year 4 day 2 p. 1 - radiation biology - syllabus, IV rok, IV rok CM UMK, Radiologia, 2014-15

//-->.pos {position:absolute; z-index: 0; left: 0px; top: 0px;}10/15/2012Day 2 p. 1RADIOLOGY ANDIAGNOSTIC IMAGINGDr. Zbigniew Serafin, MD, PhDserafin@cm.umk.plRadiation Biology and Radiation Protectionmainly based on:C. Scott Pease, MD, Allen R. Goode, MS, J. Kevin McGraw, MD, Don Baker, PhD, John Jackson, MA, Spencer B. Gay, MD: Basic Radiobiology.Radiation BiologyThe core of an atom exists precariously: massive repulsiveelectromagnetic forces between closely-assembled protons in thenucleus must be counterbalanced."Stability" thus reflects the balance of power between strong nuclearforce, weak nuclear force, and electromagnetic force. The nuclearbinding energyquantifies the energy necessary to maintain coherence.Isotopesare atoms with the sameatomic number(proton count) butdifferentatomic masses(number of neutrons). Heavier elements aremore likely to have binding energies insufficient to maintain a stablenuclear configuration. Such radioisotopes may undergo decay byemission ofenergetic quanta.110/15/2012Radiation BiologyAlpha particleslarge and positively chargedtend to cause ionizations and lose energy over a very short distanceare composed of two protons and two neutrons (i.e. a naked helium nucleus)large size & relatively high charge prevent deep penetration of matter(blocked by dead skin or paper)chronic exposure to inhaled alpha particles is a lung cancer riskare important in the uranium decay series, of which radon is a productRadiation BiologyNeutronsuncharged particlesmay carry significant kinetic energy ("Fast Neutrons")may collide with a nuclear proton, causing its ejectionproduce biologically-important ionizations and excitations due to suchcollisionsare often produced as part of fussion reactionsRadiation BiologyBeta Particlesare smaller and less energetic than alpha particleshave a negative chargeare created when a neutron transmutates into a protonare emitted during decay of iodine-131, phosphorus-32, carbon-14, andstrontium-90210/15/2012Radiation BiologyGamma rays and X-rays (photons)represent pure electromagnetic energyprogress at the speed of lighthaving no mass or charge, are neither attracted to nor repulsed by chargedparticlesgamma-rays originate from the nucleus, usually carries higher energies thanX-raysX-rays originate from electron cloudsRadiation BiologyNOTE:regardless of the type of energy carrier or the specific type of energy-matter interaction, biologic hazard ultimately results from:i.atomic ionizations (loss of one or more electrons→positively-charged ion)ii.excitations induced by electromagnetic radiation from many sources,including radiologyPhotons interact with subatomic structures in one of the following three ways:Photoelectric absorptionCompton ScatterPair productionThe particular type of interaction reflects probability statistics based on boththeenergyof the photon and theatomic numberof the traversed atom. Formost tissues of the body, average atomic number does not vary greatly –though cortical bone has the highest effective atomic number.Radiation BiologyLinear Energy Transfer– the amount of energy transferred to the matter inthe form ofionizationsandexcitations.LET indicates the potential forbiologically important damage from radiation.LET can be thought of in two ways:an average energy for a given path length traveledoran average path length for a given deposited energy.The standard unit of measure is keV/um.310/15/2012Radiation BiologyIonizationslead to chemical changes:Free radical productionBroken bonds, importantly double-strand DNA breaksSince the intracellular environment is essentially aqueous, water is the mostlikely molecule encountered by radioactive energies. Radiolysis of water mayproduce H·, OH·.Damage caused by such free radicals represents the INDIRECT action ofionizing radiation. Most biological effects of low LET radiation can beattributed to free radicals.Less commonly, nucleoproteins or DNA may be ionized directly by chargedparticles, but not electromagnetic radiations.Radiation Biologyionizing radiationsfree radicals(indirect effect)ionizations(direct effect)changes in configurationof DNA macromoleculesinterference with DNAstructure or replicationRadiation BiologyCell deathis operationally defined as loss of function, such as reproductivecapacity for stem cells or synthesis of some specific product (enzyme, hormone).Apoptosisis the process of programmed cell death – biochemical pathwayswithin a cell leading to its own organized dismantling.When DNA is damaged and not successfully repaired, the cell may die – celldeath may occur immediately (interphasedeath)or during its attempt todivide (mitoticdeath)or after a few cell divisions (abortivecolonies).410/15/2012Radiation Biologyinterference with DNAstructure and functionchromosome breakagegene mutationcell cycle influenceeffect on cell multiplicationdivision delaytissue effects(reduced growth, abortivecolonies, degeneration)interphase deathRadiation BiologyRadiation BiologyDq – quasi-threshold doseor sub-lethal dose (SLD)most radiosensitive phases:G2-phase and mitosis (M-phase)least radiosensitive phase:latter part of S-phase (synthesis of DNA)5

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