Growth in the Chicken after High-Rate and Low-Rate Cobalt-60 Gamma Irradiation

Abstract

Injury from exposure to ionizing radiation is reflected in subsequent growth. However, few attempts to quantitate the relative influence of dose and period of exposure on gross weight have been made. Studies on the chick show that, for acute mortality, the effectiveness of a single radiation dose decreases with increase in exposure time (1, 2). In these investigations, the exposure period was varied from a few minutes to 24 hours, and the amount of lethality occurring within 30 days served as the measure of radiation injury. It was found that half of the irradiated chicks died within 30 days when subjected to a single dose of approximately 920 R of Co60 y-rays delivered in 3 to 30 minutes, whereas with doses protracted over 24 hours 1620 R was required to achieve the same mortality. The survivors of an acute irradiation have sustained some injury which can be assessed with other measures of damage. We address ourselves to the question: When newly hatched chicks are irradiated, is loss in dose effectiveness with protraction measurably reflected in postirradiation growth? This report describes the accretion in weight of chicks that survive at least 30 days after a single acutely lethal exposure to Co60 y-rays. Exposure was either completed in a few minutes or extended uniformly over 24 hours. Differences between the weight-age patterns under these two exposure conditions are observed and are used to deduce a measure of the change in dose effectiveness. The statistical approach is based on the assumption that the weight-age dependence is the resultant of two deterministic processes, one characterized by a Gompertz function (3) and the other by a linear function. In 1926, Wright (4) pointed out that the growth of individual organisms could be represented by a Gompertz function. If growth is uninhibited, an exponential increase in weight with age is expected; that is, the rate of increase in weight is a constant proportion of the weight attained. Usually the magnitude of this proportion is not constant, but decreases with age at an approximately uniform percentage rate. This conceptual structure