Cancer incidence near oilfields in the Amazon basin of Ecuador revisited
San Sebastian et al1 reported an overall excess for all cancers in San Carlos, Ecuador (standardised incidence ratio (SIR) 2.26; 95% CI 0.97 to 4.46 for males), and an increased mortality due to cancers in males (standardised mortality ratio (SMR) 3.6; 95% CI 1.31 to 7.81). The authors concluded that there was an excess of cancer cases among the villagers linked to environmental pollutants stemming from oil production activities.
Since clusters present many epidemiological biases, the Centers for Disease Control issued a set of guidelines2 in 1990 to investigate clusters. Under those guidelines, we reviewed the cluster of cancer cases reported in San Carlos.
Cancer cases from 1989 to 1998 were obtained from the original article. Data on cancer occurrence and mortality in the region were obtained from the National Cancer Registry of Ecuador, Quito (1993–7),3 using GLOBOCAN. GLOBOCAN4 is a database built by the descriptive epidemiology group of the International Agency for Research on Cancer. The International Agency for Research on Cancer calculates cancer mortality from national mortality data (1998–2000) corrected for estimated completeness (72% for males, 68% for females from Ecuador) to avoid the effect of deaths not notified to health authorities. We obtained data on population for the village of San Carlos, Ecuador, from the 2001 Ecuador census, the first to include data on the population of San Carlos.
San Sebastian et al1 underestimated the population of San Carlos by almost 50%. In the absence of census data, they estimated the population to be “approximately 1000” and assumed the population to be constant during the period 1989–98, which was not the case as shown in the 2001 census where population growth in the province of Orellana was 84% during the period 1990–2001 (from 46 781 to 86 493 people). Census data showed the average population was 1471 and that it grew by 84% in 11 years.5 In males there were 8 cases of cancer observed and, according to the 2001 census, 8.8 were expected (SIR 0.91; 95% CI 0.42 to 1.72). In females, there were 2 cases observed and 8.9 expected (SIR 0.22; 95% CI 0.04 to 0.42). In males, there were 6 deaths observed and 6.1 expected (SMR 0.98; 95% CI 0.40 to 2.03). No cancer deaths were reported in females.
Using data from the 2001 census and applying the Centers for Disease Control cluster guidelines yielded no excess of cancer or cancer mortality in the village of San Carlos. Full details of the methodology used for the calculation can be obtained from the authors at http://www.riskmr.com.
Arana, A., & Arellano, F. (2007). Cancer incidence near oilfields in the Amazon basin of Ecuador revisited. Occupational and Environmental Medicine, 64(7), 490.