ABSTRACT Background Oesophageal cancer has a high mortality; it is the 6th most common cause of death due to cancer worldwide. Of the common subtypes of oesophageal cancer, it is the adenocarcinomas that have been rising rapidly in incidence throughout the western world. The incidence of adenocarcinomas now exceeds the previously common squamous cell carcinoma. These recent changes in the incidence patterns of oesophageal cancer suggests that the environmental risk factors associated with these subtypes differ, and that changes in the prevalence of these exposures over time are the most likely explanation for the observed shifts in the incidence. However, due to its low incidence until a few decades ago, the adenocarcinoma subtype has been less studied compared to squamous cell carcinoma, and the environmental factors associated with this cancer have not been so clearly defined. Smoking and alcohol have been the strongest environmental risk factors reported for oesophageal squamous cell carcinoma (OSCC) whereas for oesophageal adenocarcinoma (OAC), the effect of smoking appears to be weaker, and the evidence for an effect of alcohol is scant and inconsistent. However, epidemiologic studies consistently identify people with frequent symptoms of gastro-oesophageal reflux (GOR) as having the highest risk of OAC, but the effect of GOR on OSCC has been negligible. Furthermore, it has been argued that adenocarcinoma occurring at the gastro-oesophageal junction (GOJAC) may have different aetiology again. Together, these reports suggest the three subtypes of oesophageal cancers (OAC, GOJAC and OSCC) may arise through different mechanisms with different strengths in the impact of risk factors. This thesis investigated the independent associations of smoking, alcohol and gastro-oesophageal reflux on cancers of the oesophagus by considering the possibility of variation in the risks due to differences in the dose effect patterns of various measures such as smoking, alcohol and GOR. Method Data from a population-based case-control study of oesophageal and ovarian cancers in Australia were used. Study participants comprised histologically confirmed cases of OSCC (n=308), OAC (n=367) and GOJAC (n=426) who were frequency matched to 1580 controls from the general population. Exposure history for both cases and controls were derived from health and lifestyle questionnaires. Unconditional multivariate logistic regression was used to calculate the odds ratios and 95% confidence intervals for the risk factors analysed. In addition, generalised additive model with a logit link was also used to explore and present the non-linearity in the dose effect pattern for continuous exposures adjusting for other confounding factors. The effects of two exposures combined on these cancers were assessed by obtaining synergy index. Results Smokers were at significantly higher risk of all three subtypes of oesophageal cancer with the risk greatest for OSCC. The effect of smoking was greater for adenocarcinoma occurring at the gastro-oesophageal junction compared to that of the oesophagus. Of the various measures of smoking, duration was significantly associated with all three subtypes of cancer whereas intensity was associated with only OSCC and GOJAC and the dose effect was non-linear. Time since quitting was associated with a steady decline in risk of all three cancers emphasising the health benefits of quitting among smokers. Alcohol was not associated with OAC or GOJAC but was significantly associated with OSCC among those drinking in excess of 170g/week. The association between alcohol and OSCC was modified by smoking; the association with alcohol was significantly greater among current smokers with effect. Low to moderate wine consumption was associated with significant risk reduction for all three cancers compared to non-drinkers. Increased frequency of GOR symptoms was associated with increased risks of OAC and GOJAC, although the risk of OSCC was constrained to frequent GOR symptoms only. The effect of GOR symptoms were exacerbated by smoking whereas it was weakened by regular NSAID use. Lastly, the sensitivity analysis that assessed the effect of non-participation among controls in the estimated effect of smoking and BMI (the two risk factors most likely to be affected by non-participation) showed a slight overestimation of effect of smoking assuming higher exposure rate among non-participants but not BMI while the effect remained strong and statistically significant. Conclusion Smoking, alcohol and GOR symptoms were the environmental factors strongly associated with all subtypes of oesophageal cancers. However, the dose effect patterns of these exposures varied by cancer subtypes. Smoking and alcohol were the larger contributing factors for OSCC whereas smoking and GOR symptoms had greater impact on OAC and GOJAC. Low to moderate wine consumption and regular NSAID use reduced the risk of all three subtypes significantly. While selection bias may have led to mildly inflated risks for smoking, the effects persisted even when modelled under extreme scenarios of biased participation amongst controls, and there was no evidence that selection bias materially affected the other associations.
| Identifer | oai:union.ndltd.org:ADTP/286808 |
| Creators | Nirmala Pandeya |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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