Radiation Effects at the Tissue and Organ Level * Response to irradiation- measures the radiosensitivity of tissue or organ * Loss of moderate number of cells- does not affect function of organs * Loss of large number of cells- display clinical result * Severity of change depends on dose and thus amount of cell loss * Repairable damage- moderate dose to localized area * Death from damage to most sensitive system- comparable dose to whole organism
Short- Term Effects * Sensitivity of parenchymal cell- determines short term effect of radiation on a tissue * Lost cells in mitosis- linked death- when continuously proliferating tissues (e.g bone marrow, oral mucous membrane) are irradiated with a moderate dose * Extent of cell loss depends on damage to stem cell pools and proliferative rate of cell population * Reduction in number of mature cells- effect of irradiation to such tissues * Little or no radiation-induced hypoplasia- when tissues composed of cells that rarely or never divide (e.g muscle) are irradiated
Long- Term Effects * Damage to fine vasculature- determines long-term effect of radiation on tissue or organ * Radio sensitivity of capillaries and connective tissues- differentiating intermiotic cells and reverting postmiotic cells * Irradiation of capillaries- swelling, degeneration, necrosis * Increases capillary permeability and slow progressive fibrosis around vessels * Increased deposition of fibrous scar tissue leading to premature narrowing and eventual obliteration of vascular lumens * Impairs transport of oxygen, nutrients, and waste product which results to death in all cell tyoe * Progressive fibroatrophy of irradiated tissue- loss of cell function and reduced resistance to infection and trauma * Death of parenchymal cells after moderate exposure: (1) mitotic linked death of rapidly dividing cells in short term (2) consequences of progressive fibroatrophy on all cell types over time
Modifying Factors 1. Dose * Damage in irradiated tissues or organs depend on amount of radiation received * Below clinical threshold- no adverse effecr * Above threshold- damage in proportion to dose

2. Dose Rate- rate of exposure * Given dose at a high dose rate cause more damage than same does at low dose rate * Low dose rate- less net damage, damage can be repaired 3. Oxygen * Irradiation conducted with reduced oxygen (hypocia)- radioresistance of biologic systems increases by a factor of 2 or 3 * Greater cell damage in presence of oxygen- related to increased amounts of hydrogen peroxide and hydroperoxyl free radicals formed * Oxygen enhancement ratio- measures damage
=Dose required to achieve a given endpoint under anoxic condition Dose required to produce same endpoint under fully oxygenated condition 4. Linear Energy Transfer * LET dose required to produce certain biologic effect * High LET radiations cause more damage- high ionization density induce double- strand breakage in DNA than xray * Low LET radiation (xray)- deposit energy uniformly in absorber, more likely to cause single- strand breakage and less biologic damage * Relative biologic effectiveness (RBE)
- comparison of biologic responses to different types of radiation
- x rays used as reference