3.2 Biological Effects of Radiation & Radiobiology Principles

Understanding radiobiology—how ionizing radiation affects living tissue—is critical for nuclear medicine technologists to ensure patient and occupational safety. The ARRT exam heavily tests your knowledge of cellular damage mechanisms, dose-response relationships, and systemic radiation effects.

Mechanisms of Cellular Damage

Radiation damages cells through two primary mechanisms: direct and indirect action. Because the human body is roughly 80% water, indirect action is statistically far more common.

Direct Action

In direct action, the ionizing radiation directly strikes a critical macromolecule, usually DNA. This causes ionization of the atoms within the DNA structure, leading to single-strand or double-strand breaks. Direct action is more common with high-LET radiations like alpha particles and neutrons.

Indirect Action & Radiolysis of Water

In indirect action, the radiation interacts with water molecules in the cell, causing radiolysis (the splitting of water). H₂O + radiation → HOH⁺ + e⁻ This process ultimately produces free radicals (such as the hydroxyl radical, OH* and hydrogen peroxide, H₂O₂). Free radicals are highly reactive and unstable; they migrate through the cell and chemically attack DNA. Indirect action is the primary mechanism of damage from low-LET radiations, including the X-rays and gamma rays used in nuclear medicine.

LET and RBE

Linear Energy Transfer (LET)

LET is a measure of the rate at which energy is deposited as a charged particle travels through matter.

• Low-LET radiation: Gamma rays, X-rays, and beta particles. They travel fast and deposit energy sparsely along their track. They cause damage primarily through indirect action.
• High-LET radiation: Alpha particles and neutrons. They are bulky, travel slower, and deposit large amounts of energy in a very short distance. They are highly destructive and cause dense ionizations, leading to a high probability of direct DNA damage.

Relative Biologic Effectiveness (RBE)

RBE is a comparison of a dose of test radiation to a dose of 250 kVp X-rays that produces the same biologic response. As LET increases, RBE generally increases (up to a certain point). High-LET radiation is biologically more damaging per unit of absorbed dose than low-LET radiation.

Cellular Radiosensitivity

Not all cells respond equally to radiation. The Law of Bergonié and Tribondeau (1906) states that cells are most sensitive to radiation when they are:

1. Rapidly dividing (high mitotic rate)
2. Undifferentiated (stem cells, immature cells)
3. Have a long dividing future

Highly Radiosensitive Cells: Lymphocytes, spermatogonia, erythroblasts, intestinal crypt cells. Radioresistant Cells: Muscle cells, nerve cells (neurons).

Stochastic vs. Deterministic Effects

Radiation effects on human populations are broadly categorized into two types:

Stochastic (Probabilistic) Effects

• Nature: Occur randomly. The probability of occurrence increases with dose, but the severity of the effect is independent of the dose.
• Threshold: Assumed to have NO threshold (Linear No-Threshold, or LNT hypothesis). Any dose, no matter how small, carries some risk.
• Examples: Radiation-induced cancer (leukemia, solid tumors) and genetic mutations.
• Relevance: This is the primary concern in diagnostic nuclear medicine and the basis for ALARA (As Low As Reasonably Achievable) principles.

Deterministic (Non-Stochastic / Tissue) Effects

• Nature: Predictable effects that occur only after a specific dose threshold is reached. The severity of the effect increases as the dose increases above the threshold.
• Threshold: Has a clear, defined threshold.
• Examples: Erythema (skin reddening), epilation (hair loss), cataracts, sterility, and radiation sickness.
• Relevance: Typically seen in radiation therapy or high-dose interventional procedures, rarely in standard diagnostic nuclear medicine.

Acute Radiation Syndrome (ARS)

ARS, or radiation sickness, occurs following a massive whole-body, acute exposure to penetrating ionizing radiation (typically > 1 Gy or 100 rads). It progresses through four stages: Prodromal (initial symptoms like nausea/vomiting), Latent (symptom-free period), Manifest Illness, and Recovery or Death.

ARS presents in three distinct syndromes depending on the dose:

1. Hematopoietic (Bone Marrow) Syndrome

• Dose Range: ~1 to 10 Gy.
• Mechanism: Destruction of bone marrow stem cells leads to depletion of red and white blood cells and platelets.
• Manifestations: Infection, hemorrhage, anemia. Survival is possible with medical intervention at lower doses.

2. Gastrointestinal (GI) Syndrome

• Dose Range: ~10 to 50 Gy.
• Mechanism: Destruction of the rapidly dividing crypt cells in the intestinal lining. The GI tract loses its ability to absorb nutrients and retain fluids.
• Manifestations: Severe diarrhea, dehydration, electrolyte imbalance, sepsis. Usually fatal within weeks.

3. Cerebrovascular (CNS/Cardiovascular) Syndrome

• Dose Range: > 50 Gy.
• Mechanism: Severe damage to blood vessels, fluid leakage into the brain causing elevated intracranial pressure.
• Manifestations: Disorientation, loss of coordination, seizures, coma. Fatal within hours to a few days.

Embryo and Fetal Risks

The developing embryo/fetus is highly radiosensitive due to rapid cell division and differentiation. The risks vary significantly based on the stage of gestation at the time of exposure:

• Pre-implantation (0-2 weeks): "All-or-nothing" effect. High doses cause embryonic death (spontaneous abortion), but survivors generally develop normally.
• Organogenesis (2-8 weeks): Period of highest risk for major congenital abnormalities and structural defects (e.g., microcephaly).
• Fetal Stage (8 weeks to term): Decreased risk of structural abnormalities, but increased risk of functional defects (like intellectual disability) and childhood cancer.

Exam Tip: The dose limit for the entire gestation period for a declared pregnant worker is 0.5 rem (5 mSv), not to exceed 0.05 rem (0.5 mSv) in any single month.