Biological Effects of Radiation Part 3: Somatic Effects (ARRT Registry Review)

When radiation interacts with the human body, it unleashes a cascade of biological effects. These effects vary based on the energy of the radiation, the type of tissue involved, and the duration and intensity of exposure. In this third installment of our series on the biological effects of radiation, we zero in on radiosensitivity—the measure of how likely cells, tissues, and systems are to be damaged by ionizing radiation. For radiography students and professionals preparing for the ARRT Registry, a deep understanding of radiosensitivity is vital—not only for exam success, but for effective, ethical clinical practice.

What Are Somatic Effects?

Somatic effects are radiation-induced biological effects that appear in the exposed individual, not in their offspring. This contrasts with genetic effects, which occur in germ cells and can be passed on to future generations. Somatic effects can be early or late, acute or chronic, mild or severe, and are highly dependent on dose, LET (Linear Energy Transfer), and the specific tissue involved.

Somatic effects begin at the cellular level. When radiation ionizes molecules within a cell—particularly DNA or water—it can lead to cell malfunction, mutation, or death. These cellular changes then ripple outward, impacting tissues, organs, and entire systems.

Radiosensitivity of Cells

Not all cells are created equal in the face of ionizing radiation. The Law of Bergonie and Tribondeau tells us that radiosensitivity is highest in cells that:

• Are actively dividing

• Are less specialized

• Have a long mitotic future

This makes tissues like bone marrow, gastrointestinal lining, and reproductive organs more vulnerable to radiation than fully differentiated tissues like muscle or nerve cells.

Key high-radiosensitivity cell types include:

• Lymphocytes

• Spermatogonia

• Epithelial cells

• Hematopoietic stem cells

These cells are particularly important in radiation biology because their damage can lead to systemic effects like immune suppression, infertility, and increased cancer risk.

Radiosensitive Tissues and Organs

1. Bone Marrow

Home to hematopoietic stem cells, bone marrow is highly sensitive. Damage here can reduce blood cell counts, leading to anemia, infection, and impaired clotting.

2. Skin

Radiation-induced skin changes include erythema, dry or moist desquamation, and at higher doses, radiation burns. These can occur during high-dose procedures or prolonged fluoroscopy.

3. Thyroid

The thyroid is particularly sensitive in children. Exposure may lead to hypothyroidism or thyroid cancer, making thyroid shielding critical during pediatric imaging.

4. Breast Tissue

Highly radiosensitive, especially in younger individuals, breast tissue damage can lead to carcinogenesis. Mammography protocols are tightly regulated to minimize exposure.

5. Eyes

The lens of the eye is sensitive to radiation, with cataractogenesis occurring after prolonged exposure. Cataracts are a unique late somatic effect because they have a threshold dose but appear years later, typically following a non-linear threshold model.

6. Gonads (Ovaries and Testes)

The germ cells in these organs are highly susceptible. Radiation can cause temporary sterility (~2 Gy), permanent sterility (~5–6 Gy), and hormonal disruption. While germ cell DNA changes are less relevant for ARRT exams, somatic effects like hormonal imbalance and fertility issues are highly relevant.

Embryo and Fetal Sensitivity

During gestation, the fetus is particularly vulnerable to radiation. The earlier in development, the more profound the potential effects. These can include:

• Growth retardation

• Neurological defects

• Microcephaly

• Childhood cancers

Exposure in the first trimester (especially weeks 2–15) carries the highest risk, which is why radiologic technologists must always confirm pregnancy status before exposing potentially pregnant patients.

Carcinogenesis as a Somatic Effect

Perhaps the most feared somatic effect is cancer. Unlike deterministic effects, cancer is stochastic—there is no threshold dose and the probability of occurrence increases with dose, not the severity. This makes long-term monitoring of radiation workers and patients essential. Common radiation-induced cancers include:

• Leukemia

• Thyroid cancer

• Breast cancer

• Lung cancer

• Skin cancer

The ARRT Registry emphasizes that even low diagnostic doses can contribute to cumulative cancer risk, reinforcing the need for justification, optimization, and ALARA (As Low As Reasonably Achievable) principles in all radiographic procedures.

Early vs. Late Somatic Effects

One of the key distinctions in radiation biology is between early (acute) and late (chronic) somatic effects:

• Early effects occur within hours, days, or weeks of exposure. They are typically linked to high-dose, short-term exposures and are often deterministic in nature.

• Late effects may not appear for months or even decades. These are often stochastic, meaning they are random and dose-dependent in probability—not severity.

Examples of early effects include:

• Erythema (skin redness)

• Epilation (hair loss)

• Hematologic depression (low blood cell counts)

• Nausea or gastrointestinal distress

Examples of late effects include:

• Cancer (e.g., leukemia, breast, thyroid)

• Cataracts (non-linear threshold model)

• Fibrosis or organ atrophy

This categorization helps practitioners evaluate patient risks based on dose and expected outcome timelines.

Deterministic vs. Stochastic Effects

Another important distinction is between deterministic (tissue reactions) and stochastic effects.

Deterministic Effects

• Threshold dose exists: No observable effects below the threshold.

• Severity increases with dose.

• Occur when radiation causes sufficient damage to affect cell survival and tissue function.

• Examples: skin burns, cataracts, infertility, acute radiation syndrome (ARS).

Stochastic Effects

• No threshold: Even a single photon may, in theory, cause a biological effect.

• Probability of occurrence increases with dose—not severity.

• Caused by random mutations, especially in DNA.

• Examples: cancer, hereditary effects (though the latter are rare at diagnostic levels).

The ARRT exam often uses these models to frame graph-based questions. Deterministic effects follow a linear threshold or non-linear threshold curve, while stochastic effects follow a linear non-threshold (LNT) model.

Acute Radiation Syndrome (ARS)

Acute Radiation Syndrome is a group of clinical symptoms resulting from whole-body exposure to high doses of radiation in a short period. ARS progresses through four phases:

1. Prodromal Phase: Nausea, vomiting, fatigue, and anorexia appear within hours.

2. Latent Phase: Symptoms temporarily subside; patient feels better.

3. Manifest Illness: Full onset of syndrome symptoms based on affected system.

4. Recovery or Death: Depends on dose, supportive care, and individual health status.

There are three main types of ARS, based on the dose range and system primarily affected:

1. Hematopoietic Syndrome (1–10 Gy)

• Affects bone marrow.

• Leads to reduced white blood cells, red blood cells, and platelets.

• Symptoms: Increased infection risk, anemia, bleeding tendencies.

• Survivability: Possible with prompt medical intervention.

2. Gastrointestinal Syndrome (6–10 Gy)

• Destroys cells in the intestinal lining.

• Symptoms: Nausea, vomiting, diarrhea, electrolyte imbalance.

• Prognosis: Poor without intensive supportive care.

3. Central Nervous System (CNS) Syndrome (50+ Gy)

• Rapid onset of confusion, convulsions, coma.

• Death typically occurs within hours to days.

• Irreversible damage to the blood-brain barrier and neural tissues.

Threshold Doses and Survival Rates

The LD 50/30 and LD 50/60 metrics represent the lethal dose required to kill 50% of an exposed population within 30 or 60 days, respectively.

• For humans, LD 50/60 is approximately 3.2–4.5 Gy with minimal medical support.

• LD 50/30 is more applicable to animal models but still used historically in radiobiology.

Dose-dependent charts from sources such as NCRP Report No. 105 and various ARRT preparation resources illustrate the probability of survival at different exposure levels .

Exception: Cataractogenesis

Cataracts are a notable exception in the deterministic vs. stochastic classification. Though a late effect, cataracts:

• Have a threshold dose (~2 Gy for single exposure, ~0.5 Gy for fractionated).

• Are non-linear in their dose-response.

• Are considered deterministic, but occur years after exposure.

Because of their unusual classification, cataracts frequently appear as a test topic. Protective measures like leaded eyewear are recommended in high-exposure settings like fluoroscopy suites.

Cancer induction is a well-documented late somatic effect. The linear non-threshold (LNT) model assumes that any dose, however small, has the potential to increase cancer risk. This model is foundational to radiation safety standards worldwide.

Certain tissues and organs have a higher radiosensitivity due to:

• Rapid cell turnover

• High oxygen content

• Low differentiation

Here’s a breakdown of key tissues and their radiosensitivity:

Understanding these differences is essential in evaluating organ dose risk, particularly in pediatric and reproductive-aged patients.

Embryonic and Fetal Radiosensitivity

The developing embryo and fetus are among the most radiosensitive biological systems. Sensitivity is phase-dependent:

• Preimplantation (0–2 weeks): High lethality risk; "all-or-nothing" response.

• Organogenesis (2–8 weeks): Critical period; risks include congenital malformations and growth retardation.

• Fetal stage (8 weeks–birth): Lower risk of malformations, but increased risk of carcinogenesis and CNS damage with exposure.

Dose limits for pregnant workers are strictly regulated (e.g., 0.5 mSv/month) to prevent deterministic effects and minimize stochastic risk. When imaging is necessary during pregnancy, justification, shielding, and optimized protocols are essential.

Short-Term vs. Long-Term Exposure Effects

Short-term exposure refers to a single or acute dose, while long-term exposure involves chronic low doses over time. While the deterministic effects (skin burns, GI syndrome) require acute high doses, long-term exposure may contribute to:

• Chronic fatigue and immune suppression

• Elevated cancer incidence in specific tissues

• Premature aging or fibrosis in irradiated organs

• Cataracts and cardiovascular changes in occupational settings

This duality is essential in balancing therapeutic benefits with risk in radiation therapy, nuclear medicine, and interventional radiology.

Radiation Protection Principles and Clinical Implications

To mitigate somatic effects, radiologic technologists must adhere to fundamental protection principles:

1. Time: Minimize exposure duration.

2. Distance: Double the distance to reduce intensity by a factor of four (inverse square law).

3. Shielding: Use lead aprons, thyroid collars, and leaded glass eyewear as appropriate.

4. Collimation: Limit beam to area of interest to reduce unnecessary exposure.

5. Technique optimization: Use high kVp/low mAs combinations where clinically appropriate.

6. Pregnancy protocols: Screen, shield, and defer elective imaging when possible.

These guidelines reflect standards set by NCRP Report 105, ensuring occupational exposures remain well below thresholds for deterministic damage while limiting cumulative stochastic risk.

Conclusion: Radiosensitivity in Context

Understanding somatic effects of radiation is vital for clinical safety, patient counseling, and passing the ARRT Registry Exam. Radiosensitivity is influenced by:

• Cell and tissue type

• Radiation quality (LET, RBE)

• Oxygenation (OER)

• Developmental stage

• Exposure duration and dose

Early effects like radiation syndrome, and late effects such as cancer and cataracts, offer a comprehensive view of the biological footprint of ionizing radiation. Though diagnostic levels pose minimal risk, adherence to protection principles ensures a safe practice environment for both patients and providers.

Stay informed, stay protected—and stay ready for your ARRT exam.

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