Answer to Question #285 Submitted to "Ask the Experts"

Q

Would you please help me on my school project by telling me what the effects of radiation on bone tissue are? Thank you.

A

In discussing effects of irradiation on bone tissue, it is important to recognize that bone contains nongrowing, mineral bone and limited regions where bone growth occurs and it has cavities of bone marrow containing hematopoietic stem cells (cells that form blood cells). The typical long bone consists of a shaft of compact bone (the diaphysis) surrounding the marrow cavity and shaft ends (epiphyses) consisting of spongy bone covered with a layer of compact bone. In immature animals, bone growth occurs near the ends of the bone in a cartilage plate between the these two regions (the epiphyseal-diaphyseal plate). We don't have any more recent references on bone radiobiology; perhaps other readers of this feature can provide more recent references. We found material in Till and Meyer (1983), Casarett (1968), and ICRP Publications 11 and 41. Till and Meyer: Mineral bone, the largest component of the skeleton, is not very radiosensitive. At levels associated with occupational and environmental radiation exposure, nongrowing mineral bone is not considered at risk; only the soft tissues of the of the skeleton are of concern. There is general agreement that there are two radiosensitive tissues of the skeleton (ICRP-1968):

• The active marrow--Blood cells are formed in the bone marrow; developing cells are found in various stages of maturation within the active marrow. The active marrow is of primary concern as a target tissue with respect to leukemia induction.
   
• The osteogenic (bone-forming) cells, particularly those on the endosteal surfaces of bone–The osteogenic cells are the precursors of cells involved in the formation of new bone and the resorption of bone. Osteogenic cells are target of concern with regard to bone cancer induction.

Casarett: At higher radiation doses, the nongrowing portions of the bone are relatively radioresistant, but damage can occur to the cartilage plate in growing bones. For example, if radiation is given to the knee region of 30-day-old rats, mitosis (cell division) ceases within two or three days, and many of the cells of the cartilage plate disappear or are swollen and degenerate. Within a week, many of the remaining cells are dead or degenerate. Changes in bone, cartilage, fat, and the blood vessels of the marrow are observed. Regeneration may start two to six weeks after exposure. Although recovery of the plate may appear to be complete several months after irradiation, there will usually be a shortening of the bone as a result of the period of growth inhibition If large doses of radiation are given to growing bone, most cartilage cells degenerate, bone-forming cells (osteoblasts) are destroyed, and a bone-like substance without cells or blood vessels may be formed from the cartilage. This will gradually be resorbed and replaced with a solid plate incapable of growth and result in a shortened bone. Irradiation of formed bone may result in an upset of the normal process of bone resorption and new bone formation resulting in either an excessive absorption or an overgrowth of bone. In irradiated bone, fractures are common due to either structural damage to the bone matrix or to defective bone mineralization. The effects of radiation on bone are of concern (1) when dealing with depositions of large quantities of radioactive isotopes of elements such as plutonium, radium, and strontium which localize in the bone or (2) in localized radiation therapy which may result in massive doses to bones. ICRP Report 41 (1984): Mature bone and cartilage are relatively nonradioresponsive; however in the proliferative (growing) state, as in growing children or during the healing of fractures, these tissues show a heightened response. In children, for example, some retardation of growth may result from doses as low as 1 Gy (100 rad), depending upon the age at irradiation and the conditions of exposure. Other skeletal changes have been observed after therapeutic irradiation in childhood at doses exceeding 20 Gy (2,000 rad). In adults, mature cartilage will tolerate 40 Gy (4,000 rad) fractionated over four weeks or more than 70 Gy (7,000 rad) fractionated over 10-12 weeks and mature bone will tolerate as much as 65 Gy (6,500 rad) fractionated over 608 weeks. However, the susceptibility of these tissues to subsequent trauma months or years later may be increased, but precise dose-response data for such long-term effects are fragmentary. References:

• Casarett, A.P. Radiation biology. Englewood Cliffs, NJ: Prentice-Hall, Inc.; 1968: 191-195.
   
• International Commission on Radiological Protection. A review of the radiosensitivity of the tissues in bone. Oxford: Pergamon Press; ICRP Publication 11; 1968.
   
• International Commission on Radiological Protection. Nonstochastic effects of ionizing radiation. Oxford: Pergamon Press; ICRP Publication 41: 22-25; 1984.
   
• Till, J.E.; Meyer, H.R. Radiological assessment—A textbook on environmental dose analysis. Washington, DC: U.S. Nuclear Regulatory Commission; NUREG/CR- 3332, ORNL-5968;1983: 7.3.5.1.

C.E. Roessler, Ph.D., CHP