Chapter 4: Cancer incidence in Canada: trends and projections (1983-2032) – Leukemia - HPCDP: Volume 35, Supplement 1, Spring 2015

Chapter 4: Projections by Cancer Site

23. Leukemia

Leukemia is the second most common lymphohematopoietic cancer diagnosed in Canadians, after NHL. During 2003–2007, the average annual number of new leukemia cases was 2570 for males and 1875 for females, making up 3.2% and 2.5% of all new male and female cancer cases, respectively (Tables 4.23.1 and 4.23.2).

The lifetime risk of developing leukemia is 1 in 53 for males and 1 in 72 for females.^{Endnote 1} The lifetime probability of dying from the disease is 1 in 96 for males and 1 in 125 for females. The 5-year relative survival rate for leukemia is intermediate among cancers, at 59% in males and females for 2006–2008 in Canada.^{Endnote 1}

Leukemia is the most frequently diagnosed childhood malignancy, representing 72% of the lymphohematopoietic cancers in children and youth aged 0 to 14, and 33% of all childhood cancers in Canada during 2003–2007 (data not shown). Figure 4.23.1 shows how age-specific incidence rates varied with age in 2003–2007. The incidence rate had a small peak in children aged 0 to 4, then decreased and remained stable until age 30 when it started to increase slowly to the early 50s. The rate then escalated rapidly and reached its peak in people aged 85 and over. Almost three-quarters (74%) of leukemia cases occurred in those aged 55 or older. Males and females had similar rates up to age 50, after which the rates were higher in males and the divergence between sexes increased consistently with advancing age (Figure 4.23.1). The male-to- female ratio of ASIRs increased steadily from 1.3:1 in the under 45 age group to 1.9:1 in those aged 85 or older.

The overall ASIRs for leukemia decreased slightly from 1983–1987 until 1993–1997 (Figure 4.23.2). Thereafter, the incidence rates increased slightly to the same level in 2003–2007 as in the earliest observation period 20 years before. During 1998–2007, leukemia ASIRs increased non-significantly in males by 0.6% per year and increased significantly in females by 1.2% per year (Figures 3.1 and 3.2). Most of the increases occurred in men aged 65 or older and in women aged 65 to 74. The regional trends in ASIRs of leukemia were very similar for males and females (Figure 4.23.2). In the last observation period (2003–2007), the internal ranking of the regional ASIRs was similar for males and females, with significantly low rates in Atlantic Canada and elevated rates in the Prairies.

The predicted regional trends of leukemia ASIRs are similar in males and females (Figure 4.23.2). The ASIRs are expected to increase slightly in British Columbia, in the Atlantic region, and most evidently in Ontario. The rates will decrease marginally or stabilize in the Prairies and Quebec. The ranking of the regions in ASIRs will be the same for males and females. The rates for Ontario are projected to surpass those for the Prairies within the next 5 years. The Atlantic region will continue to experience the lowest rates. Figure 4.23.3 shows that the ASIRs of leukemia are predicted to increase or level off across the age groups but the difference between males and females persists over time.

From 2003–2007 to 2028–2032, leukemia ASIRs for Canada are projected to increase by 5% in males, from 15.1 to 15.8 per 100 000, and by 7% in females, from 9.2 to 9.8 per 100 000 (Tables 4.23.3 and 4.23.4).

The annual number of newly diagnosed male cases is projected to increase by 98%, from 2570 to 5095, and the number of new female cases, by 88%, from 1875 to 3520 (Tables 4.23.1 and 4.23.2).

Comments

Leukemia is a cancer of the white blood cells. It has 4 main subtypes: acute lymphocytic leukemia (ALL), chronic lymphocytic leukemia (CLL), acute myeloid leukemia (AML) and chronic myeloid leukemia (CML).

The overall observed incidence rates for leukemia were generally modest, decreasing slightly during 1983–1997 and then increasing slightly, a pattern consistent with that observed in the US.^{Endnote 293} The etiology of leukemia remains largely unknown. The established and suspected risk factors include environmental exposures, genetic (inherited) susceptibility and lifestyle category.

In 2008, IARC documented that exposure to ionizing radiation, alkylating agents (used in chemotherapy) and benzene had been consistently associated with leukemia risk.^{Endnote 86} Ionizing radiation is linked to increased incidence of AML, ALL and CML, but not CLL. Exposures include atomic bomb exposure (Hiroshima and Nagasaki), nuclear power plant accidents and therapeutic radiation exposure. ^{Endnote 52, Endnote 53, Endnote 86, Endnote 294, Endnote 295} Findings from exposure to electromagnetic fields are inconsistent. IARC has concluded that occupational benzene exposure causes leukemia, particularly AML.^{Endnote 47, Endnote 86} Benzene is also present in tobacco smoke and gasoline. Smoking, accounting for 20% of AML cases, is associated with a 2-fold increase in AML risk including in a Canadian cohort.^{Endnote 52, Endnote 111, Endnote 295, Endnote 296} The prevalence of these exposures has been stable or decreasing during the study period and can explain the relative stable and the decreased component of the incidence trend of leukemia.

Viruses, particularly retroviruses, have been linked to the development of leukemia. IARC considers HTLV-1 infection to be causally associated with leukemia.^{Endnote 47} HTLV-1 infects 5 to 10 million people in the world, ^{Endnote 297} with clusters in certain ethnic/ geographical concentrations in Japan, Africa, South America, the Caribbean and Melanesia.^{Endnote 297, Endnote 298} EBV is the etiological agent of acute infectious mononucleosis and is associated with adult ALL and aggressive NK-cell leukemia.^{Endnote 53, Endnote 298, Endnote 299}

Some identified chromosomal anomalies are related to leukemia. CLL, the most common adult leukemia in the West, is estimated to have one of the highest familial risks for a hematological malignancy. ^{Endnote 300} A 7.5-fold increased risk of CLL has been observed in people with a first degree relative with CLL.^{Endnote 301} Cytogenetic abnormalities occur in around 50% of CLL cases.^{Endnote 302} A reciprocal chromosomal translocation known as the Philadelphia chromosome is present in 95% of CML cases.^{Endnote 303} Philadelphia chromosome is also seen in about 20% to 30% of adult ALL cases.^{Endnote 304} Children with Down syndrome have a 10- to 30-fold increased risk of ALL and AML.^{Endnote 52, Endnote 53, Endnote 111, Endnote 294, Endnote 305} Inherited genetic syndromes have been linked to 5% of ALL and AML cases.^{Endnote 306}

Lifestyle factors have been implicated in the development of leukemia.Endnote 307 A meta-analysis showed an increased risk for each of the 4 major subtypes of leukemia in obese people.Endnote 308 A Canadian case-control study estimated that, compared with normal weight, the odds ratios of leukemia were 1.3 (95% CI: 1.1–1.5) for overweight and 1.6 (CI: 1.3–2.0) for obesity.Endnote 309 Overweight and obesity together accounted for 18% of all leukemias in Canada—9% from overweight and 8% from obesity.Endnote 309 The prevalence of obesity has increased in Canada.Endnote 51, Endnote 72, Endnote 73, Endnote 110 However, current data are not sufficient to conclude whether the increased prevalence of obesity explains some of the recent increase in leukemia incidence. Based on the same populationbased Canadian data, the researchers observed a 25% risk reduction for adult leukemia associated with the highest category of vigorous physical activity (OR = 0.75; CI: 0.57–0.99). Risk reductions associated with high levels of vigorous activity were greater with higher BMI.Endnote 310

Changes in coding practice and the clinical ability to identify leukemia may contribute to some of the increase. In addition, changes in classification could also influence the trends.^{Endnote 295}