Radiation Biology, Dose Units & Protection

Dose quantities

Quantity	SI unit (traditional)	What it describes
Exposure (air kerma)	C/kg or Gy in air (roentgen)	Ionization the beam produces in air
Absorbed dose	gray, Gy = 1 J/kg (1 Gy = 100 rad)	Energy absorbed per mass of tissue
Equivalent dose	sievert, Sv (1 Sv = 100 rem)	Absorbed dose × radiation weighting factor (1 for x-rays)
Effective dose	sievert, Sv	Sum of equivalent doses × tissue weighting factors; compares risk between exams

Exam	Approximate effective dose	Days of natural background (at about 3 mSv/yr)
4 bitewings, rectangular collimation, digital or F-speed	About 5 µSv	Under 1 day
Full-mouth series, rectangular collimation	About 35 µSv	About 4 days
Panoramic	About 10–25 µSv	About 1–3 days
CBCT, small field of view	Often tens of µSv; large fields can reach hundreds	Days to weeks

Textbooks commonly cite natural background as about 3 mSv per year (NCRP Report 160: about 3.1 mSv); NCRP Report 184 gives a somewhat lower estimate.

Most damage is indirect: x-rays ionize water and the resulting free radicals damage DNA. Direct hits on DNA are a smaller share.
Radiosensitive cells are rapidly dividing and undifferentiated (lymphocytes, bone marrow, intestinal crypts, germ cells); mature nerve and muscle are resistant (law of Bergonié and Tribondeau).
Stochastic effects (cancer, heritable effects): no threshold assumed; probability rises with dose, severity does not. This is the concern in diagnostic imaging.
Deterministic (tissue) effects: threshold; severity rises with dose (mucositis, xerostomia, osteoradionecrosis, cataract), seen after radiotherapy, not dental imaging.

Principle: ALARA, now often stated as ALADA or ALADAIP (as low as diagnostically acceptable, indication-oriented and patient-specific).
Biggest dose reducers: selection criteria (no routine radiographs), fastest receptor (digital or F-speed film), rectangular collimation, correct technique to avoid retakes.
ADA 2024 radiation-safety recommendations: lead aprons and thyroid collars are no longer recommended for dental imaging (collars can block the beam and cause retakes); state rules may still require them, so follow local law.
Operator: never hold the receptor or tube head; stand behind a barrier or at least 6 ft away at 90–135° to the beam; wear a dosimeter if required.
Dose limits (NCRP): occupational 50 mSv/year; public 1 mSv/year; fetus of a declared pregnant worker 0.5 mSv/month.
Pregnancy: radiographs indicated for diagnosis are taken; the selection criteria do not change.

Mixing gray (absorbed dose) and sievert (effective dose).
Telling patients a thyroid collar is always required: the ADA (2024) no longer recommends aprons or collars, though some state rules still do.
Calling cancer risk from imaging deterministic: it is stochastic.
Delaying needed radiographs solely because the patient is pregnant.

Pick an answer to see right away whether it is correct and why.

Which unit expresses effective dose?
1 Gy equals:
Most biologic damage from diagnostic x-rays comes from:
Which statement about stochastic effects is correct?
Under the ADA's 2024 recommendations, a thyroid collar for an intraoral exam is:

0 of 5 answered

Answer key: 1 D · 2 A · 3 B · 4 B · 5 C