Xrayfund
7/22/99
From Molecules to Humans
· All x-rays are harmful
· Effects result from atomic interactions
o Ionization
o Electron excitation
· Chemical binding properties change
· Molecule may break, function improperly, or cease to function
· At any stage it is possible to recover from radiation damage
Human Biology
· Atomic composition of the body determines the type of radiation interaction
· 85% of the body is hydrogen and oxygen
· Five types of molecules in the body
o 80% water
o 15% protein
o 2% lipids
o 1% CHO
o 1% nucleic acid
o 1% other
Law of Bergonie and Tribondeau
· 1906, two French scientists
· Radiosensitivity is a function of the metabolic state of the tissue being irradiated
Physical Factors Affecting Radiosensitivity
· Linear energy transfer (LET)—should be high
· Relative biologic effectiveness (RBE)--
· Fractionation and protraction
o For lung and prostate tumors approximately 6800-7000R/case
o 180-200 R/treat
Biologic Factors Affecting Radiosensitivity
· Oxygen effect
o Oxygen enhancement ratio (OER)
· Age
o Most sensitive before birth
o In old age we become more radiosensitive
· Sex
o Females are less radiosensitive than males
· Recovery
Radiation Dose-Response Relationships
· One of these graphs will be on the final
· Linear, nonthreshold
o Used by Beir committee for risk estimates and protection guidance
o 0 radiation—no one exposed, no cancer
o A little radiation—a little cancer
o More radiation—more incidence of cancer
o They decide how much lead in the walls
· Linear, threshold
o Can increase the dose to a certain point w/no increased incidence
o At a certain point, incidence increases
· Nonlinear, nonthreshold
o Starts at zero
o W/little exposure see lots of cases
o
Levels
off at some point
· Nonlinear, threshold
o Doesn't start at 0
Molecular/Cellular Radiobiology
· Irradiation of macromolecules
o Main-chain scission
o Cross-linking
o Point lesions
Distance
· Better to take a picture at 72" than 40"—dose pt slightly less
· Time doesn't matter if using the same mAs
· Put operator as far from the x-ray source as possible (inverse square law)
· Radioactive—as far away as possible and up wind
Shielding
· Can't shield pt in the area that you need to see
Badges
· Name badge--$12/yr/person
o Film in badge that can rate A, B, g radiation
o Higher optical density, more exposure
o Processing like dental films
o Least expensive way
o Not the most effective
· Lithium fluoride crystals—TLD (thermal luminescent dosimeter)
o Heat crystals to 400°
o Lease stored energy as light
o Comparable to amount of exposure
o Not affected by heat of the user
o Encapsulated so water and humidity do not affect
o Can be physically lost
o $2/person/badge--$24 a year
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