Rad T 110 Sherer Biologic Effects of Radiation Exposure

Energy Transfer Determinants Linear energy transfer –LET Relative biologic effectiveness –RBE Oxygen enhancement ratio –OER

Low energy LET X-rays and gamma rays Penetrating and randomly interactive Most damage is thru indirect effect and is sublethal; therefore repairable

High energy LET Particulate radiation is the most common form of this Typically have both mass and electrical charge More destructive than low LET radiation

Interaction with DNA Low LET x-rays have a low probability of interacting with DNA. –Most damage is indirect –Damage can usually be repaired High LET particles have a higher probability of interacting with DNA and damaging it –Non-repairable

RBE Takes into account specific cells and tissues For diagnostic radiography weighting factors are used.

OER OER is less with high LET radiation (particles) –Most damage is direct OER is higher with low LET x-rays –Oxygen is producing peroxides

Direct Effect The incoming photon strikes the DNA molecule directly causing ionization of the DNA.

Radiolysis of Water The photon strikes a water molecule and creates an ion pair. –H +, hydrogen ion –OH -, hydroxyl ion –H *, hydrogen radical –OH *, hydroxyl radical

Free radicals Contain an unpaired outer shell electron and are unstable and reactive They can also travel and transfer energy (damaging) other cells

More bad news Combining of free radicals into caustic agents –OH * + OH * = H 2 O 2 Hydrogen peroxide –O 2 + H * = HO 2 * Hydroperoxyl radical

Indirect Effect The previously mentioned agents produce indirect effects because the damage is not caused by the radiation but the indirect effect of the radiation.

Effects on DNA Single strand break –Point mutation, commonly occur with low LET Easy type of damage to repair Read about the other types of effects

Target Theory Basically, the DNA molecule is the most important molecule Therefore, if the DNA molecule is hit and inactivated the cell will most likely die

Cellular Effects of Irradiation Instant death –1000 Gy in a few seconds Reproductive death at the cellular level –10 Gy Apoptosis –Cells die without attempting cell division

Cellular Effects of Irradiation Mitotic death –Cell death occurs after cell division Mitotic delay –Mitosis occurs after missing a division cycle Loss of function Chromosome breakage –Can lead to genetic effects Changes are passed down

Cell Sensitivity Immature cells –Non-specialized and rapid cell division –Both conditions that encourage radio- sensitivity

Lethal Dose Humans –300 – 400 rads, 3 – 4 grays Expressed as LD 50/30 in most mammals or LD 50/60 in humans –The first number is percentage of effect (death) second number is how many days for the effect to occur

Dose-Response Curves Linear or non-linear Threshold or non-threshold Type of curve depends on the effect being observed or reported

Radiation Protection Based on a linear non-threshold curve Any radiation has the potential for problems This is the most cautious curve.

Deterministic or Non-Stochastic Effects These are based on a threshold curve and can be linear or non-linear.

Early non-stochastic effects Occur within weeks of exposure –Nausea –Fatigue –Erythema –Epilation –Symptoms of these effects Generally, classified as Acute Radiation Syndromes

ARS Stages –Prodromal Symptoms will be present –Latent Symptoms will resolve or be dormant –Manifest You got problems

Types of ARS Hematopoietic –100 – 1000 rads GI –600 – 1000 rads Cerebrovascular –Up to 5000 rads Death in 2 to 3 days

Late Non-stochastic Effects Cataracts Fibrosis Organ atrophy –Loss of parenchymal cells Decreased fertility –sterility

Late Stochastic Effects Cancer Genetic defects

Embryologic Effects Most sensitive during organogenesis –10 days to 6 weeks –2 weeks to 8 weeks