5 Gy to the lens. Stochastic effects — cancer and heritable effects — follow the linear no-threshold (LNT) model: probability rises with dose while severity does not depend on it. The principles of protection are justification, optimisation, and dose limits for workers and the public only, never for patients. For radiation workers the limit is 20 mSv a year, averaged across 5 years, with no more than 50 mSv in a single year; for the public it is 1 mSv per year.

Key points

Exposure C/kg, absorbed dose Gy, equivalent and effective dose Sv.
wR: X-rays and γ 1, protons 2, alpha 20.
Deterministic — threshold; stochastic — linear, no threshold.
Justification, optimisation, and dose limits for workers and the public.
Worker 20 mSv per year (5-year average), public 1 mSv per year.

Summary table

Quantity	What it measures	Unit	Exam note
Exposure	Ionisation in air	C/kg	Does not express biological harm
Absorbed dose	Energy per kg of tissue	Gy	Basis of all calculations
Equivalent dose	Absorbed dose times wR	Sv	Alpha 20, protons 2, X-rays and γ 1
Effective dose	Sum of equivalent doses times wT	Sv	For comparing examinations, not personal risk
CTDIvol and DLP	CT output per slice and over the scan length	mGy and mGy·cm	Basis of diagnostic reference levels (DRL)

A free exam question

נבדק א' נחשף ל 200mGy קרינת X . נבדק ב'נחשף ל mGy 40 קרינת אלפא . נבדק ג' נחשף ל mGy 100 קרינת גאמא . ערך ה Sv - שיתקבל יהיה: (the question as examined, in Hebrew)

א. הגבוה יותר בנבדק א'
ב. הגבוה יותר בנבדק ב'
ג. הגבוה יותר בנבדק ג'
ד. זהה בנבדקים א 'ו-ג'

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Sources

ICRP Publication 103: The 2007 Recommendations of the International Commission on Radiological Protection, 2007 — Quantities used in radiological protection
ICRP Publication 118: Statement on tissue reactions, 2012 — Threshold doses for tissue reactions
ACR–SPR Practice Parameter for Diagnostic Reference Levels and Achievable Doses in Medical X-Ray Imaging, 2023 — Dose quantities and reference levels

Sources are given for further reading; the text is written afresh and not copied from them.

Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.