Biological Response to Small Discrete Highly Radioactive Sources II. Morphogenesis of Microlesions in Rat Lungs from Intravenously Injected 238PuO2 Microspheres

Evaluation of risk from high specific-activity particulate nuclear fuels, being used in thermoelectric generators, by the organ-mean dose concept seems dubious. A maximum permissible lung burden may consist of a single or at most a few particles from which all the energy is deposited in very small tissue volumes. This paper presents the progressive gross and histological development of microlesions produced in rat lung tissue from highly-radioactive (about 108 and 103 rads/hr surface dose for alpha and X-rays, respectively) microspheres injected into the femoral vein and lodged in the lung vasculature. Histological sections were prepared from animals killed on days 1, 3, 7, 14, 21 and 28 and at monthly intervals for the following 6 months. Cellular changes were apparent as early as 24 hr. The microlesion, characterized by concentric regions of cellular debris and collagenous tissue surrounding the microsphere, became more complex with time but appeared to be self limiting. Curiously, some cells in areas receiving 10,000–100,000 rads over 3 days appeared viable. No alterations were seen in control rats given tungsten carbide spheres.