Our findings from the present study are frequent tooth brushing may lower the occurrence of head and neck cancer even after adjusting for important confounding factors. In addition, poor oral health evidenced by a greater number of tooth loss was independently related to future occurrence of head and neck cancer.
In previous studies, infrequent tooth brushing was associated with head and neck malignancies, particularly in tongue, gum, and other areas of the mouth [6, 28, 29]. Tooth brushing less than once a day was also associated with risk of tongue cancer (odds ratio: 2.1) and cancer in other parts of the mouth (odds ratio: 2.4) [30]. In a case-control study of Chinese subjects, the risk of oral cancer was 6.9-fold greater for males and 2.5-fold for females in subjects who infrequently brushed their teeth [31]. Results in another European study indicated daily brushing was a protective factor against developing oral cancer (odds ratio: 0.41) [32]. The results from the present study are in agreement with the above-mentioned studies and provide additional information for longitudinal relationship of tooth brushing with decreased occurrence of head and neck cancer.
In the present study, a positive association between loss of multiple teeth and the occurrence of head and neck cancer was observed. These association has been consistently reported in previous studies. In a meta-analysis of case-control and cross-sectional studies, the odds ratio for tooth loss more than 5 of head and neck malignancy was 2.0 [33]. In addition, in a dose-response meta-analysis of prospective cohort studies, tooth loss was significantly related to a higher occurrence of head and neck cancer [34]. Tooth loss may be closely related to failure of maintaining oral hygiene (infrequent tooth brushing, dental visits, and expertised dental cleaning), exposure to cancer-inducing factors such as smoking, nitrosamines, or alcohol, and secondary inflammation or infection of the oral cavity [35, 36]. Because these factors may promote malignancy [37, 38], our hypothesis is plausible.
To investigate of which site head and neck cancer is more associated with oral hygiene, we analyzed the site specific risk according to oral disease and hygiene. For the oral cavity cancers, the results were quite similar to the main result of present study. On the other hand, for the tonsil or pharynx cancer, risk of cancer occurrence was not associated with oral health and oral hygiene, statistically significantly. This result seems supportive of the hypothesis that, poor oral hygiene can mediate cancer mainly in oral cavity.
In the present study, visiting the dentist and expertised dental cleaning were associated with decreased occurrence of head and neck cancer. However, infrequent dental visits was related to an increased occurrence of head and neck cancer based on univariable analysis. These results are consistent with previous studies [6, 7, 39, 40]. Conversely, after adjusting for various important confounding factors, the association between dental visits and expertised dental cleaning and occurrence of malignancy was not statistically significant. These results may be due to other factors, which are more likely associated with malignancy than dental visits and expertised dental cleaning.
Paradoxically, the presence of periodontal disease was decreased occurrence of head and neck cancer. Subjects with periodontal disease visit hospitals more often than subjects without periodontal disease and receive more frequent health care. Moreover, in our study, definition of periodontal disease using the ICD-10 code does not apply recently published classification criteria and definitions of case for periodontal disease [41]. In addition, the discrepancy of age, sex, income level between the included subjects and excluded those may explain these results.
The present study had several limitations. First, although this was a longitudinal study and the cohort was retrospectively analyzed without intervention, therefore, the information bias could not be excluded and causal relationships were not confirmative. Second, many subjects were excluded before analysis due to the missing value in the oral health status data. Because the oral examination was not a mandatory test item, this exclusion may lead to a selection bias, leaving the subjects with healthier behavior in the study dataset. Third, although we used a wash-out period of one year before the index date, a hidden malignancy at baseline remains a possibility. Fourth, the frequency of tooth brushings and expertised dental cleanings were assessed based on a self-reported survey, which may be subject to recall bias. Fifth, although parameters of oral health can be altered during follow up, our study did not take into account these time-varying factors. Sixth, although HPV infection and their treatment were important covariates for occurrence of head and neck cancer, because treatment for HPV infections was not covered by national insurance, our health claim data could not adjust the HPV-related parameters. Seventh, the NHIS-HEALS cohort dataset does not supply information about the education or occupations of the subjects. Therefore, income level was the only adjusted variable that reflects socioeconomic status. Finally, because this is a cohort study with a large number of subjects without a sample-size calculation, we cannot completely rule out the possibility that the association between oral health and the occurrence of head and neck cancer is statistically significant only due to the large sample size. Nevertheless, the present study had several strengths. Contrary to previous studies, which were mostly case-control studies, this was a nationwide, general population-based longitudinal study. In addition, this study included a relatively large sample size. Distinct from previous studies, various risk factors were included such as comorbidities, laboratory findings, alcohol intake, smoking status, and oral hygiene, and adjusted in multivariable analysis in this study.