Breast, cervical and colorectal cancer survival rates for northern Saskatchewan residents and First Nations

Abstract

This descriptive study was done 1) to explore and describe the proportional distribution of breast, cervical and colorectal cancers by stage (a proxy measure of availability, access, and utilization of secondary prevention strategies) in northern Saskatchewan First Nations and non-First Nations in comparison to southern Saskatchewan First Nations and non-First Nations; 2) to assess the impact of stage and age on the survival patterns for these cancers in northerners and First Nations whose survival patterns have been shown by previous research to be equal or poorer in comparison to southerners. Univariate and multivariate survival analyses were carried out to ascertain the impact of the different proportions of stage for each study group on survival. Stage at time of diagnosis is a proxy assessment of secondary prevention services, which include formal screening programs. Data for this study was obtained from the Saskatchewan Cancer Registry, which has been maintaining cancer data since 1932. Cancer stage at time of diagnosis information is complete in the registry for different years for each cancer site. Hence data for breast cancer was for the years 1970 to 1995; cervical cancer data for the years 1980 to 1995; colorectal cancer data for the years 1990 to 1995. The proportion of cancer cases for each site by TNM stage and age were compared among the four study groups. First Nation and northern populations were found to have a larger proportion of diagnoses at a later stage in comparison to the southern non-First Nation group. Using Cox's proportional hazards model, both stage and age at time of diagnosis were found to be significant predictors of survival for all study groups. Age and stage adjusted relative risks were calculated and found to be significant in comparison to the southern non-First Nation group for cancer of the breast (RR =1.81 P=0.013). For cervical cancer the relative risk of dying of cervical cancer for southern First Nations in comparison to southern non­-First Nations was found to be 1.38 but this was not statistically significant (p = 0.097). For colorectal cancer, the relative risk of dying of colorectal cancer was found to be better for northern First Nations in comparison to southern non-First Nations (RR = 0.59), however this was not statistically significant (p = 0.45). This study showed that despite adjusting for stage and age at time of diagnosis, there were still some unexplained differences in the survival pattern of northern First Nations, northern non-First Nations and southern First Nations in comparison to southern non-First Nations. Hypotheses as to what these unexplained differences are have been offered. These include differences in socio-economic status as well as availability, accessibility, attitudes towards and knowledge of secondary prevention strategies. Further study into these unexplained differences should be carried out.