RADIOBIOLOGY

Principles of Radiobiology based on n Random nature of radiation interactions n how radiation interacts at cellular level n factors that can alter these interactions

REMEMBER n All radiation damage occurs at cellular or subcellular level n visible effects are seen after millions of cells are impacted

Characteristics of ionizing radiation n Charge n Mass n Energy Vary among different types of radiation

Ionizing radiation causes injury through these concepts n LET n RBE n OER

LET n High and low LET (chart 6-1, pg 108 Sherer) n LET IS AVERAGED DUE TO POLYENERGETIC NATURE OF IONIZING RADIATION

RBE Background information n QF-How much biologic damage one type of radiation does vs. type of radiation n What is the QF of x-rays? n What about Alpha? n QF=rounded estimate of RBE

RBE (Pg. 495-Bushong) n Compares: dose of standard radiation necessary to produce a given effect dose of test radiation necessary to produce same effect n Standard radiation is 250 KeV n RBE’s of diagnostic is 1 n Max. is approx. 3 n As LET increases; so does RBE n Alpha and Beta have high RBE

OER The ratio of radiation damage done with oxygen vs. without oxygen

OXYGEN EFFECT n Tissue more sensitive to rad when oxygenated or in aerobic state n VS n anoxic or hypoxic n Oxygen produces free radicals n Why would this be important in radiation therapy?

n Alpha-insensitive to oxygen (Low OER) n Gamma and x-rays more sensitive (High OER) n Therefore Low LET is associated with ____OER (LOW OR HIGH)

IONIZING RADATION AND BIOLOGIC DAMAGE n Direct or indirect n occurs more with high LET radiation n Relative Biologic Effectiveness

Direct n Can be fatal to cell n high LET

A FREE RADICAL n Hydroperoxl n Hydroxyl nH2O2nH2O2 n 2/3 of all biologic damage is from H 2 O 2 which is… n hydrogen peroxide

INDIRECT n RADIOLYSIS OF WATER n Breakdown of H 2 O n produces an ion pair of HOH and e- n These can attach to uncharged water molecules (unstable)= free radical

CHROMOSOMAL EFFECT n Biologic damage occurs during breakage of chromosomal backbone or to DNA n Point mutations n Frameshift mutation

RADIATION MUTATION CHARACTERISTICS n Germ cells-carry on to future generations n Somatic cells-effect only occurs once n undesirable n probably cumulative n threshold n effects are not specific to radiation

TARGET THEORY ADDRESSES LETHAL DAMAGE THERE ARE AREAS ON THE DNA CHAIN THAT, IF DAMAGED, ARE LETHALTO THE CELL

UNDERSTANDING THE “TARGET” n CELLS ARE MADE UP MOLECULES n OVERABUNDANCE OF MOLECULES n CRITICAL MOLECULE n “WHEN RADIATION INTERACTS WITH OR NEAR A CRITICAL MOLECULE, THIS SENSITIVE AREA IS CALLED ‘TARGET’.” n NUCLEUS VS CYTOPLASM

Principles of Radiobiology based on n Random nature of radiation interactions n how radiation interacts at cellular level n factors that can alter these interactions

NOW ON TO THE THEORIES n DNA SEQUENCE VITAL-DAMAGE TO ANY TARGET(SEQUENCE) WILL KILL THE CELL n NO SINGLE SEQUENCE IS IMPORTANT-2 TO 3 TARGETS (SEQUENCES) NEED TO BE HIT

n TARGET ONLY NEEDS ONE HIT TO CAUSE FETAL DAMAGE n TARGET NEEDS TO BE HIT TWICE OR MORE TO CAUSE DAMAGE n

Radiation damage n Cannot be distinguished from other sources (chemical, heat or trauma)