3.1 Biological Effects of Radiation Exposure

Nuclear medicine technologists work daily with unsealed sources that emit photons, electrons, or alpha particles. Domain II of the CNMT exam expects you to distinguish how radiation harms tissue, who is being protected (patient vs worker vs fetus), and which effects drive regulation. This section builds the biological foundation for ALARA, dose limits, and surveys covered later in the chapter.

Deterministic (Tissue) Effects

Deterministic effects—also called tissue reactions—appear only after a practical threshold dose is exceeded. Above that threshold, severity increases with dose, and there is often a latency of hours to weeks depending on the tissue.

Classic examples relevant to medical radiation:

In nuclear medicine, diagnostic procedures are designed so that deterministic thresholds are not approached in patients or staff. Therapeutic radiopharmaceuticals (high-activity I-131, Lu-177, Ra-223) can deliver large local doses into deterministic territory on purpose while whole-body stochastic risk is managed separately.

Key exam phrase: deterministic = threshold + severity scales with dose.

Stochastic Effects

Stochastic effects are random in occurrence. Classic endpoints are cancer (somatic) and, theoretically, heritable effects. For protection purposes:

• There is no known practical threshold below which risk is proven zero.
• The probability of effect rises with dose; cancer severity does not “scale” the way burn severity does.
• U.S. radiation protection (NRC/NCRP philosophy behind 10 CFR 20) uses the linear no-threshold (LNT) model: low-dose risk is assumed proportional to dose with no safe cutoff.

LNT is a conservative regulatory model, not a claim that every mrem causes cancer. Exam items test that you apply LNT for ALARA and dose limits, not epidemiology debates.

Patient vs Technologist Contexts

Patients receive intentional medical exposure under a licensed authorized user; that justification does not excuse poor staff technique. Assay, administer, and shield so the patient gets the prescribed activity and staff receive as little dose as reasonably achievable.

Short-Term vs Long-Term Effects

Short-term (early) high-dose effects include nausea, fatigue, erythema, and marrow depression (hours to weeks)—deterministic when thresholds are crossed. Long-term (late) effects include cancer (years–decades latency), delayed cataracts, fibrosis, and potential heritable risk. Occupational NM work is managed almost entirely around late stochastic risk and fetal protection, not acute radiation sickness.

Exam time frames: hours–days after multi-Gy whole-body → prodromal/marrow syndromes; weeks after high skin dose → moist desquamation; years after lower cumulative doses → LNT cancer-risk framing.

Radiosensitivity: Law of Bergonié and Tribondeau

The Law of Bergonié and Tribondeau states that radiosensitivity is greater when cells are (1) highly mitotic, (2) have a long proliferative future, and (3) are poorly differentiated.

High sensitivity: lymphoid tissue, bone marrow stem cells, GI crypt cells, germ cells, and embryo/fetus. Lower sensitivity: muscle, mature bone/cartilage, and highly differentiated adult neural tissue (developing brain in utero is an important exception).

Cell-cycle note: late G2/M cells are generally more radiosensitive than late S-phase cells. After high whole-body dose, rapidly renewing tissues (marrow, then GI) fail first—classic Bergonié and Tribondeau application.

Fetal Sensitivity

Risk depends on gestational age and dose:

• Preimplantation (~0–9 days): “all-or-none” lethality risk at high doses.
• Organogenesis (~weeks 2–8): elevated teratogenic/structural risk at high doses.
• Fetal period, especially ~8–15 weeks for CNS: concern for intellectual disability and growth effects after significant fetal dose. Appropriate diagnostic NM doses are usually far below those thresholds.

Once a worker declares pregnancy in writing, the licensee must keep embryo/fetus dose ≤ 0.5 rem (5 mSv) for the entire gestation (10 CFR 20.1208)—far below the adult occupational 5 rem (50 mSv)/year TEDE limit, reflecting fetal radiosensitivity.

Practical points: screen women of childbearing age before high-risk exams or therapy when protocol requires; never administer I-131 without pregnancy-status verification per policy; diagnostic studies may proceed when clinically justified with agent-specific fetal dose awareness.

Exam Hooks

• Extreme local skin injury → deterministic (severity ↑ with dose)
• Lifetime cancer risk after chronic mrem → stochastic / LNT
• Marrow fails before muscle after acute whole-body dose → Bergonié and Tribondeau
• Fetal limit 0.5 rem/gestation after declaration → high fetal radiosensitivity + separate limit