Radiation is a form of energy that travels as waves or high-speed particles. It can be non-ionizing or ionizing. Ionizing radiation is the most harmful because it has enough energy to remove tightly bound electrons from atoms, causing dangerous chemical changes in the body.
Types of Radiation
1. Non-Ionizing Radiation
Includes UV light, infrared light, microwaves, and sound waves. This type can make atoms vibrate but cannot remove electrons. It causes mild tissue damage, like sunburn.
2. Ionizing Radiation
This radiation removes electrons and creates ionization. Main sources include:
- X-rays and gamma rays
- High-energy neutrons
- Alpha particles (2 protons + 2 neutrons)
- Beta particles (high-speed electrons)
Ionizing radiation is widely used in medicine (diagnostic imaging, radiotherapy) but can cause severe tissue injury, mutations, fibrosis, cancer, and birth defects.
Radiation Units
Curie (Ci)
Measures how much radiation a source emits. One Ci = 3.7 × 1010 disintegrations per second.
Gray (Gy)
Measures the amount of radiation energy absorbed by tissue. 1 Gy = absorption of 1 joule per kilogram of tissue.
Sievert (Sv)
Measures biological effect of absorbed dose. It considers type of radiation and tissue sensitivity. Medical imaging doses are usually measured in millisieverts (mSv).
Main Determinants of Biological Damage
How harmful radiation is depends on several factors:
1. Rate of Delivery
Smaller, divided doses cause less damage because cells get time to repair. Radiotherapy uses “fractionated dosing” so healthy cells can recover but tumor cells die.
2. Field Size
Small, shielded fields can tolerate high doses, but large exposures can be fatal even at lower doses.
3. Cell Proliferation
Rapidly dividing cells are most sensitive. These include:
- Gonadal cells
- Bone marrow
- Lymphoid tissues
- GI tract mucosa
4. Oxygen Effects and Hypoxia
Radiation damage occurs mainly through reactive oxygen species (ROS). Hypoxic (low oxygen) tissues—like the center of fast-growing tumors—are less sensitive to radiation therapy.
5. Vascular Damage
Radiation injures endothelial cells (blood vessel lining), causing narrowed vessels and chronic ischemia. Long-term effects include fibrosis and organ atrophy.
Total Body Irradiation
Exposure of large parts of the body to radiation causes systemic damage. Doses below 1 Sv may cause minimal symptoms, but higher exposures cause acute radiation syndrome, involving:
1. Hematopoietic System
- Early lymphopenia (within hours)
- Bone marrow suppression → aplasia
- Granulocyte count drops to zero by week 2
- Platelet drop → bleeding
- Anemia after 2–3 weeks
Recovery may take weeks to months if exposure is not lethal.
2. Gastrointestinal System
High doses destroy GI mucosa → diarrhea, bleeding, fluid loss, infections.
3. CNS System
Very high doses (>20–30 Sv) cause brain edema, vascular leakage, and rapid death.
Fibrosis
Radiation commonly causes late fibrosis in exposed tissues. It occurs due to:
- Vascular injury
- Loss of stem cells
- Release of cytokines promoting chronic inflammation
Common sites include lungs, salivary glands, and pelvic organs after radiotherapy.
DNA Damage and Carcinogenesis
Ionizing radiation damages DNA by causing:
- Single-strand breaks
- Double-strand breaks (most dangerous)
- Base damage
- Cross-linking
If cells fail to repair DNA properly, mutations accumulate. Double-strand breaks may cause deletions, duplications, or chromosomal translocations, eventually leading to cancer.
Radiation also has a bystander effect, where irradiated cells release signals that affect nearby non-irradiated cells.
Cancer Risk from Low-Level Radiation
Any dividing cell exposed to mutation can become cancerous. Cancers seen after radiation exposure include:
- Leukemia
- Thyroid cancer
- Breast cancer
- Lung cancer
High cancer rates have been documented in:
- Hiroshima and Nagasaki survivors
- Chernobyl survivors
- Marshall Islands residents exposed to fallout
- Patients receiving childhood radiotherapy
Repeated exposure above 50–100 mSv increases long-term cancer risk. Radon gas exposure in miners is linked to lung cancer due to inhalation of radioactive polonium particles.
Detailed Notes:
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