A total of 106 cases were contacted and invited for interview, of whom 81 (76%) agreed to participate. After exclusion of six in whom the diagnosis of cancer had been made more than five years before interview, there were 75 cases of head and neck cancer (). There were 493 controls contacted and invited for interview, of whom 324 (66%) agreed to participate. Five control subjects declined to provide their income and were excluded, leaving 319 control subjects for the analysis. None of these five controls reported using cannabis. The characteristics of the cases and controls are shown in .
Anatomic site of malignancy by ICD code
The frequency distribution of cases and controls for selected variables
The risk of head and neck cancer did not vary with age because of the controls being frequency matched on the age of cases in five-year age groups to improve the efficiency of the study. A family history of upper respiratory tract cancer was not significantly associated with an increased risk of head and neck cancer (relative ratio [RR] = 0.6; 95% confidence interval [CI], 0.2-1.9) (). Males had a significantly increased risk of cancers of the head and neck compared with females (RR = 3.4, 95% CI, 1.7-6.7). In the age group studied, the increased relative risk of head and neck cancer for Maori and Pacific Island people compared with all other ethnicities was not statistically significant when adjusted for age, sex, pack years of tobacco smoking, joint years of marijuana use, alcohol consumption, and income level (RR = 1.9; 95% CI, 0.8-4.4) (data not shown). When ethnic groups were further divided into New Zealand European, Chinese or Indian, Maori, and other ethnic groups, the relative risks for Maori and Chinese or Indian ethnic groups compared with New Zealand Europeans were 2.2 (95% CI, 1.0-5.2) and 3.9 (95% CI, 0.6-24.8), respectively ().
Tobacco use, cannabis use, and alcohol consumption and risk of head and neck cancer
A low level of income was strongly associated with an increased risk of cancer of the head and neck, after adjustment for potential confounders, with the risk for those earning more than $70,000 per annum about one fifth of those earning less than or equal to $25,000 per annum (RR = 0.2; 95% CI, 0.1-0.4).
The relative risk from ever smoking cigarettes was statistically significant (RR = 2.1; 95% CI, 1.1-4.1) (). The relative risk of head and neck cancer increased with successive increases in cigarette smoking, with the highest quintile of pack years of cigarette smoking having a relative risk of 4.9 (95% CI, 1.9-12.4) after adjusting for confounding variables. This increased risk was 4% for each pack year of exposure (95% CI, 2%-6%) after adjustment for confounding variables (). A significant increase in the relative risk with increasing alcohol consumption was observed (test for trend, P < 0.01), and the relative risk of head and neck cancer from heavy alcohol consumption compared with nondrinkers was 5.7 (95% CI, 1.2-25.9).
Cannabis use and tobacco use as continuous variables and relative risk of head and neck cancer
The median duration of cannabis use was 10.5 years among controls (range, 0.25 to 29 years) and 25 years among cases (range, two to 32 years). Ever use of cannabis was not associated with a significantly increased risk of head and neck cancer (). The risk associated with the highest tertile of cannabis use (>8.3 joint-years of exposure) was not statistically significant, RR=1.6 (95% CI, 0.5 to 5.2) after adjustment for confounding variables including tobacco smoking, alcohol consumption, and level of income. When cannabis use was fitted as a continuous variable, the estimated 4% increase in the risk of head and neck cancer for each joint year of exposure (RR = 1.04; 95% CI, 0.97-1.11) was not statistically significant, after adjustment for confounding variables (). When cannabis use in the five years before diagnosis or reference date was excluded, the risk from each joint year of exposure increased but again was not statistically significant (RR = 1.08; 95% CI, 0.77-1.53). Adjustment for whether cannabis use was solely from joints or combined with other methods of use did not appreciably alter these results. The age at which subjects started smoking cannabis was not associated with the risk of head and neck cancer (data not shown). Adjustment for occupational risk of respiratory tract cancer or the consumption of different food groups did not appreciably alter the estimate obtained for the relative risk of head and neck cancer from cannabis use.