Lecture_3.pptx

Radiobiology & Protection RMI 212 Lecture 3 Radiation Response Slide 1 fchs.ac.ae  Physical and biological factors affecting cell radio sensitivity (radiation response): The factors affecting cell response include 1. Linear Energy Transfer (LET), 2. Re...

Radiobiology & Protection RMI 212 Lecture 3 Radiation Response Slide 1 fchs.ac.ae  Physical and biological factors affecting cell radio sensitivity (radiation response): The factors affecting cell response include 1. Linear Energy Transfer (LET), 2. Relative Biologic Effectives (RBE), 3. Oxygen Enhancement Ratio (OER) 4. Age fchs.ac.ae  Linear Energy Transfer (LET) LET describes a measure of the rate at which energy is deposited as a charged particle travels through matter. LET is described in terms of KeV/micrometer. As electromagnetic radiation (x- and gamma- rays) considered as low LET radiation. Compared with EM, particular radiations (for example, alpha particles and neutrons), which are highly ionizing and have large mass and charge, are more likely to interact with tissues. Alpha and neutron radiation are considered high LET radiations. The higher the LET radiation, the greater the chance for a biologic interaction. Diagnostic X-rays, which have an LET of 3KeV/mm, are consider to be low when compared with all radiations. fchs.ac.ae Relative Biologic Effectives (RBE) The relative effect of LET is described by the RBE. RBE is a comparison of dose of test radiation to a dose of 250 KeV X-ray that produces the same biologic response. The constant is the biologic response not the radiation dose. RBE= Dose in rads from 250 KeV X-ray necessary to produce a given effect Dose in rads of test of radiation necessary to produce the same effect fchs.ac.ae  The RBE measures the biologic effectiveness of radiations having different LETs Factors that influence RBE include radiation type, cell or tissues type and the radiation dose rate In comparing LET and RBEs, as LET increases, RBE increases also. Diagnostic X-ray are considered to have an RBE of approximately 1. Course: FRD1011 Radiologic Physics & Radiation Protection Lecture 1: Essential Concepts of Slide 5 fchs.ac.ae  LET RBE fchs.ac.ae Oxygen Enhancement Ratio (OER) The response of biologic tissue to radiation is greater when irradiated in the oxygenated state than when irradiated in anoxic or hypoxic conditions. This is known as oxygen effect. OER depends on LET. The OER is most noticeable for low LET radiation, and is less effective with high LET radiation. Because of the physical differences between high and low LET radiation, the quantity of damage done by high LET radiation would be beyond repair. Thus, having oxygen present would not intensify the response to radiation to the same magnitude, as would be the case with low LET radiation. fchs.ac.ae LET OER fchs.ac.ae  Age Age influences radiosensitivity. We are most radiosensitive before birth (Fetus). Radiosensitivity declines until maturity, the time when we are most radioresistant. With old age humans again tend to become more radiosensitive. These concepts are consistent with the Law of Bergonie and Tribondeaue. fchs.ac.ae Sensitivity of radiation with age fchs.ac.ae  SEX The female is more radioresistant in some species possibly due to high levels of estrogens, some of which have radioprotective properties. fchs.ac.ae  Cell cycle The generation time from one cell division to the next, known as the cell cycle, is dependent on species, tissue type, age, and environmental influences. fchs.ac.ae Cell cycle fchs.ac.ae  Cell Cycle Cell radiosensitivity depends upon what part of the cell cycle the cell is in. Mitosis, and the passage from the late G1 into early S-phase, are judged the most radiosensitive phase of the cell cycle. Late S-phase is considered to be the most radioresistant cell cycle phase. fchs.ac.ae Cell Cycle Experiments have determined that the nucleus of a cell is considerably more radiosensitive than is the cytoplasm of the cell. DNA is the most radiosensitive part of the cell. RNA radiosensitivity is intermediate between that of DNA and protein. Chromosome produced radiation damage can be analyzed during the metaphase portion of the cell cycle. fchs.ac.ae  Cellular Effects of Irradiation. Damage to the cell’s nucleus reveals itself in one of the following ways… 1. Instant death. 2. Reproductive death. 3. Mitotic, or genetic, death. 4. Mitotic delay. 5. Interference of function. 6. Chromosome breakage. fchs.ac.ae  Instant Death. Instant death of large numbers of cells occurs when a volume is irradiated with an x-ray or gamma-ray dose of about 1000Gy (100,000 rads) in a period of seconds or a few minutes. Radiation doses high enough to cause this damage are greater than those used in diagnostic or even therapeutic treatments. fchs.ac.ae  Reproductive Failure Reproductive death generally results from exposure of cells to doses of ionizing radiation in the range of 1 to 10 Gy (100 to 1000 rads). The cell loses its ability to multiply but does not die. fchs.ac.ae  Mitotic Death. Mitotic, or genetic, death occurs when the cell dies after one or more divisions. Even relatively small doses of radiation can cause this type of cell death. The radiation dose required to produce mitotic death is less than the dose needed to produce slowly dividing cells or nondividing cells. ( delay) fchs.ac.ae Mitotic death NORMAL IRRADIATED fchs.ac.ae  Mitotic Delay. Exposing a cell to as little as 0.01 Gy (1 rad) of ionizing radiation just before it begins dividing can cause mitotic delay, the failure of the cell to start dividing on time. After this delay, the cell may resume its normal mitotic function. fchs.ac.ae  Interphase death In many cell types, radiation induced damage results in cell death only if the cell goes into mitosis,, death occurs in interphase most probably due to radiation induced membrane damage. fchs.ac.ae fchs.ac.ae Law of Bergonie’ and Tribondeau fchs.ac.ae Law of Bergonie’ and Tribondeau  It was developed in 1906 by two French radiobiologist, Bergonie’ and Tribondeau  about the relative sensitivity of two different types of cells or tissues to radiation fchs.ac.ae  Law of Bergonie and Tribondeau By 1906 Bergonie and Tribondeau realized that cells were most sensitive to radiation when they are:  Rapidly dividing  Undifferentiated  Have a long mitotic future fchs.ac.ae  LAW The OF BERGONIE law of Bergonie andAND TRIBONDEAU Tribondeau states that : 1. Immature cells are more radiosensitive than mature cells. 2. Younger tissues and organs are more radiosensitive than older tissues and organs. 3. The higher the metabolic cell activity, the more radiosensitive it is. 4. The greater the growth rate for tissues, the greater the radiosensitivity. This law concludes that compared to a child or mature adult, the fetus is most radiosensitive. fchs.ac.ae