Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

326 CHAPTER 7
the very steep gradient of absorbed dose around a radioactive particle. Those cells
very close to the radioactive particles would be killed by the high radiation dose,
while those cells further away would get a sublethal dose, but a dose large enough
to initiate an oncogenic lesion. Numerous laboratory studies showed that this was
not the case. Particulate radioactivity in the lungs was found to be no greater hazard
than the same amount of radioactivity distributed throughout the lungs. Accordingly,
the lung models used for setting radiation safety standards for inhaled radioactivity,
including particulate matter, are based on mean doses to the radiosensitive tissues
in the respiratory tract.
Lung cancer is also associated with overexposure to external radiation. Among
4111 patients who had received X-ray therapy for ankylosing spondylitis, 88 deaths
from lung cancer were observed, while only 59 deaths were expected. A similar increase
in lung-cancer incidence was found among a very large population of women
who had undergone radiation therapy for cancer of the uterine cervix. An increased
incidence of lung cancer was also seen among the Japanese survivors of the nuclear
bombings who had been exposed to high doses of radiation. According to the data,
a dose of 1 Gy (100 rads) is estimated to increase the likelihood of dying from lung
cancer by about 32% for males and about 140% for females (Table 7-9).
Thyroid Cancer
Increased incidence rates of thyroid cancer in children and adolescents following
radiation overexposure have been well documented. An increased incidence rate was
seen among children who had been therapeutically treated with X-rays for enlarged
thymus glands, ringworm of the scalp, and acne. Radiogenic thyroid cancer was also
seen among children who had been overexposed to both internal radiation from
inhaled radionuclides of iodine and external radiation from the fallout from the
BRAVO nuclear bomb test in the Marshall Islands. Thyroid cancer was the first of
the solid tumors observed at a higher than expected incidence rate among Japanese
survivors of the nuclear bombings. In all these cases, the latency period between
exposure and the appearance of the excess thyroid cancers was measured in years.
A relatively large number of children were exposed to radioiodine in the fallout
from the nuclear weapons testing program in Nevada. A total of 1378 children
who had been exposed to the fallout were followed for 14 years, through 1971.
No difference was found between the thyroid cancer incidence rate of the exposed
children and that of unexposed controls. However, the data suggested a greater
incidence rate of other thyroid abnormalities in the exposed group than in the
control groups.
More recently, a sharp rise in the incidence rate of thyroid cancers was reported
among young people in the Ukraine following the Chernobyl nuclear power plant
disaster in April 1986. In 1986, only 15 confirmed thyroid cancers were reported
among Ukrainians who were 18 years of age or less at the time of the reactor accident.
In 1989, this figure increased to 36, and it reached 108 during 1992 and 101 in 1993.
In this case, there seems to be little doubt that the increased thyroid cancer incidence
rate is probably due to the exposure received as a result of the Chernobyl accident.
During the years 1944–1957, about 740,000 Ci (2.7 × 10 16 Bq) 131 I were released
to the atmosphere at the Hanford Laboratories. A study of 3190 persons who had
been born during the years 1940–1946 were studied when they reached the ages
of 46–57. Most of them still resided in the Hanford region. The thyroid doses were