Ans. c (Gray) (Ref. Grainger Radiology 4th/232)The old unit of radiation absorption is rad & new (SI) unit is gray.Radiation unitsThe activity of a radioactive material is the number of nuclear disintegration per unit of time. The unit of activity is a becquerel (Bq); 1 Bq is equal to 1 disintegration per second. Formerly, the unit of avtivity was curie (Ci) and 1 Bq corresponds approximately to 27 picocuries.The potency of radiation is measured in THREE (three)ways (1) Roentgen: Roentgen is the unit of exposure. It is the amount of radiation absorbed in air at a given point, i.e., number of ions produced in 1 ml of air. (2) Rad: Rad is the unit of absorbed dose. It is the amount of radioactive energy absorbed per gram of tissue or any material. 1 mrad = 0.0001 rad (3) Rem: Rem is the product of the absorbed dose and the modifying factors. The rem indicates the degree of potential danger to health. The radiation to which the average citizen is expected is made up almost of the fast moving, highly penetrating X-rays and gamma rays, where rem and rad are equal (2).The radiation units (viz roentgen, rad rem) are replaced by the new SI Units (International System of Units) which are: (a) Coulomb per kilogram (C/kg) replacing the reontgen. 1 roentgen is equal to 2.58 x 10'4 C/kg1. It is the unit for exposure. There is no special name for this, (b) Gray (Gy) replacing the rad. It is the unit of absorbed dose, defined as the dose of ionizing radiation that impart 1 joule of energy to 1 kg of absorbing material. 1 rad is equal to 0.01 Gy. (1), and (c) Sievert (Sv) replacing the rem. It is the SI unit of dose equivalent. The dose equivalent of 1 sievert is equal to 100 rems.Dose equivalent (H): As all types of radiation do not produce the same biological effect per unit of energy absorbed, the concept of dose equivalent has been introduced. The dose equivalent, M (Sieverts) is equal to the absorbed dose, D (grays), M (Sieverts) is equal to the absorbed dose, D (grays), multiplied by a quality factor Q which depends upon the density of ionization produced in the tissue by the radiation.H = DQThe factor Q for X-rays and Y-rays and electrons is equal to 1, whereas for a particle it is 20 (3). ARADIATION UNITSConventional UnitsNew SI Units:These are such units as the "three Rs" & "C": 1) Roentgen (Unit of exposure),(1 mrad = 0.001 rad)2) Rad (Unit of absorption)3) Rem (Unit of equvivalent dose)4) Curie (Unit of activity formerly) (1 Bq = 27 picocuries)1) Coulomb/kg - Replaced Roentgen2) Gray - Replaced Rad (1 rad = 0.01 Gy)3) Sievert - Replaced Rem (1 Sv = 100 rem)4) Becquerel-Unit of activity(lBq=l disinteg/sec)A. Radioactivity# The number of times each second a radioactive material decays and releases radiation. The old unit is curile curie & new unit is Bq.B. Absorbed Dose# The amount of radiation energy absorbed into a given mass of tissue.# Absorbed dose is the radiation quantity used to express the concentration of radiation energy actually absorbed in a specific tissue. This is the quantity that is most directly related to biological effects.# Dose values can be in the traditional unit of the rad or the SI unit of the gray (Gy).# The rad is equivalent to 100 ergs of energy absorbed in a gram of tissue and the gray is one joule of energy absorbed per kilogram of tissue.1 gray (Gy) = 100 rads10 mGy = 1 rad1 mGy = 100 mradC. Dose (Equivalent)# Measures the energy per unit mass times adjustments for the type of radiation involved (quality factor) and the biological response in the tissue (a weighting factor).# Equivalent dose converts dose into an estimate of risk.# They are quantities that can be measured and expressed in terms of the more fundamental physical quantities like energy. Dose Equivalent, in the unit, sievert (Sv), is a quantity that expresses the relative biological impact of the radiation by including a radiation weighting factor (wR).# The relationship is: Dose Equivalent (Sv) = Dose (Gy) x wR. The value of the radiation weighting factor (wR) is a characteristic of each specific type of radiation.D. Effective Dose# It is actually a simple and very logical concept. It takes into account the specific organs and areas of the body that are exposed.# The point is that all parts of the body and organs are not equally sensitive to the possible adverse effects of radiation, such as cancer induction and mutations.# For the purpose of determining effective dose, the different areas and organs have been assigned tissue weighting factor (wT) values.# For a specific organ or body area the effective dose is: Efective Dose (Gy) = Absorbed Dose (Gy) x wTEducational PointsCELL CYCLE AND RADIATION# G2-M interphase is most radiosensitive. (G2M interphase > M > G2)# The cell is vulnerable to radiation in the stage of mitosis (M), less so during synthesis (S) and relatively insensitive during resting periods.# 'S' phase is the most radioresistant phase of cell cycle.# But, 'S' phase is the most chemosensitive phase of cell cycle.# Radiosensitivity of a cell also depends on its histological type and oxygenation of the tissues.RADIOBIOLOGY# Four important processes that occur after radiation exposure can be summarized as the ' four R's" of radiobiology- The first is repair. Repair is temperature dependent and is thought to represent the enzymatic mechanisms for healing intracellular injury.- The second R is reoxygenation, a process whereby oxygen (and other nutrients) are actually better distributed to viable cells following radiation injury and cell killing.- The third R is repopulation, the ability of the cell population to continue to divide and to replace dying and dead cells.- The fourth R is redistribution, which reflects the variability of a cell's radiosensitivity over the cell cycle.