the body. At this level of radiation dose, death is a certainty and medical
help is of no use.
Long-Term Effects of Radiation
The long-term or late effects of radiation cause various syndromes long
after the radiation exposure. These may appear after acute radiation syn-
dromes subside following exposure to a single large dose or after exposure
to many smaller doses over a period. The late effects may be somatic or
genetic, depending on the respective cells involved. Somatic effects are seen
in the form of carcinogenesis, life-shortening, cataractogenesis, and embryo-
logic damage. On the other hand, genetic effects result in abnormalities in
the offspring.
Somatic Effects
Carcinogenesis
Cancer develops in three stages: initiation, promotion, and progression. Ini-
tiation of cancer is caused by various agents such as chemicals, ultraviolet
rays, radiation, and viruses. In the case of radiation, cancer is initiated by
the action of radiation on the DNA molecule resulting in the mutation of
the cell. Cancer promoters are those agents that cannot initiate the cancer
but simply promote it once it is started. Examples of tumor promoters are
estrogen, phorbol ester, excessive fat, and radiation. Radiation acts as a pro-
moter by inactivating tumor suppressive genes through the interaction of
the free radicals produced in the cytoplasm by radiation. In these two
stages, mutated cells proliferate at the site of cancer growth. One or more
of these cells become aggressive and are likely to spread to other organs.
This stage is called the progression or metastasis of the cancer.
At the cellular level, carcinogenesis is thought to be controlled by two
types of genes: oncogenes and suppressor genes. There is evidence that
oncogenes are responsible for the growth and proliferation of tumor cells,
while suppressor genes inhibit the tumor cell growth. Most oncogenes
have their counterpart, proto-oncogenes in normal cells, that are present
throughout their eukaryotic evolution. Radiation or other carcinogens acti-
vate normal proto-oncogenes to several cancer-causing oncogenes and inac-
tivate suppressor genes, resulting in cell proliferation to cause cancer.
Chromosome aberrations (deletion or translocation) caused by radiation
are responsible for oncogene activation. There are about 100 oncogenes
identified that are associated with various human cancers. For example,
deletion of a part of the chromosome is responsible for the activation of N-
rasoncogene associated with neuroblastoma. Similarly, a translocation
between chromosomes 8 and 14 in humans activates the C-myconcogene
in B-cell lymphoma.
Long-Term Effects of Radiation 249