5.2 Radiation Effects, Risks & Dose Quantities

5.2 Radiation Effects, Risks & Dose Quantities

Quick Answer: Deterministic effects appear above practical thresholds (skin burns, cataracts at high doses); stochastic effects (cancer risk) are probability-based and managed with ALARA under linear no-threshold thinking. Report absorbed dose in Gy and risk-weighted doses in Sv (usually mGy/mSv in diagnostic work). Know relative dose ranks and pregnancy radiosensitivity—not invented exact CAMRT dose charts.

RTR.1.6 expects technologists to explain radiation effects and risks at a professional level: what types of harm exist, which quantities describe dose, how sensitive tissues and embryos are, and how diagnostic exposures compare in order of magnitude. Exam questions rarely need research-paper precision; they need correct categories, units, and clinical judgment.

Stochastic vs Deterministic Effects

Radiation bioeffects are classically divided into two categories:

Deterministic effects in context

Diagnostic general radiography almost never approaches thresholds for skin burns. Prolonged fluoroscopy and interventional procedures can deposit high local skin dose (measured conceptually in Gy to skin). That is why fluoro time, collimation, dose spreading (changing beam angle), and last-image-hold matter clinically—not only ALARA philosophy.

Stochastic effects in context

For chest x-rays, extremities, and most routine studies, the concern discussed with patients is small incremental cancer risk, framed carefully: benefit of a justified exam outweighs the small risk when optimization is applied. Avoid fear-based statements (“this will cause cancer”) and avoid dismissive ones (“radiation is completely harmless”). Professional tone: justified, optimized imaging has a very small risk compared with the clinical information gained.

Radiosensitivity

Law of Bergonié and Tribondeau (classic teaching): radiosensitivity tends to be greater in cells that are undifferentiated, highly mitotic, and have a long proliferative future. Practically:

• Higher sensitivity: lymphocytes, bone marrow, reproductive germ cells, intestinal crypt cells, embryonic tissues.
• Lower sensitivity: muscle, mature bone, nervous tissue (though high doses still injure).
• Age: children generally carry higher lifetime stochastic risk per unit dose than older adults because of longer life expectancy and more sensitive developing tissues.
• Organ weighting: effective dose uses tissue weighting factors so that dose to more sensitive or cancer-relevant organs contributes more to whole-body risk metrics.

Pregnancy and the embryo/fetus

Pregnancy is a high-yield radiation-protection topic:

1. Screening: Ask patients of childbearing potential about pregnancy per facility policy (last menstrual period, pregnancy test when indicated, signs, and documentation).
2. Risk timing: Sensitivity is especially significant during organogenesis (roughly weeks 3–8 post-conception for major organ formation) and remains important throughout fetal development for growth and CNS considerations at higher doses. Diagnostic doses are usually far below thresholds for deterministic fetal effects, but stochastic risk framing and ALARA still apply.
3. Management: If the patient is or may be pregnant, confirm justification with the clinical team, consider alternative modalities without ionizing radiation when clinically appropriate (e.g., ultrasound or MRI when they answer the question), optimize projections, collimate strictly, and use shielding that does not compromise the exam if the beam is remote from the fetus. Never simply cancel a potentially life-saving exam without medical consultation.
4. Occupational pregnancy: Declared pregnant workers follow facility fetal dose monitoring policies (additional dosimeter at waist under apron is a common pattern—details in 5.3). Work restrictions are policy- and regulation-driven; technologists should know to report pregnancy declaration promptly.

Exam trap: “Refuse all x-rays in pregnancy” is incorrect. Correct path: verify need, optimize, document, involve the responsible provider.

Dose Quantities and Units

Use SI units consistently:

Why both Gy and Sv appear

• Gy answers “how much energy was absorbed in tissue/skin?”
• Sv answers “how does this compare for biological risk accounting?” after weighting.

For photons and electrons, radiation weighting is 1, so 1 mGy absorbed in a tissue corresponds to 1 mSv equivalent dose to that tissue. Effective dose still differs because only some organs are irradiated and tissue weights differ. Do not treat effective dose as a measured badge reading; it is a calculated risk-related quantity for populations and exam comparison.

Prefixes you will see

• mGy / mSv — milligray / millisievert (10⁻³) — common for CT organ doses and effective dose of many exams.
• µSv — microsievert (10⁻⁶) — common for extremity or very low-dose projections and background comparisons.
• Gy without milli — concerning for interventional skin dose or radiotherapy territory, not routine chest x-ray effective dose.

Typical Diagnostic Dose Ranges (Order of Magnitude)

Exact values vary by technique, body habitus, equipment, and protocol. For exam readiness, learn relative ranking and rough orders of magnitude rather than claiming a single official CAMRT number:

Natural background radiation in Canada is often discussed in the few mSv per year range (location-dependent). A single chest x-ray is typically a small fraction of annual background; a multiphase body CT can be comparable to or higher than a year of background. Use this for communication, not for refusing justified CT.

Patient communication phrases (professional)

• “This exam uses a low amount of radiation compared with the benefit of answering your doctor’s question.”
• “We use the smallest field and appropriate settings to keep dose low.”
• “If you might be pregnant, tell us before we start so we can plan safely.”

Avoid quoting made-up precise cancer probabilities on the exam unless a vignette supplies numbers; qualitative comparison and ALARA actions are safer.

BEIR and Linear No-Threshold (LNT) Framing

Advisory bodies (including BEIR committee reports widely cited in radiation protection teaching) support a linear no-threshold model for low-dose stochastic risk for radiation protection purposes: risk is assumed to increase proportionally with dose without a threshold, down through the diagnostic range. Key teaching points:

1. LNT is a protection model, not a claim that every microSievert has a proven, measurable cancer in an individual patient.
2. It justifies ALARA—even small unnecessary doses are worth avoiding when no clinical benefit is gained.
3. Uncertainty is real at very low doses; science continues to refine risk estimates, but practice does not wait for perfect certainty to collimate and justify.
4. Do not invent exact numerical risk coefficients as if they were CAMRT blueprint facts. If a question asks for the model used in protection, LNT / proportional risk with dose is the expected framework.
5. Hormesis or threshold-only models may appear as distractors; standard entry-to-practice teaching for occupational and medical protection remains LNT-aligned ALARA.

Putting risk in exam scenarios

• Repeat film because of poor collimation: unnecessary stochastic risk + wasted resources—preventable.
• Fluoro with hand in beam: deterministic skin risk to hand + stochastic risk—stop and use tools.
• Pregnant patient, clinically essential CT: optimize, justify, document; do not apply a non-pregnant adult protocol mindlessly if a lower-dose protocol exists and answers the question.
• Comparing two justified exams: choose the lower-dose modality only when it is clinically equivalent—not when it fails to answer the diagnostic question.

Integrating Effects Knowledge into RTR Practice

Your operational translation of this section:

1. Classify concerns: skin/fluoro time → think deterministic local dose; routine radiography communication → stochastic + ALARA.
2. Use correct units in documentation and discussion: mGy for local/CTDI-style or organ contexts as taught; mSv for effective-dose comparisons.
3. Treat children and pregnant patients with heightened optimization, not panic.
4. Rank common exams so you can answer “which has higher dose?” items without false precision.
5. Let LNT support a culture of avoiding frivolous exposure while defending necessary imaging.

Bottom Line for RTR.1.6

Know the two effect categories, the Gy/Sv language, who is more radiosensitive, how to handle pregnancy questions, and order-of-magnitude dose ranks. Frame low-dose risk with BEIR/LNT-informed ALARA. The CAMRT exam rewards accurate concepts and safe clinical choices—not fabricated numeric tables presented as official absolutes.