Analysis of the morbidity and mortality of children irradiated in fetal life

A review of the types of childhood cancer associated with a medical X-ray examination of the pregnant mother

PURPOSE

For 65 years the interpretation of the statistical association between the risk of cancer in a child and a prior diagnostic X-ray examination of the abdomen of the pregnant mother has been debated. The objections to a direct cause-and-effect explanation of the association vary in their strength, but one of the most notable grounds for controversy is the finding from the first and largest case-control study reporting the association, the Oxford Survey of Childhood Cancers (OSCC), of an almost uniformly raised relative risk (RR) for nearly all of the types of cancer that are most frequent in children. Here we compare the antenatal X-ray associations found in the OSCC for different types of childhood cancer with the results of all other case-control and case-cohort studies appropriately combined in meta-analyses, and we also review the findings of the few cohort studies that have been conducted.

CONCLUSIONS

From the case-control/case-cohort studies other than the OSCC there are consistent and clear elevations of risk for all types of childhood cancer combined, all leukemia, and all cancers except leukemia combined. This compatibility of the findings of the OSCC with those of the combined other studies is less clear, or effectively absent, when some categories containing smaller numbers of incident cases/deaths are considered, but lack of precision of risk estimates due to sparse data presents inferential challenges, although there is a consistent absence of an association for bone tumors. Further, more recent studies almost certainly address lower intrauterine doses, with an anticipated decrease in estimated risks, which could be misleading when comparisons involve a limited number of studies that are mainly from later years, and a similar problem arises when having to employ all types of antenatal X-ray exposures for a study because data for abdominal exposures are absent. The problem of low statistical power is greater for cohort studies, and this, together with other shortcomings identified in the studies, limits the interpretational value of results. The findings of non-medical intrauterine exposure studies are constrained by sparse data and make a limited contribution to an understanding of the association. Certain aspects of the various studies require a need for caution in interpretation, but overall, the appropriate combination of all case-control/case-cohort studies other than the OSCC lends support to the inference that low-level exposure to radiation in utero proportionally increases the risk of the typical cancers of childhood to around the same level.

Carcinogenic risks of prenatal ionizing radiation

The risk of cancer in offspring who have been exposed to diagnostic X-ray procedures while in utero has been debated for 55 years. High doses at high dose rates to the embryo or fetus (e.g. >0.5 Gy) increase the risk of cancer. This has been demonstrated in human epidemiology studies as well as in mammalian animal studies. Most pregnant women exposed to diagnostic X-ray procedures or the diagnostic use of radionuclides receive doses to the embryo or fetus <0.1 Gy. The risk of cancer in offspring exposed in utero at a low dose such as <0.1 Gy is controversial and has not been determined.

How radiosensitive are the developing embryo and fetus?

In its 1990 recommendations, the ICRP considered the radiation risks after exposure during prenatal development. This report is a critical review of new experimental animal data on biological effects and evaluations of human studies after prenatal radiation published since the 1990 recommendations.

Thus, the report discusses the effects after radiation exposure during pre-implantation, organogenesis, and fetogenesis. The aetiology of long-term effects on brain development is discussed, as well as evidence from studies in man on the effects of in-utero radiation exposure on neurological and mental processes. Animal studies of carcinogenic risk from in-utero radiation and the epidemiology of childhood cancer are discussed, and the carcinogenic risk to man from in-utero radiation is assessed. Open questions and needs for future research are elaborated.

The report reiterates that the mammalian embryo and fetus are highly radiosensitive. The nature and sensitivity of induced biological effects depend upon dose and developmental stage at irradiation. The various effects, as studied in experimental systems and in man, are discussed in detail. It is concluded that the findings in the report strengthen and supplement the 1990 recommendations of the ICRP.

Percutaneous balloon mitral commissurotomy during pregnancy

OBJECTIVE

To evaluate the effectiveness and safety of percutaneous balloon mitral commissurotomy for the treatment of pregnant women with severe mitral stenosis over a period of six years.

DESIGN

Analysis of clinical, haemodynamic, and echocardiographic data before and immediately after the procedure, the pregnancy outcome, and the fate of newborn babies.

SETTING

Academic cardiovascular centre in Monastir, Tunisia.

PATIENTS

44 pregnant patients who underwent percutaneous transvenous dilatation of the mitral valve between January 1990 and February 1996. Grade 2 mitral regurgitation was present in two patients and densely calcific valves in three (7%).

RESULTS

Commissurotomy was successfully achieved in all cases. The total mean (SD) duration of teh procedure was 72 (18) minutes and that of fluoroscopy 16 (7) minutes. Left atrial pressure decreased from 28 (10) to 14 (7) mm Hg, mitral pressure gradient fell from 22 (8) to 5 (3) mm Hg. Cardiac output increased from 4.8 (1.1) to 6.3 (1.2) l/min and Gorlin mitral valve area from 0.96 (0.21) to 2.4 (0.4) cm2 (all P < < 0.001). Cross sectional echocardiographic mitral valve area increased from 1.07 (0.21) to 2.32 (0.36) cm2. There were no maternal or fetal deaths. Complications included a grade 4 mitral regurgitation in one patient that required early valve replacement. All patients delivered at full term, 42 vaginally and two (5%) by caesarean section; 41 babies were normal and three whose mothers had the procedure near term were relatively hypotrophic. At a mean follow up of 28 (12) months (range 2 to 26) all children had normal growth.

CONCLUSIONS

During pregnancy, balloon mitral commissurotomy is the treatment of choice of severe pliable mitral stenosis in patients who are refractory to medical treatment.

Risk of childhood cancer from fetal irradiation

The association between the low dose of ionizing radiation received by the fetus in utero from diagnostic radiography, particularly in the last trimester of pregnancy, and the subsequent risk of cancer in childhood provides direct evidence against the existence of a threshold dose below which no excess risk arises, and has led to changes in medical practice. Initially reported in 1956, a consistent association has been found in many case-control studies in different countries. The excess relative risk obtained from combining the results of these studies has high statistical significance and suggests that, in the past, a radiographic examination of the abdomen of a pregnant woman produced a proportional increase in risk of about 40%. A corresponding causal relationship is not universally accepted and this interpretation has been challenged on four grounds. On review, the evidence against bias and confounding as alternative explanations for the association is strong. Scrutiny of the objections to causality suggests that they are not, or may not be, valid. A causal explanation is supported by evidence indicating an appropriate dose-response relationship and by animal experiments. It is concluded that radiation doses of the order of 10 mGy received by the fetus in utero produce a consequent increase in the risk of childhood cancer. The excess absolute risk coefficient at this level of exposure is approximately 6% per gray, although the exact value of this risk coefficient remains uncertain.

The effects of ionizing radiation, microwaves, and ultrasound on the developing embryo: clinical interpretations and applications of the data

The term "radiation" evokes emotional responses both from lay individuals and from professionals. Many spokespersons are unfamiliar with radiation biology or the quantitative nature of the risks. Frequently, microwave, ultrasound, and ionizing radiation risks are confused. Although it is impossible to prove no risk for any environmental hazard, it appears that exposure to microwave radiation below the maximal permissible levels present no measurable risk to the embryo. Ultrasound exposure from diagnostic ultrasonographic imaging equipment also is quite innocuous. It is true that continued surveillance and research into potential risks of these low-level exposures should continue, but at present ultrasound not only improves obstetric care but also reduces the necessity of diagnostic x-ray procedure. In the field of ionizing radiation, we have as good a comprehension of the biologic effects and the quantitative maximum risks as of any other environmental hazard. Although the animal and human data support the conclusion that no increases in the incidence of gross congenital malformations, intrauterine growth retardation, or abortion will occur with exposures below 5 rad, that does not mean that there are definitely no risks to the embryo exposed to lower doses of radiation. Whether there exists a linear or exponential dose-response relationship or a threshold exposure for genetic, carcinogenic, cell-depleting, and life-shortening effects has not been determined. In establishing maximum permissible levels for the embryo at low exposures, we use the information in Tables 2, 3, 4, 7, 10, and 14. It is obvious that the risks of 1-rad or 5-rad acute exposure are far below the spontaneous risks of the developing embryo, since 15% of human embryos abort, 2.7%-3.0% of human embryos have major malformations, 4% have intrauterine growth retardation, and 8%-10% have early- or late-onset genetic disease. The maximum risk attributed to a 1-rad exposure, approximately 0.003%, is thousands of times smaller than the spontaneous risks of malformations, abortion, or genetic disease (see Table 10). Thus, the present maximum permissible occupational exposures of 0.5 rem for pregnant women and 5 rem for medical exposure are extremely conservative. Medically indicated diagnostic roentgenograms are appropriate for pregnant women, and there is no medical justification for terminating a pregnancy in women exposed to 5 rad or less because of a radiation exposure.(ABSTRACT TRUNCATED AT 400 WORDS)