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primitive/progenitor cells and other rapidly dividing cells, while slower growing and
more mature cells are generally radio-resistant. The most radiosensitive mammalian cells,
in decreasing order, include the following:
• Spermatogonia.
• Lymphocytes and oocytes.
• Erythroblasts.
• Other hematopoietic tissue.
• Small-intestine crypt cells.
• Hair follicles.
All of these cells contain rapidly dividing cell lines. Clinicians are familiar with the
effects of radiotherapy and chemotherapy on these tissues, as sterility, bone marrow
damage, diarrhea, and hair loss all involve radiosensitive stem cell lines. With survivable
radiation doses, some stem cells survive, and their cell lines regenerate. Microvascular
injury can result in dramatic systemic symptoms and in permanent and irreversible
damage such as local radiation injury.
Clinically detectable effects first appear at doses >0.2 Gy (≥20 cGy or 20 rad). These
effects include decreased sperm count, chromosome abnormalities, and mild bone
marrow depression. Whole-body radiation doses >0.7 Gy can cause clinical illness. The
lethal dose 50 (LD 50 ) for penetrating radiation is approximately 3.5 Gy for untreated
patients and 5 Gy for those receiving full medical treatment. The LD 50 is the dose of
radiation that will kill half the exposed population.
Clinical Stages
All health effects from radiation exposure tend to follow a similar clinical pattern that can
be divided into a series of time-dependent stages: prodrome, latent period, and manifest
illness. At higher radiation doses, these stages are associated with shorter time of onset,
more severe signs and symptoms, and decreased survival. However, there is individual
variation, and symptoms may not occur in all patients.
Prodrome. The initial stage of prodromal symptoms (prodrome) begins within the first
few hours to 2 days after exposure. Symptoms include nausea and vomiting, with
subsequent malaise, fatigue, and weakness. This is a nonspecific clinical response to
acute radiation exposure caused by the cell membrane and free-radical effects of radiation
energy, as mediator chemicals such as histamine, interleukins, and cytokines are released.
Latent period. On recovery from the prodrome, there is usually a latent period during
which most symptoms subside, although fatigue and weakness may remain.
Manifest illness. This is the full disease picture that develops from the clinical signs and
symptoms associated with damage to major organ systems (e.g., blood-forming elements,
intestine, cardiovascular, CNS). The molecular cause of disease is DNA damage. Death is
usually caused by sepsis.
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