RT 7 notes

• Radiobiology is the study of the effects of ionizing radiation on biologic tissue
• ALARA stands for As Low As Reasonably Achievable
• The effect of x-rays on humans is due to interactions at the atomic level, resulting in ionization or excitation of orbital electrons and energy deposition in tissue
• Ionization involves the removal of an orbital electron from an atom, leading to breakage of molecules, changes in chemical properties, or relocation of atoms within molecules
• Excitation is the addition of energy to a system by raising the energy of electrons with x-rays
• Abnormal molecules from radiation may lead to improper function or cell death, but radiation damage can be repaired at every stage
• The human body consists of five principal types of molecules: proteins, lipids, carbohydrates, nucleic acids, and water
• Proteins, lipids, and carbohydrates are organic molecules supporting life and containing carbon
• Nucleic acids, like DNA, are critical and radiosensitive target molecules concentrated in the cell nucleus
• Water, the most abundant molecule in the body, plays a role in delivering energy to target molecules, contributing to radiation effects
• Radiation interactions at the atomic level can lead to molecular changes affecting cell growth and metabolism
• The human cell consists of the nucleus and cytoplasm, with DNA as the principal molecular component of the nucleus
• Cell functions include being the basic unit of life, protection and support, movement, communication, metabolism, energy release, and inheritance
• The human body has somatic cells and genetic cells, with somatic cells undergoing mitosis and genetic cells undergoing meiosis
• Mitosis involves four subphases: prophase, metaphase, anaphase, and telophase, with interphase being the period of growth between divisions
• Meiosis reduces the number of chromosomes in genetic cells to half for marriage compatibility, ensuring daughter cells have the correct chromosome count
• Law of Bergonie and Tribondeau states that stem cells are radiosensitive, younger tissues are radiosensitive, and tissues with high metabolic activity are radiosensitive
• Linear Energy Transfer (LET) measures the rate of energy transfer from ionizing radiation to soft tissue, affecting radiation quality and the radiation weighting factor
• Relative Biologic Effectiveness (RBE) quantitatively describes the ability of radiation to produce biological damage, with higher LET radiation having a higher RBE
• Protraction and Fractionation of radiation doses affect the biologic response, with protracted doses delivered continuously at a lower rate and fractionated doses delivered in equal portions at regular intervals
• Biologic factors affecting radiosensitivity include the oxygen effect, where tissue is more sensitive to radiation in oxygenated conditions, and age, with humans being most radiosensitive in early development
• Factors affecting chromosome damage:

  • Total Dose:
  • The number of doses is directly proportional to the dose.
  • Increasing the dose increases the number of breaks in the chromosome.
  • Dose Rate:
  • Refers to the rate at which radiation is delivered.
  • Low-dose rates result in a decreased number of complex aberrations, while high-dose rates increase the frequency of complex aberrations.
  • LET (Linear Energy Transfer):
  • Low LET radiations produce more simple aberrations than complex ones.
  • High LET radiation has a greater probability of producing more complex aberrations
• There are two types of ionizing radiation interaction with the human body:

  1. Indirect Action:

  • Effects produced by free radicals created by the interaction of radiation with water molecules.
  • Radiolysis of water results in the production of four free radicals: Hydroxyl Radical, Hydrogen Radical, Hydrogen Peroxide, and Hydroperoxyl Radical.

  2. Direct Action:

  • Biologic damage occurs as a result of ionization of atoms on essential molecules such as macromolecules, DNA, and chromosomes.
  • Irradiation of macromolecules can lead to main-chain scission, cross-linking, and point lesions
• DNA Irradiation can result in:

  • Single strand break
  • Double Strand Break
  • Change or Loss of base
  • Breakage of hydrogen bonds
  • Main-Chain Scission
• Chromosomal Irradiation can lead to different types of chromosomal mutations:

  1. Single Break Effect:

  • Restitution
  • Acentric Chromosome
  • Dicentric Chromosome
  • Translocation

  2. Double Break Effect:

  • Deletion
  • Inversion

  3. Chromosomal Stickiness:

  • Occurs in cells already in division
  • Results in clumping of chromosomes together
  • Believed to be caused by alterations in the chemical composition of the protein component of the chromosome by irradiation
• Cell Irradiation can have five possible effects:

  • Instant death
  • Reproductive death
  • Apoptosis
  • Mitotic or genetic death
  • Mitotic delay
• Acute Radiation Lethality:

  • Measured quantitatively by LD 50/30
  • LD 50/30: dose of radiation to the whole body resulting in death within 30 days for 50% of subjects
  • LD 50/30 for humans is approximately 300 rad (3 Gy)
  • LD 10/30 and LD 90/30 indicate doses resulting in 10% or 90% lethality within 30 days respectively
  • LD 50/60: lethal dose to 50% when survival time is extended for 60 days
  • Mean Survival Time decreases as whole-body radiation increases
• Local Tissue Damage:

  • When only part of the body is irradiated, a higher dose is needed to produce a response
  • Effects on skin:
  • Skin cells are replaced at a rate of approximately 2% per day
  • Basal cells are the stem cells of the epidermis
  • Skin effects follow a nonlinear, threshold dose-response
  • Erythema is the first observed response to radiation exposure
  • SED50 = 500 rad (5 Gy) required to affect 50% of the population
• Effects on Gonads:

  • Ovaries:
  • Irradiation reduces size through germ cell death
  • Sensitivity to radiation increases with maturity
  • Different doses lead to delays in menstruation, temporary infertility, or permanent sterility
  • Testes:
  • Irradiation reduces sperm count, leading to temporary infertility or permanent sterility
  • Doses affect male procreation for a specific period
• Hematologic Effects:

  • Hemopoietic cell survival decreases with increasing dose
  • Granulocytopenia and thrombocytopenia occur, with recovery times of approximately 2 months
  • Erythrocytes are less sensitive and recover over 6 months to a year
• Cytogenetic Effects:

  • Nearly every type of chromosome aberration can be radiation-induced
  • Lymphocytes are often used for cytogenetic analysis
  • Single- and multi-hit chromosome aberrations occur, leading to visible derangements
• Stochastic Effects of Radiation:

  • Response not observed immediately, but over months or years
  • Incidence of radiation response increases with dose
  • Result of low doses over a long period
  • Can lead to radiation-induced malignancy and genetic effects
• Radiation and Pregnancy:

  • Damage to the embryo depends on the stage of gestation
  • Three stages: Pre-implantation, Major Organogenesis, Fetal Stage
  • Specific abnormalities occur at different doses during each stage
• Acute Radiation Lethality


  Measured quantitatively by LD 50/30
• LD 50/30


  The dose of radiation to the whole body that will result in death within 30 days to 50% of the subjects being irradiated
• The LD 50/30 for humans is estimated to be approximately 300 rad (3 Gy)
• LD 10/30 and LD 90/30


  Indicates a dose resulting in 10% lethality or 90 % lethality within 30 days respectively
• LD 50/60


  The lethal dose to 50% when the observe survival time is extended for 60 days
• Mean Survival Time


  As the whole body radiation increases, the average time between exposure and death decreases
• Local Tissue Damage


  • When only part of the body is irradiated, in contrast to whole-body irradiation, a higher dose is required to produce a response