Radiation Interaction with Matter

• Four major organic substances include carbohydrates, lipids, nucleic acids, and proteins.
• Two major inorganic substances are water and mineral salts, also known as electrolytes.
• The safest approach regarding ionizing radiation exposure is to minimize the dose as much as possible.

Interaction of Radiation with Matter

• Ionization occurs when an atom gains or loses electrons, resulting in a charged atom, most often positively charged.
• X-rays, gamma rays, and alpha rays differ in radiation weighting factor and particulate nature; electromagnetic radiation and alpha particles have distinct properties.
• Linear Energy Transfer (LET), Relative Biologic Effectiveness (RBE), and Oxygen Enhancement Ratio (OER) are key concepts for understanding how ionizing radiation injures and affects biological tissue.
• The two radiation categories based on LET are low-linear energy transfer (X-rays, gamma) and High-linear energy transfer radiation (alpha).
• Low-LET radiation causes sublethal DNA damage through indirect action, often reversible by repair enzymes.
• High-LET radiation has a higher likelihood of direct DNA interaction, causing potential irreparable damage.
• The relationship between LET and RBE in X-rays is directly proportional.
• The relationship between LET and OER is inverse.
• Diagnostic X-rays primarily involve low-LET interactions due to their higher frequency.

Direct and Indirect Action

• Ionizing radiation interacts with cells either directly, such as with DNA, or indirectly, such as with water (H2O).
• Due to the human body consisting of 80% water and less than 1% DNA, low-LET irradiation mainly produces effects through indirect action.
• Direct action occurs when ionizing particles interact with vital biological macromolecules like DNA, RNA, proteins, and enzymes.
• Direct action is more likely with high-LET radiation like alpha particles.
• Radiolysis of water involves the dissociation of water molecules by ionizing radiation.
• Radiation interacting with water results in H+ and OH- ions, plus hydrogen (H*) and hydroxyl (OH*) free radicals.
• Energetic hydrogen and hydroxyl free radicals can trigger unwanted chemical reactions.
• Approximately two-thirds of radiation-induced damage is attributed to the hydroxyl free radical (OH*).
• Radiation-induced chromosome breaks in somatic and reproductive cells can lead to chromosomal fragments, aberrations, and anomalies.

Target Theory

• Target Theory states that if ionizing radiation hits macromolecules like DNA, it can cause irreparable, permanent damage.
• Every cell contains a master molecule crucial for maintaining normal cell function.
• Effects of irradiation on the entire cell include causing damage to the cell’s nucleus.
• Damage to the cell’s nucleus can result in instant death, reproductive death, apoptosis (interphase death), mitotic or genetic death, mitotic delay, or interference with function.
• Cells have a greater chance of recovery when interacting with low-LET radiation.
• Cells exposed to high-LET radiation have little to no chance of recovery.
• Higher doses of radiation correlates to less chance of recovery.

Cell Radiosensitivity

• Immature, undifferentiated cells undergoing rapid division are radiosensitive, whereas differentiated, specialized cells dividing slowly or not at all are radioinsensitive.
• Oxygen enhances the impact of ionizing radiation on biologic tissue by increasing tissue radiosensitivity.
• Blood cells and blood-producing organs are the most radiosensitive.
• Radiosensitive cells (in order of decreasing sensitivity): bone marrow, gonads, spleen, skin outer layer, and intestinal crypt cells (epithelial cells).
• The Law of Bergoiné and Tribondeau states that cell radiosensitivity is directly proportional to reproductive activity and inversely proportional to differentiation.
• Aerobic conditions have normal oxygen levels, hypoxic conditions have decreased oxygen, and anoxic conditions have no oxygen.

Radiation Units

• Exposure is measured in roentgen (R), which is being replaced by coulombs per kilogram (C/kg) in the SI system. Air kerma is measured in Gray (Gya).
• Absorbed dose (D) is measured in rad, replaced by Gray (Gyt) in the SI system.
• Equivalent dose (EqD) is measured in rem, replaced by Sievert (Sv) in the SI system.
• Effective dose (EfD) is measured in rem, replaced by Sievert (Sv) in the SI system.