The fact that therapeutic radiation poses a significant risk to the patient in most instances is rec- ognized by all, but, as noted above, the benefit in most instances far outweighs the risk. Several examples of carcinogenesis resulting from therapeutic radiation have already been discussed. The association of subsequent cancers with treatment of cervical cancer, Hodgkin disease, and a number of childhood cancers has been described (cf. Boice, 1981), as well as the increased risk of cancer developing in the contralateral breast after radiation treatment for cancer in the af- fected breast (see above). In addition, a series of women who received radiation treatment for acute postpartum mastitis have also shown a significantly increased risk for developing breast cancer (Shore et al., 1986). In women who had received radiation therapy for breast cancer, there was an increased risk of lung cancer developing 10 years later even in nonsmokers, but a multi- plicative effect was observed in smokers (Neugut et al., 1994).
Less apparent is the risk of cancer development from diagnostic radiation, in most in- stances for routine diagnoses. Diagnostic radiography was found to have only a small influence on the occurrence of leukemia and breast cancer in women so exposed (Evans et al., 1986) as well as in several studies investigating the radiation risk of mammography. Essentially no excess cancer risk was determined for this diagnostic procedure, especially in relation to the potential benefit obtained by mammographic screening (Feig, 1984). In addition to the relative safety of diagnostic x-rays, the use of radioactive iodine in determining abnormalities in the thyroid gland by techniques of nuclear medicine is not carcinogenic to the human, as evidenced in one large cohort subjected to these diagnostic procedures (Holm et al., 1988). On the other hand, Bross et al. (1979) argued that small subgroups in the general population are particularly sensitive to ion- izing radiation, and this factor should be taken into account in determining safe exposures.
Furthermore, not all diagnostic radiation is free from risk. There is now substantial evi- dence that children irradiated in utero with ionizing radiation for diagnostic purposes have an excess risk of childhood cancer of about 40% (cf. Wakeford, 1995). Another series supporting this finding reported approximately a twofold increase in risk of childhood cancer after prenatal x-ray exposure (Harvey et al., 1985). The use of Thorotrast (Chapter 11) as an x-ray contrast medium for diagnosing the presence of lesions in the liver, kidney, and other solid organs has now clearly been associated with the development of angiosarcoma of the liver as well as leuke- mia from the alpha radiation given off by the decay of thorium (Andersson and Storm, 1992). There has also been a report that diagnostic radiation may increase the risk of salivary gland neoplasms (Preston-Martin et al., 1988). Still, despite these few examples of the sensitivity of the exposed individual (the fetus) and the chronic intense localized dose of radiation such as that from iodine 131, diagnostic radiation is a comparatively safe procedure in which the extensive benefits generally far outweigh the minimal risk.