Biomedical Engineering Reference
In-Depth Information
The 1990 BEIR V Report (see Section 13.15) states than an acute X-ray dose of
0.15 Gy to the human testes interrupts spermatozoa production to the extent that
temporary infertility results. An X-ray dose of 3 to 5 Gy, either acute of fractionated
over several weeks, can cause permanent sterility.
In the adult human female, all germ cells are present as ooctyes soon after birth.
There are no (oogonial) stem cells, and there is no cell division. The BEIR V Report
states than an acute dose of 0.65 to 1.5 Gy to the human ovary impairs fertility
temporarily. Fractionation of the dose to the ovaries over several weeks considerably
increases the tolerance to radiation. The threshold for permanent sterility in the
adult human female for X irradiation of the ovaries is in the range from 2.5 to 6 Gy
for acute exposure and is about 6 Gy for protracted exposure.
Every normal cell in the human has 46 paired chromosomes, half derived from
the father and half from the mother. Each chromosome contains genes that code
for functional characteristics or traits of an individual. The genes, which are seg-
ments of deoxyribonucleic acid (DNA), are ordered in linear fashion along a chro-
mosome. The DNA itself is a macromolecule whose structure is a linear array of
four varieties of bases, hydrogen bonded in pairs into a double-helical structure.
The particular sequence of bases in the DNA encodes the entire genetic informa-
tion for an individual. The human genome contains about
6
10
9
base pairs and
×
perhaps 50,000 to 100,000 genes.
Mutations occur naturally and spontaneously among living things. Various esti-
mates indicate that no more than about 5% of all natural mutations in man are as-
cribable to background radiation. Radicals produced by metabolism, random ther-
mal agitation, chemicals, and drugs, for example, contribute more.
A useful, quantitative benchmark for characterizoing radiation-induced muta-
tion rates is the
doubling dose
. It is defined as the amount of radiation that produces
in a generation as many mutations as arise spontaneously. For low-dose-rate, low-
LET radiation, the BEIR V Report estimated the doubling dose for mice to be about
1 Gy for various genetic endpoints. It noted that this level is not inconsistent with
what might be inferred for man from the atomic-bomb survivors. The BEIR VII
Report reviews and discusses doubling-dose estimates, which have been almost
exclusively based on both spontaneous and radiation induced rates in mice. The
Committee concludes that extrapolation of the doubling dose based on mice for
risk estimation in humans should be made with the
human
spontaneous rate. It
reports a revised estimate of 0.82
0.29 Gy, and suggests retaining the value 1 Gy
for the doubling dose as an average rate for mutations.
Radiation-induced genetic changes can result from gene mutations and from
chromosome alterations. A gene mutation occurs when the DNA is altered, even by
a loss or substitution of a single base. The mutation is called a point mutation when
there is a change at a single gene locus. Radiation can also cause breakage and
other damage to chromosomes. Some mutations involve a deletion of a portion of a
chromosome. Broken chromosomes can rejoin in various ways, introducing errors
into the normal arrangement. Figure 13.9 shows two examples of chromosome
aberrations induced in human lymphocytes by radiation. Chromosome aberrations
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