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Medical Physics

Medical Physics. Chapter (4) Radiation Protection. Radiation protection in medicine. About 20% of background radiation comes from natural radioactivity in our bodies primarily potassium 40 ( 40 K) . Since a sizable amount of this radiation comes from

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Medical Physics

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  1. Medical Physics Chapter (4) Radiation Protection

  2. Radiation protection in medicine • About 20% of background radiation comes from • natural radioactivity in our bodies primarily • potassium 40 (40K). • Since a sizable amount of this radiation comes from • the soil and building materials, it should not be • surprising that background radiation varies with • geographical location, There are a few places, • including one area in Brazil and one area in India, • where the background radiation is a factor of 3 to 40 • times greater than average. • Typically,30 to 40% of the background radiation comes • from cosmic rays from outer space.

  3. The atmosphere acts as a shield to absorb some of this • radiation as well as hazardous components of the • ultraviolet spectrum. • At high elevations some of this protection is lost, and • cities such as Denver (≈ 1000 m) and Mexico City • (≈2300m ) have a higher than average background due • to this effect. • The early background at 3000 m elevation is about • 20% higher than that at sea level.

  4. Onesourceofnaturalradiationistheairwe • breathe.Radongas,oneoftheradioactivedaughter • productsoftheradiumfamily,ispresentintheair.It • decayswithahalf-lifeofaboutdaysintoasolidthatis • depositedonanysurfacethatishandy,suchasdust • particles. • Aswebreathe,someoftheseradioactivedust • particlessticktotheliningofthelungs,andthusthe • lungsreceivemoreradiationthantherestofthe • body.

  5. Ontheaverage,theannualdoseequivalenttothe • lungsisaboutninetimestheamounttotherestofthe • body. • Theamountofradonintheairinsideabuilding • dependsonthecompositionofthebuilding. • Theradiationtothelungsduetoradoninawooden • buildingishalfthatinabrickbuildingandone-third • thatinaconcretebuilding.

  6. Tobaccoleavescollectradioactivitywhiledrying.Ifyou • areacigarettesmoker,thesmokethatenteryour • lungsmaygivethemuptofivetimestheradiationa • nonsmokerreceives.Thismaybeacontributingfactor • tolungcancerincigarettesmokers. • Medicalradiationexposurescomefrom • therapeuticand diagnosticusesofX-rays, and • diagnosticusesofradioactivity(nuclearmedicine),

  7. BiologicalEffectsofIonizingRadiation • Thebiologicaleffectsofionizingradiationareoftwo • generaltypesSomaticand Genetic. • Somatic effectsaffectanindividualdirectly(lossof • hair,reddeningoftheskin,etc.),whilegenetic effects • consistofmutationsinthereproductivecellsthataffect • latergenerations. • Sincegeneticeffectsoccuronlywhenreproductivecells • areirradiated,thegonadsالغدد التناسلية و الخلاياshould • beshieldedduringX-raystudieswhenpossible.

  8. In order lo evaluate the genetic effects of X-rays on the • population, the concept of Genetically Significant Dose • (GSD) is useful. • The GSD due to an exposure depends on: •  1- the dose to the individual's ovaries المبايض or testes •  2- the individual's age, which determines the probability • of that person becoming a parent in subsequent years.

  9. ThusX-rayingwomenover50yearsold,whonormally • havelittlechanceofhavingbabies,contributesيساهم • littletotheGSDofthepopulation. • Exposureofthereproductiveorgansofchildrenresults • inthemaximumcontributiontotheGSDofthe • population,sincetheirpotentialforproducing • offspringذريهisatmaximum.

  10. AnX-raythatincludesthetestes,whichhave little • shieldingtissue,resultsinalargergeneticdosethana • similarX-raythatincludestheovaries. • Ontheotherhand,itiseasytoshieldthetestes • withoutlosinganydiagnosticinformation,while • thisisnottruefortheovaries. Somaticeffectsdependon 1-theamountofradiation, 2-thepartofthebodyirradiated • 3-theageofthepatient. • Ingeneral,theyoungertheperson,themore • hazardous مصدرللخطر theradiation.

  11. In fact, the most dangerous period to receive radiation • is before birth. At certain periods in the development of • the fetusالجنين , radiation can produce deformitiesتشوه . • Much information on the effects of radiation on various • organs has been obtained as a product of the treatment • of cancer with radiation. • It is usually the somatic effects of radiation therapy to • normal tissues near the tumor that limit the amount of • radiation that can be given to the tumor.

  12. Someofthecommonsomaticeffectsofradiationare • Reddeningoftheskin(erythema) • Lossofhair,ulceration تقرح • Stiffening(fibrosis تليف )ofthelung • Formationofholes(fistulasناصور )intissues • Reduction of white blood cells (leukemia), and • Induction of cataracts in the eyes. • Perhaps the most feared somatic effect of radiation is • carcinogenesis, the induction of cancer. It has been • found that radiation can induce many types of cancer • besides skin cancer. • Radiation to the thyroid has caused thyroid cancer, • and radiation to the blood-forming organ (bone • marrow) has caused blood cancer, or leukemia.

  13. Radiation Protection Units and Limits • The roentgen (R)is a unit for measuring exposure of • X-rays orgamma rays. • The units of radioactivity are the becquerel (Bq) and • the curie (Ci) • The units, for absorbed dose, the gray (Gy) and the rad. • The rem (RadEquivalent Man), which is used in • radiationprotection. • Fortunately, it is simply related to the rad, and for most purposes rads and rems are numerically equal. • The rem is a unit for the quantity dose equivalent • (DE). • Definition of Dose Equivalent (DE): The DE is • defined as therads times the quality factor (QF) of • the radiation, or DE = (radsXQF)

  14. The QF takes into account the increased • damagedone by certain types of radiation. • A radof densely ionizing radiation (protons& • Neutrons)does much more damageto a cellthan a • rad of X-rays, gamma rays,or beta rays and is assigned • a larger QF. • The QF for X-rays, gamma rays, and beta rays • (electrons) is unity so for these common types of • radiation rads equal rems. Also, for X-rays and gamma • rays the exposure in roentgens is almost equal to the • dose in rads in soft tissue.

  15. Thus for the most important man-made radiations • roentgens, rads, and rems ran be considered to • have the same values. • Federallawsrestricttheamountofman-maderadiation • tothepublic(excludingmedicalX-rays)toanaverageof • 0.17rem/year. • Individualnumbersofthepublicmayreceiveupto • 0.5 rem/year,andradiationworkersareallowed • 5rems/year.

  16. Thesevaluesapplytothereproductiveorgansandthe • eyes.Lessradiation-sensitiveorganssuchasthehands • andfeetareallowedmuchmore,upto75rems/yearfor • workers. • Thesevalues,referredtoastheMaximumPermissible • Doses(MPDs).Theyhavebeenthesubjectofmuch • controversyجدالsincethereisnoassurancethatthese • amountsaresafe.

  17. TheMPDshavebeenjudgedbyradiationexpertstobe • acceptablerisks.Therehasbeenatendencytoreduce • theMPDswithtime. • Forexample,theMPDtothepopulation السكانfroma • nuclear powerplantisnowonly0.005rems/year. • Sincebackgroundradiationistypically0.125 • rem/year,variationsinbackgroundinanareawitha • nuclear-powerplantمنشائات الطاقة النووية mayexceedthe • MPD.

  18. Recommendedmaximumpermissibledoses(MPDs)forradiationworkersandthegeneralpopulation,excludinginternationalmedicalexposures.Recommendedmaximumpermissibledoses(MPDs)forradiationworkersandthegeneralpopulation,excludinginternationalmedicalexposures.

  19. Radiation Protection in Radiation Therapy • Sincetheradiationtherapyareaofahospitalcontains • intenseradiationsources,itistypicallysurroundedby • concretewallsabout0.5mthick.Also,thescattered • radiationduringaradiotherapytreatmentislarge. • Toprotectindividualswhomightinadvertently • enterيدخل بشكل غير مقصود theroomduringa • treatment,thedoorhasaswitch(interlock)that • turnsoffthemachinewhenthedoor isopened. • Theradiationsourcesthemselvesmustbeadequately • shielded.Wecancalculatetheamountofshielding • thatisneededtoreducethegammaradiationfroma • 60Cosourcetoasafelevel.

  20. Wefirstexplainhowtocalculatetheexposurerate • fromthegammaraysfromaparticular radionuclide • source. • ThebasicequationforIγ,theradiationintensityin • roentgensperhour,is Iγ=ГA/D2 • whereDisthedistanceinmetersfromasourceofA • megabecquerelsorcuriesofradioactivityandГ • (gamma)isaconstantfortheparticularradionuclides • thatdependsonthe numberandenergiesofthe • gammaraysemittedperdisintegration(e.g.3.5x10-5 • Rm2/MBqhfor60Co).

  21. The equationtellsusthattheradiationatagiven • distancefroma radioactivesourceisproportionalto • theintensityofthesourceandinverselyproportionalto • the distancesquared. • Usingthisequationwecancalculate,forexample,that • theradiationintensityat1mfroma185TBq(about • 5000.Ci)60CoTeletherapysourceisabout 6000R/h.

  22. Theamountofshieldingthatisnecessarytoreducethe • radiationtoasafe-. (2mR/hat1m)-canbecalculated • inseveralways. • Perhapstheeasiestinvolvesusingthethicknessoflead • thatisahalf-valuelayer(HVL)fortheradiationinvolved • Eachhalf-valuelayerreducestheintensitybyafactorof • 2.Thususing5HVLsreducestheintensitybyafactorof • 25,or32,andusing10HVLsreducestheintensitybya • factorof210,or1024.

  23. once you know by what factor you wish to reduce the • radiation intensity, you can simply calculate the number • of half-value layers are necessary • If the source is doubled the amount of shielding is not • doubled but an additional half-value layer must be • added • an alternate way to calculate the thickness of the • shielding is to use the equation for exponential • attenuation • I = Io e - µx • where I is the intensity after passing through a thickness • x , Io is the initial intensity at the same point without • any absorber e which is equal to 2.718 is the base of • the natural logarithm

  24. and µ is the linear attenuation coefficient, • which is related to the half-value layer by the equation • µ = 0. 693/HVL • This equation can be rewritten in a more convenient • form to calculate shielding • X = 2.3log(I/I0)/ µ

  25. Thenuclearmedicinephysicianshould: 1-Determinewhetherthestudyisnecessaryضروري ordesirable (إختيارى) . 2-Usetherightradiopharmaceutical;sometimes differentradiopharmaceuticalsareinsimilar containers. 3-Usetherightamountoftheradiopharmaceutical; for99mTcit isnecessarytohavea calibrationdevice. 4-Givetheradiopharmaceuticaltotherightpatient;ina busydepartmentitiseasytomakemistakes. 5-Makesurethedetectionequipmentisworking correctly;therearestandardtestproceduresfor gammacamerasandscanners.

  26. Inanuclearmedicinelaboratorythe • containersofradioactivityaregenerally • placedbehindawallof5cmthickleadbricks. • Thereisusuallyalaboratorymonitorinthe • workareatoindicate,theradiationlevelona • meterandalsotogiveanaudiblesignalofthe • radiationlevel. • Itisalsocustomarytoplacefilmbadgesata • fewplacesinthestoragearea("hotlab")to • monitorradiationlevels.

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