Mantel-Haenszel analysis of Oxford data. II. Independent effects of fetal irradiation subfactors.

A Mantel-Haenszel analysis of fetal irradiation subfactors indicated that most of the "extra" X-rayed cases in the Oxford Survey of Childhood Cancers were radiation induced. First trimester exposures were rare but probably ten times more dangerous than later exposures. Ratios of observed: expected numbers of cancer deaths were lower for children with abnormal X-rays than for other X-rayed children, and lower for recent than remote exposures. The first of these differences was probably due to several antenatal conditions having positive associations with obstetric radiography and several causes of early (noncancer) deaths; the second one was probably due to a progressive lowering of film doses between 1940 and the present time. A rare cause of fetal irradiation (hydramnios), whose associations with congenital defects are well documented, led to the discovery that two faults in the International Classification of Diseases and Causes of Death have contributed to mistaken ideas about the etiology of childhood cancers: Neoplasms were not listed among the official causes of stillbirths, and cystic tumors of the kidneys and lungs of infants were not listed as neoplasms.