Week 7 B Chapter 34 & 35 Fundamentals of Radiobiology 80

Week 7 B Chapter 34 & 35 Fundamentals of Radiobiology 80

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Chapter 34 Principles of Radiobiology In 1906, two French scientists, Bergonie and Tribondeau, theorized and observed that radiosensitivity was a function of the metabolic state of the tissue being irradiated. There observations became the Law of Bergonie & Tribondeau
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Law of Bergonie & Tribondeau Stem cells are radiosensitive; mature cells are radio-resistant. Younger tissues and organs are radiosensitive. Tissues with high metabolic activity are radiosenstive. High proliferation rate for cells and high growth rate for tissues result in increased radiosensitivity.
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Law of Bergonie & Tribondeau Basically it states that radiosensitivity of living tissue varies with maturity and metabolism. In diagnostic imaging the law serves to remind us that a fetus is considerably more sensitive to radiation exposure than a child or mature adult.
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Physical factors Affecting Radiosensitivity When tissue is irradiated, the response of the tissue is determined principally by the amount of energy deposited per unit of mass: the dose in Rads (Gy). Even under controlled conditions, the response to like exposures may be different.
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Physical factors Affecting Radiosensitivity Physical property factors L inear e nergy t ransfer ( LET ) R elative B iological E ffectiveness ( RBE ) Fractionation and Protraction
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Linear Energy Transfer The rate which energy is transferred from ionization to soft tissue is the Linear Energy Transfer (LET). It is another method of expressing radiation quality and determining the value of the tissue weight factor used in radiation protection. It is expressed in the units of kiloelectron volts of energy transferred per micrometer of track length in soft tissue.
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Linear Energy Transfer The ability of ionizing radiation to produce a biologic response increases as the LET of the radiation increases. The LET of diagnostic x-rays is approximately 3 keV/µm.
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Relative Biologic Effectiveness As the LET of the radiation increases, the ability to produce biologic damage also increases. This quantification is referred to as the Relative Biologic Effects (RBE). The RBE of diagnostic x-ray is 1. Radiations with a lower LET will have a RBE of less than 1. Radiations with a higher LET will have a RBE greater than 1.
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The LET & RBE of Various Radiations Type of Radiation LET RBE 25 MV x-rays 0.2 0.8 60 Co rays 0.3 0.9 1MeV electrons 0.3 0.9 Diagnostic X-ray 3.0 1.0 10 MeV protons 4.0 5.0 Fast Neutrons 50.0 10 5 MeV Alpha Particles 100.0 20
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LET & RBE Graph As the LET increases, the RBE also increases but a maximum level is reached followed by a reduction due to overkill.
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Fractionation & Protraction If the dose is administered over a long time rather than quickly, the effects of that dose will be less.
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