4. DISCUSSION
Childhood cancers are a multifactorial group of disease that have both genetic and environmental causes. Many studies on the causes of childhood cancers have limitations due to the rare occurrence of these cancers, reliance on retrospective evaluation based on parental statements, which can lead to biases in recalling and reporting, and inability to design appropriate studies due to the presence of multiple factors in the cancer mechanism 6,7. For this reason, there are studies in the literature containing many different results on risk factors that may play a role in the etiology of childhood cancers6-10. Therefore, we aimed to account for as many confounding factors as possible in order to evaluate the effect of ETS exposure on childhood cancers in details.
In our study, we observed that a positive family history of malignancy was present in 51% of the case group while 36.4% of the control group (p < 0.05). Among the cancer patients 9.1% of individuals with malignancy were first-degree relatives, whereas 90.8% were second-degree or more distant relatives. In contrast, all individuals with malignancy in the control group were second degree or more distantly related. Considering that environmental and hereditary factors play a role in the etiology of cancer, it was an expected finding that the rate of relatives with malignancy in cancer patients was significantly higher.
Greaves and Kinlen suggested that early exposure to infection, especially during and before infancy, reduces the risk of developing leukemia 8. They stated that children who attend kindergarten and have an older sibling are more likely to encounter such infectious agents at an earlier and more frequently, and this will reduce the risk of developing leukemia. Although there are both supportive and contradictory findings regarding this claim. In our study, revealed a significant disparity between the case and control groups with regard to the proportion of children who had at least one older sibling, it was significantly higher in case group(59.6%; n=59) than the children in the control group (36.7%; n=36) (p < 0.05).
This finding does not support Greaves and Kinlen’s hypothesis. Present study showed that a significantly higher percentage of patients in the case group (13%) resided in rural areas compared to those in the control group (1%) (p < 0.05).This would be inconsistent with Greaves and Kinlen’s hypothesis of early exposure to infection, assuming that rural residents may be infected more frequently and earlier; however, an increased risk of insecticide and pesticide exposure or exposure to other pathological infectious agents may explain the higher rate of rural living in cancer patients. The fact that many infectious agents are no longer present and undetectable at the time of diagnosis makes it very difficult to evaluate the cancer-infection relationship. For this reason, it may be useful to question indirect indicators such as birth order, whether he went to a kindergarten, a history of infection requiring hospitalization, and where he lived.
The fact that cigarettes contain many carcinogenic and toxic substances and that these substances are detected in fetal blood, placenta, and urine of exposed children lead us to consider smoking as a possible risk factor in the etiology of childhood cancers 2. The most important source of children’s exposure to cigarette smoke is parental smoking. For this reason, many studies on this subject focus on smoking behaviors of parents 1,2,6,11-14. The fact that smoking has been demonstrated to cause damage to almost every cell and tissue by various mechanisms including DNA damage, epigenetic changes, oxidative damage, inflammation among others, it is crucial to evaluate the consequences of smoking from the prenatal stage. In our study, pre-conceptional, during pregnancy and current smoking status of the mother and father were examined in the case and control groups. In the United Kingdom Childhood Cancer Study, the pre-conceptional period was considered as the one-year period before the pregnancy of the index case and was accepted as such in our study 2. For this reason, parents were also asked whether they smoked regularly at any time in their lives, and their smoking behaviors outside of these three periods were also taken into account.
In a retrospective cohort study published by Kessous et al. in 2019, an increased risk of benign neoplasm (hemangioma, thyroid cyst, etc.) was found in the children of mothers who smoked during pregnancy; however, no increased risk of malignant neoplasm was detected14. In a cohort study conducted in Canada between 2006 and 2016, children whose mothers’ used alcohol, drugs and cigarettes during pregnancy were examined. Maternal smoking during pregnancy was associated with the risk of AML and fibrosarcoma, and weakly associated with neuroblastoma and renal tumors 11. A Danish study found no association between maternal smoking during pregnancy and an increased risk of childhood cancer 12. We found no difference between the case and control groups in terms of pre-conceptional, during pregnancy and current smoking behaviors of the mother according to questionnaire (p > 0,05).
In our study, more than half of the fathers in the case and control groups were smokers, and there was no significant difference between two groups in this regard. All fathers who smoked during the pre-conceptional period continued to smoke during pregnancy in both groups. The rate of fathers who smoked in the pre-conceptional period and during pregnancy in the control group (75.8%) was significantly higher than the cancer group (61.7%) (p < 0.05). At the same time, 81.8% of the fathers in the control group stated that they used to smoke regularly in any period of their lives, and this rate was significantly higher than the cancer group (68.1%) (p<0.05). It would be wrong to interpret this situation as paternal smoking has a protective effect on childhood cancers. As our objective measurement findings show and we will talk about later, this may be a manifestation of subjective methods such as surveys that can cause biases or a result of our small sample size. Social desirability bias of cancer patients’ parents and socio-economic differences between two groups may also be a reason.
Many methodological limitations make it difficult to establish a relationship between childhood cancers and ETS exposure. Therefore, we evaluated ETS exposure with the measurement of cotinine, which is an objective marker. According to questionnaire data, the ETS exposure rate of children with cancer (68.8%) was found to be significantly lower than the rate of children in the control group (83.8%) (p<0.05). Many studies define ETS exposure as ’cigarette smoking in the child’s environment’ 15. The American Academy of Pediatrics, on the other hand, defines a person’s exposure to second-hand and/or third-hand cigarettes as ”involuntary exposure to tobacco smoke” 16. As we mentioned before, the children in the study were considered to have exposure to ETS in the presence of at least one of the smoking conditions of their mother, father, another individual in the house or another individual (friend, etc.) whom they frequently encounter. However, the extent of exposure to cigarette smoke depends on many factors such as the number of smokers, the amount of cigarettes smoked, proximity to smokers, the size of the environment and ventilation conditions, and the duration of exposure17. The fact that the definition of ETS exposure does not cover these factors may be a reason for the higher ETS exposure in the control group.
Cotinine is the primary metabolite of nicotine and is the most reliable biomarker of tobacco smoke exposure according to Benowitz et al17. Cotinine levels can be measured from blood, saliva, urine and hair. Plasma, saliva, and urine measurements cannot detect long-term exposure due to their short half-lives, leading to difficulties in sample storage. In contrast, hair cotinine analysis allows for cross-sectional and long-term cumulative evaluation as cotinine accumulates in the hair shaft as hair grows. While individual differences such as age, gender and race may impact cotinine metabolism, the meta-analysis of Florescu et al. aimed to determine the most suitable hair cotinine threshold value for certain age groups and pregnant women. The results of meta-analysis suggest that 0.2 ng/mg is the most appropriate for distinguishing children exposed to ETS based on its high sensitivity and specificity 5. The mean cotinine value of the case group was found to be 0.224±0.088 ng/mg, and the mean cotinine value of the control group was 0.136±0.048 ng/mg. Hair cotinine levels were found to be significantly higher in children with cancer (p<0.001). When the exposure to ETS was evaluated with the hair cotinine analysis, 59.4% of the children with cancer and 14.1% of the children in the control group were found to be exposed to cigarette smoke according to the threshold value. Cotinine analysis shows that ETS exposure was significantly higher in cancer patients (p<0.001). Our findings support the prediction that smoking may be among the factors that cause childhood cancers.
In conclusion, in this study, which we evaluated the cigarette smoke exposure of 104 children with cancer and 99 children without cancer diagnosis, using a questionnaire and hair cotinine levels, supports the idea that parental smoking may cause childhood cancer. As seen in this study, the evaluation of cigarette smoke exposure by subjective methods such as questionnaires can cause biases. Hair cotinine analysis is a good test in the evaluation of cigarette smoke exposure, as it is an easy to apply, non-invasive test and also shows cross-sectional and long-term exposures. We believe that the use of hair cotinine analysis together with valid and reliable questionnaires in studies aimed at evaluating the effects of smoking on childhood cancers would prove highly beneficial in preventing biases.