Socioeconomic status and incidence of pediatric leukemia in Canada: 1992–2010

Highlights

• Childhood leukemia incidence in Canada remained consistent over the study period.

• Pediatric leukemia incidence had no significant association with household income.

• Incidence of childhood leukemia was not significantly associated with education.

• Policies should focus on mitigating risk factors that are independent of SES.

Abstract

Background

Leukemia is the most common cancer among Canadian children, representing about a third of pediatric cancers in Canada and is responsible for about one-third of pediatric cancer deaths. Understanding the effect of socioeconomic status (SES) on pediatric leukemia incidence provides valuable information for cancer control and interventions in Canada.

Methods

Using a linked data from the Canadian Cancer Registry (CCR), Canadian Census of Population (CCP) and National Household Survey (NHS) we aimed to quantify socioeconomic inequalities in the incidence of pediatric leukemia from 1992 to 2010. We used the concentration index (C) approach to quantify income- and education-related inequalities in the incidence of pediatric leukemia over time.

Results

Though there were fluctuations in incidence over the study period, our results showed that the total incidence of pediatric leukemia in Canada was generally consistent from 1992 to 2010. Incidence rate of 47 per 1,000,000 as at 1992 rose to 57 per 1,000,000 in 2010. The estimated values of the C over the study period failed to show any significant association between pediatric leukemia incidence and household income or education status.

Conclusions

Although pediatric leukemia incidence is not rising significantly, it is not reducing significantly either. The incidence of pediatric leukemia showed no significant association with socioeconomic status. Future cancer control interventions should focus more on mitigating risk factors that are independent of socioeconomic status.

Introduction

Leukemia is a malignancy of the blood that is generally classified according to clinical presentation (acute or chronic) and cell lineage of origin (lymphoid or myeloid) [1]. It is the most common cancer in children between 0–14 years of age [2], representing about 32% of pediatric cancers in Canada [3]. Of the different types of leukemia, acute lymphocytic leukemia (ALL) is the most common among children and it represents about 80% of childhood leukemia cases [[4], [5], [6]]. Though modern treatment has improved, with a cure rate as high as 80–90% [2], leukemia is still responsible for about 30% of pediatric cancer deaths [7]. Furthermore, the toxic and traumatic nature of the treatments means that there are short and long-term health consequences for surviving children and their families [8]. As more children survive, the need for long-term monitoring and follow-up care will only continue to increase [2].

The etiology of leukemia is quite complex and not totally understood, with numerous theories proposed over the years. Genetics play a key role in the incidence of leukemia as evidenced by the numerous translocations (e.g. TEL-AML1 and BCR-ABL), copy number alterations (e.g. CDKN2A deletion), point mutations (e.g. RAS mutations in AML), and epigenetic modifications that have been identified over the years [2,[8], [9], [10]]. Environmental factors also seem to contribute to the increased incidence of childhood leukemia. Parental exposure to pesticides, benzene, tobacco smoke (both during and after pregnancy), paints and solvents, petroleum products at home or work, organic pollutants as well as traffic emissions have all been associated with an increased risk of leukemia in children [2,[11], [12], [13]]. Pediatric leukemia, particularly ALL has also been associated with delayed exposure to infections at birth [5,8]. This infection theory suggests that delay in exposure of a child to common infections leads to a deficient immune system that is more susceptible to leukemia, especially ALL [5,14]. Another infection theory suggests that maternal infection during pregnancy increases the risk of childhood ALL [5,15]. The third well-known theory is Kinlen’s population mixing theory which proposes that children in more isolated or less densely populated communities are more likely to possess an immune system that has been exposed to a less variety of infectious agents than their counterparts in less isolated communities. These children are thought to be more likely to develop leukemia upon exposure to new infections from inward migrants [16,17]. Other factors like maternal folate supplementation during pregnancy as well as breastfeeding for more than six months have been observed to reduce the risk of ALL in children [2,8,14], while factors like Down Syndrome, fanconi anemia, hereditary immunodeficiency, exposure to ionizing radiation and caesarean delivery at birth have been observed to show a positive association with pediatric leukemia [2,8,14,17]. It is evident that a plethora of modifiable and non-modifiable risk factors contribute to the incidence of leukemia. Addressing modifiable risk factors at a population level might help reduce the incidence of pediatric leukemia in Canada.

The emphasis on the social determinants of health over the last few decades has led to an increased interest in assessing the impact of social inequalities on health status. In adults, studies have suggested that lower socioeconomic status (SES) is associated with an increased risk of colorectal, lung and cervical cancer while higher SES is associated with increased risk of breast cancer, melanoma and prostate cancer [[18], [19], [20], [21], [22], [23]]. Literature examining the effect of SES on pediatric leukemia reports mixed findings; while some reported childhood leukemia to be more frequent among individuals of low SES, other studies show high SES to be a risk factor for pediatric leukemia [14,24,25]. A Canadian study conducted in 2005 utilized data from the Canadian Cancer Registry (CCR) and demonstrated that higher income is a risk factor for pediatric ALL [26]. A review study by Poole et al. in 2006 observed a negative association when the incidence of childhood leukemia was measured against family income or parental education [27].

Although some of the current literature have investigated the association between childhood leukemia and SES, to the best of our knowledge, no study measures socioeconomic inequalities in the incidence of pediatric leukemias in Canada over time. Using a linked data from the Canadian Cancer Registry (CCR), Canadian Census of Population (CCP) and National Household Survey (NHS) we aimed to quantify income- and education-related inequalities in incidence of pediatric leukemia from 1992 to 2010. Understanding the distribution of the incidence of pediatric leukemia among different SES groups can provide valuable information for cancer control and intervention in Canada.