In utero exposure to x-rays has been identified as a risk factor for childhood leukemia (18
). Our results suggest that childhood RMS could be added to the list of cancers associated with in utero
diagnostic x-irradiation. The magnitude of risk of RMS associated with x-rays during pregnancy appears to be somewhat higher than that reported for leukemia and other solid tumors, however more studies are needed to confirm this.
In our study, all x-ray examinations of subjects’ mothers during pregnancy were considered. This led to the observation that x-ray exposure other than those of the abdomen and pelvis might be associated with an increased risk of RMS. For example, eleven case mothers reported having had dental x-rays (three of which were done with lead shielding), but only three control mothers reported this. Six of the eleven case mothers reporting dental x-rays had the examinations done during their second or third trimester of pregnancy; all three of the exposed control mothers had their x-ray examinations done in the last two trimesters of pregnancy. This is somewhat surprising, since most dental x-rays are not done on an emergency basis and thus, could be deferred until after pregnancy. Although dental x-ray beams are focused on the teeth and supporting bony structures, there is considerable opportunity for scatter of the rays, particularly when downward-angled beams are used or when old equipment is used or films are taken by inadequately trained personnel. The same opportunity for scatter of x-rays might exist also for the three other non-truncal x-rays reported by case mothers (two lower extremity and one sinus film) and the two reported by control mothers (two lower extremity films). In the absence of systematic data on shielding during these examinations we can only conjecture about these exposures.
Although the exact mechanism by which ionizing radiation exposure of the fetus produces childhood cancers is not known, radiation induces genome instability leading to accumulation of DNA damage and alteration in DNA and histone methylation (27
). Rodents have been validated as good models for prediction of risk of perinatal x-ray-induced carcinogenesis in humans (29
), despite marked differences in the physiology of gestation (30
). A possible mechanism for the induction of RMS by radiation exposure is suggested by a report of RMS and radiation hypersensitivity in a mouse model of Gorlin (nevoid basal cell carcinoma) syndrome (31
). Gorlin syndrome is characterized by a constellation of clinical problems including generalized overgrowth of the body, cysts, skeletal abnormalities and a predisposition to benign and malignant tumors. Hahn et al., studied the multiple-pass transmembrane receptor patched (ptc) in a mouse model. Ptc is thought to be a tumor suppressor gene and patients with Gorlin syndrome have germline mutations in ptc (33
). They found that mice heterozygous for ptc inactivation develop features consistent with Gorlin syndrome and also have a high incidence of RMS of the embryonal type only. It has recently been proposed in Ptc1 deficient mice, which develop basal cell carcinoma, that deregulation of the Shh/Ptc1 pathway represents the initiating step, but that additional genetic damage by radiation in genes serving critical functions in the regulation of cell proliferation, apoptosis, response to DNA damage, such as p53, is required for tumor formation (34
). The evidence that the Ptc pathway may play an important role in RMS development in humans in not limited to Gorlin syndrome patients who possess germline Ptc mutations. In addition, 30% of sporadic RMS show genomic loss of Ptc1 at locus 9q22 (35
). Ptc is a receptor for sonic hedgehog (Shh) and Shh/Ptc pathways play a key role in hypaxial skeletal muscle development, and is required for normal differentiation of early developing muscle (36
). Deregulation of this pathway is detected in a substantial percentage of sporadic RMS (37
A previous paper from our case-control study of RMS, reported an increased frequency of neurofibromatosis and major malformations in RMS cases (20
). Several case series of RMS patients have also found an increased frequency of malformations (20
). RMS patients were found to have a particularly increased frequency of central nervous system anomalies (20
). The observations of an increased risk of RMS in children exposed to ionizing radiation in utero
and of an increased occurrence of malformations in children with RMS raise the question of whether our findings may be related to the ptc receptor and the signaling pathway of muscle development in which it is involved.
In searching for additional indirect support for this question in our data, we examined the risks associated with x-ray exposure when stratified by histologic type of RMS. This was done to see if embryonal RMS, the histologic type observed to occur spontaneously in the mouse model, was more strongly associated with x-ray exposure in humans than were the other histologic types. shows the results of this analysis. The highest odds ratio (12.1) was observed for first trimester x-ray exposure in embryonal RMS cases. This is based on 10 exposed cases versus 1 exposed control, hence the wide 95% confidence interval which nonetheless excludes the null value. Our observation of a very high OR for first trimester exposure is consistent with the suggestion of Zhan and Helman (1998)(39
) that perhaps there is a critical window of time in which developing skeletal muscle is sensitive to ptc alterations (39
). Nevertheless, observations of an increased frequency of major malformations in RMS and of ionizing radiation induced RMS, particularly embryonal RMS, suggest that ptc alterations might play a role in the etiology of this malformation. Although the ORs are generally highest for embryonal RMS, the ORs appear to be elevated for other types of RMS, as well. Similar to our findings of the highest risk among the embryonal tumor subtype, a recent study of solid tumor occurrence after in utero x-ray exposure revealed that the increased risk of brain tumor development was confined to the embryonal subtype (PTEN) (40
Despite these observations, there are several limitations to our study. Even though this represents one of the largest case-control studies ever conducted on this rare tumor, the infrequency of the exposure precluded detailed analyses. Only 13.5% of cases and 6.8% of controls reported any x-ray exposure during pregnancy. Further, there is concern of recall bias; mothers of case children may tend to over-report exposure compared to mothers of control children. In addition, differential recall bias among cases is possible. For example, case mothers might more accurately recall an x-ray exposure that would be more readily perceived as causal (abdominal/pelvic x-ray) than a more distant exposure (dental x-ray). On the other hand, it is also possible that they would be less likely to report an exposure they may consider causal. It is therefore impossible to address whether differential recall bias played a role in our study. However, it is reassuring that we found an association between x-ray exposure and risk of one (embryonal) but not both RMS subtypes. Namely, there is no reason to believe that a mother of a child with embryonal RMS would recall the exposure differently than a mother of a child with alveolar RMS. We also did not collect information regarding shielding. As we noted, there was an average of 8 years between exposure and interview. Despite these concerns, we found a strong positive association between maternal x-ray exposure and embryonal RMS (less so with other RMS), which is congruent with the animal data.
A recent large analysis (>1800 cases) of childhood acute lymphoblastic leukemia (ALL) found little evidence of an association with prenatal diagnostic x-ray exposure (41
). The authors conclude that declining exposure levels through declines in radiation levels in x-rays and a medical practice decrease in x-rays provided to pregnant women may explain these null findings. Mothers in our study were pregnant with study subjects approximately during the 1970s and 1980s, thus covering a period of time when radiation dosages were higher and x-ray practices for women of childhood bearing age may have been less adhered to. For example, comparison of two UK dental surveys, one carried out in the late 1980s and the other in the late 1990s, showed that the mean patient entrance dose caused by dental x-rays has decreased by 40% (42
Although Alice Stewart and colleagues reported on the dangers of in utero
x-ray exposures fifty years ago, insufficient attention has been paid to the consideration that women of child-bearing age should be assumed to be pregnant when it comes to radiographic examinations. The use of lead apron shielding of uninvolved body areas during x-ray studies has grown widespread but lapses still occur. In addition, concerns remain that a routinely used leaded apron of 0.25 mm lead thickness may not provide sufficient protection (43
). Prior to routine shielding of the neck, downward pointing beams from dental x-rays could reach the gravid uterus. While routine lead shielding of the neck (thyroid) has been added by most dentists, in the current age of digital x-rays the radiation exposure has been significantly decreased (45
), which may reduce adherence to this practice.
In conclusion, our study supports the association between in utero x-ray exposure and the risk of RMS and supports the view that in addition to brain tumors, other solid tumors may occur after such exposure. This is the largest case-control study of RMS to date and the exposure levels were surprisingly high given the public awareness of possible carcinogenic effect of in utero x-ray exposure. Therefore findings from our study reinforce prevention guidelines to avoid the potential hazards of diagnostic x-ray exposure to unborn children.