Radiation Biology and Protection

Core Review

Ionizing Radiation and Biological Damage

X-rays are a form of ionizing radiation — they carry sufficient energy to eject electrons from atoms, creating ions. This ionization can cause biological damage through two mechanisms:

Direct Effect: The X-ray photon directly strikes and ionizes a critical molecule (e.g., DNA), causing immediate molecular damage. This is relatively uncommon but highly damaging when it occurs.

Indirect Effect: The X-ray photon ionizes water molecules (approximately 70-80% of cells are water), producing free radicals (particularly hydroxyl radicals, OH-). These free radicals then damage critical molecules like DNA. This is the primary mechanism of radiation-induced biological damage at diagnostic dose levels.

DNA Damage Outcomes:

• Complete repair with no residual damage (most common at diagnostic doses)
• Repair with mutation (may lead to carcinogenesis over time)
• Cell death (apoptosis or necrosis, more common at higher doses)
• Incorrect repair leading to chromosomal aberrations

Stochastic vs. Deterministic Effects

This distinction is fundamental to radiation protection and is heavily tested on the NBDHE:

Stochastic Effects (Probabilistic):

• Occur randomly; probability increases with dose, but severity does NOT
• No known threshold dose — theoretically, even a single X-ray photon could cause an effect
• Primary concern: cancer induction and heritable (genetic) effects
• All diagnostic dental radiation doses are in the stochastic range
• Examples: radiation-induced cancer, leukemia, genetic mutations
• The linear no-threshold (LNT) model assumes any dose carries some risk

Deterministic Effects (Tissue Reactions):

• Severity increases with dose once a threshold is exceeded
• Have a threshold dose below which the effect does not occur
• Result from substantial cell killing in a tissue
• Examples: skin erythema (threshold ~2 Gy), cataract formation (threshold ~0.5 Gy for lens), mucositis, bone marrow suppression
• Threshold doses are far above those used in diagnostic dental radiology (dental exposures deliver microGray [µGy] doses, while thresholds are in the Gray [Gy] range)
• Exception: cumulative occupational exposure over many years could theoretically approach deterministic thresholds; hence the importance of protection

Key NBDHE Distinction: Dental radiology primarily concerns stochastic effects (cancer risk), not deterministic effects, because dental doses are far below deterministic thresholds.

Radiosensitivity

Tissues and organs vary in their sensitivity to ionizing radiation. The law of Bergonié and Tribondeau states that cells are more radiosensitive when they are:

• Actively dividing (high mitotic activity)
• Undifferentiated (immature)
• Have a long mitotic future

Most Radiosensitive Tissues (in order):

1. Lymphoid tissue (lymphocytes)
2. Bone marrow (hematopoietic stem cells)
3. Reproductive cells (testes, ovaries)
4. Intestinal epithelium
5. Skin (basal layer)
6. Lens of the eye
7. Oral mucosa

Least Radiosensitive Tissues:

• Mature bone
• Mature cartilage
• Muscle
• Nerve tissue
• Mature connective tissue

Clinical Relevance: The thyroid gland (moderately radiosensitive), bone marrow (highly radiosensitive — mandible in the primary beam), and lens of the eye (moderately sensitive) are organs of concern in dental radiography. This is why thyroid collars and collimation are essential.

Radiation Protection Principles

ALARA (As Low As Reasonably Achievable): The cornerstone principle of radiation protection. Every effort should be made to minimize radiation dose without compromising diagnostic quality. ALARA applies to both patients and operators.

ALADA (As Low As Diagnostically Acceptable): An evolution of ALARA specific to dentistry. It emphasizes that the "reasonable" dose is the minimum dose that provides diagnostically acceptable images. The distinction is that ALADA focuses specifically on diagnostic acceptability rather than a broader array of "reasonable" factors.

Three Cardinal Principles of Radiation Protection:

1. Time: Minimize exposure time. Use the shortest exposure time consistent with diagnostic image quality. Fast image receptors (F-speed film or digital sensors) reduce required exposure time.

1. Distance: Maximize distance from the radiation source. Operator should stand at least 6 feet from the source, preferably behind a protective barrier. The inverse square law applies — doubling the distance reduces exposure to 1/4.

1. Shielding: Use appropriate shielding materials. Lead aprons (0.25 mm lead equivalent), thyroid collars (0.5 mm lead equivalent for thyroid), and protective barriers. Shielding is the last line of defense after time and distance have been minimized.

Beam-Limiting Devices

Collimation: Restricts the size and shape of the X-ray beam to the area of clinical interest.

• Round collimator: Traditional design, produces a circular beam ~2.75 inches (7 cm) in diameter at skin surface
• Rectangular collimator: Produces a rectangular beam matched to the size of the image receptor (slightly larger than a #2 film). Reduces patient dose by 60-70% compared to round collimation by decreasing the irradiated tissue volume. This is the current STANDARD of care.
• The collimator should be lined with lead to absorb off-focus radiation.

Filtration: Removes low-energy (soft) X-rays that would be absorbed by the patient without contributing to the image. These photons increase patient dose without diagnostic benefit.

• Inherent filtration: The glass housing and insulating oil provide approximately 0.5-1.0 mm aluminum equivalent
• Added filtration: Aluminum disks placed at the tube window provide additional filtration
• Total filtration requirements:
  • Below 50 kVp: 0.5 mm aluminum equivalent
  • 50-70 kVp: 1.5 mm aluminum equivalent
  • Above 70 kVp: 2.5 mm aluminum equivalent (federal requirement)

PID (Position-Indicating Device):

• Conventional PID: Open-ended plastic cylinder; preferred over closed-ended (which can produce backscatter)
• Rectangular PID: Combines PID and rectangular collimation, providing optimal beam limitation
• PID length: typically 8, 12, or 16 inches. Longer PIDs (16 inches) produce images with less magnification and geometric distortion due to increased source-to-object distance, but require higher mAs (inverse square law).

Patient Protection

Selection Criteria: Radiographs should only be prescribed when the diagnostic yield justifies the radiation risk. The ADA/FDA have established evidence-based selection criteria. Never take routine "screening" radiographs without clinical indication.

Selection Criteria by Patient Type (ADA/FDA guidelines):

• New adult patient: Posterior bitewings + selected periapicals or panoramic + posterior bitewings (based on clinical examination)
• Recall adult patient with no clinical caries or risk factors: Posterior bitewings at 24-36 month intervals
• Recall adult patient with clinical caries or risk factors: Posterior bitewings at 6-18 month intervals
• Child (primary dentition, closed contacts): Posterior bitewings if contacts are closed and proximal surfaces cannot be visualized
• Child (mixed dentition): Posterior bitewings + selected periapicals based on clinical findings
• Adolescent (permanent dentition, no caries): Posterior bitewings at 18-36 month intervals
• Periodontal assessment: Selected vertical bitewings or periapicals; full mouth series if indicated
• Edentulous patient: Not routinely indicated; periapicals or panoramic only if specific pathology is suspected

Image Receptor Selection:

• Speed: F-speed film is the fastest currently available intraoral film. It reduces patient dose by approximately 60% compared to D-speed, and 20-25% compared to E-speed. There is no clinically significant loss of diagnostic quality with faster film.
• Digital sensors: Reduce dose by 50-90% compared to D-speed film. Digital sensors are considered the dose-minimizing standard.
• Intensifying screens for extraoral: Rare-earth screens (gadolinium, lanthanum) are faster than calcium tungstate and reduce patient dose.

Other Patient Protection Measures:

• Thyroid collar (0.5 mm lead equivalent): Recommended for all intraoral exposures, especially in children and women of childbearing age. Must not interfere with the image (important for panoramic).
• Lead apron (0.25 mm lead equivalent): Recommended when the gonads are in or near the primary beam. In modern dental radiography with appropriate collimation and technique, the gonadal dose is negligible, but the apron provides additional reassurance and is considered standard of care.
• Proper technique (no retakes): Retakes double the patient's dose. Correct film/sensor placement, angulation, and exposure settings minimize retakes.
• Fast image receptors: Should always be used.
• Rectangular collimation: Standard of care.

Operator Protection

• Distance: Stand at least 6 feet from the X-ray source. The operator should ideally be positioned at a 90-135 degree angle to the primary beam (scatter is least at these angles).
• Position: Never hold the film or tube head during exposure for any patient. If a patient requires stabilization, use a film-holding device or have a family member (not a pregnant person) assist after providing appropriate protective equipment.
• Shielding: Stand behind a protective barrier (lead-lined wall) or wear a lead apron if a barrier is unavailable.
• Monitoring: Wear a personal radiation dosimeter (film badge) at collar level, outside the lead apron. Dosimeters should be changed according to schedule (monthly or quarterly).
• Dose limits for occupational exposure (NCRP):
  • Effective dose: 50 mSv per year (5 rem/year)
  • Cumulative dose: 10 mSv × age in years
  • Lens of eye: 150 mSv per year
  • Skin/extremities: 500 mSv per year
  • Pregnant worker (declared): 0.5 mSv per month (5 mSv for the entire pregnancy)

Important: The NBDHE is not likely to ask for specific numeric dose limits, but you should understand the principle of dose monitoring and the special considerations for pregnant radiation workers.

Measurement Units

• Exposure (Roentgen, R): Measure of ionization in air. Used for calibration.
• Absorbed Dose (Gray, Gy): Energy deposited per unit mass. 1 Gy = 100 rad.
• Equivalent Dose (Sievert, Sv): Absorbed dose adjusted for the biological effectiveness of the radiation type (radiation weighting factor). For X-rays, the factor is 1, so Gy = Sv.
• Effective Dose (Sievert, Sv): Equivalent dose adjusted for the radiosensitivity of the exposed tissues (tissue weighting factor). This is the most clinically meaningful measure of overall risk.

Typical effective doses in dental radiography:

• Single intraoral (F-speed, rectangular collimation): ~1-5 µSv
• Full mouth series (F-speed, rectangular collimation): ~35-170 µSv
• Panoramic: ~10-25 µSv
• CBCT (limited field): ~50-200 µSv

For perspective: annual background radiation in the US is approximately 3,000 µSv (3 mSv).

Clinical Application

A 45-year-old new patient presents with no radiographs available. Clinical examination reveals moderate plaque and calculus, probing depths of 3-5 mm, and no obvious caries. According to selection criteria, posterior bitewings are indicated (to evaluate for interproximal caries and alveolar bone levels) along with selected periapicals for areas with probing depths ≥5 mm or other clinical findings. You would NOT prescribe a full mouth series simply because the patient is new — selection criteria should guide prescription. Use rectangular collimation, F-speed film or a digital sensor, and a thyroid collar.

Common Traps

• TRAP: Confusing stochastic and deterministic effects. If the question asks about effects with a threshold or where severity depends on dose, the answer is deterministic/tissue reactions.
• TRAP: Thinking lead aprons are primarily for gonad protection. In modern dentistry with collimated beams, the gonadal dose from head-and-neck imaging is already negligible. The apron provides psychological reassurance and additional safety margin.
• TRAP: Prescribing radiographs based on time since last films rather than clinical indication. Selection criteria are based on risk factors, not arbitrary time intervals.
• TRAP: Forgetting that thyroid shielding is critical. The thyroid gland is more radiosensitive than many tissues and is frequently in or near the primary beam during intraoral exposures.