Pooled analysis of two case-control studies in New Caledonia and French Polynesia of body mass index and differentiated thyroid cancer: the importance of body surface area.

Background: New Caledonia and French Polynesia have among of the world highest thyroid cancer incidence rates. Studies have demonstrated a relationship between anthropometric parameters and the prevalence of cancer. In this study we evaluated further the relationship between body mass index (BMI) and other anthropometric parameters on the incidence of thyroid cancer in the New Caledonia and French Polynesia populations. Methods: We performed a pooled analysis of two case-control studies in New Caledonia and French Polynesia. We included a total of 554 cases (65 men and 489 women) of differentiated thyroid cancers and 776 population control subjects matched on sex, age, and study. Anthropometric factors (height, weight, BMI, body fat percentage [BF%], and body surface area [BSA]), at age 18 and before diagnosis, were analyzed by conditional logistic regression, adjusting for other independent risk factors. Results: A high proportion of cases (73%) were overweight (25-29.9 kg/m(2)) or obese (>= 30 kg/m(2)) before diagnosis of thyroid cancer (against 57% of control subjects). An increased risk of thyroid cancer was observed with greater height, weight, BMI, BF%, and BSA. The association of thyroid cancer risk with height, weight, BMI, and BF% did not remain when adjustment was made for BSA. By comparison, the odds ratios for the highest versus the lowest quartile of BSA at age 18 were 3.97 (95% confidence interval, 2.57-6.15; p < 0.001) for women and 4.06 (95% confidence interval, 1.03-16.06; p = 0.04) for men. The association between thyroid cancer risk and each of anthropometric factors did not depend on tumor size or menopausal status before diagnosis. Conclusion: Among anthropometric factors, BSA plays a dominant role in thyroid cancer risk and explains the apparent role of BMI.