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

Chapter 4: Projections by Cancer Site

14. Prostate cancer

Prostate cancer is the most common form of cancer and the third leading cause of cancer death in Canadian males. One in 7 can expect to be diagnosed with prostate cancer in their lifetime, and 1 in 28 males to die from it.^{Endnote 1} The annual number of new cases was 21 460 in 2003–2007, constituting 13.8% of all new Canadian cases of cancer and 26.6% of those in males (Table 4.14.1, Figure 3.9).

During 2001–2007, the ASIRs for prostate cancer were stable (Figure 3.1). The risk of developing the disease increases with age more than for any other cancer (Table 4.14.2). Between 2003 and 2007, 66% of the overall cases were in men aged 55 to 74, while only 8% were in males younger than 55. A high 5-year relative survival rate of 96% was reported for 2006–2008.^{Endnote 1}

Overall incidence rates of prostate cancer increased steadily until 1993–1997, showed additional but smaller increases to 1998–2002 and then levelled off (Figure 4.14.1).

This pattern was observed in the Prairies, the Atlantic region and Ontario. The rates in British Columbia reached their first peak one period sooner than in other regions, then gradually levelled off and decreased thereafter. Quebec experienced much lower rates in the last 10 observation years than other regions. The trends of age-specific ASIRs show that recent incidence rates of prostate cancer in Canada increased in the younger age groups (<75 years) and decreased in the older age groups (75+) (Figure 4.14.2).

The Nordpred method produces extreme increases in prostate cancer incidence rates and counts, so we used a two-step approach of the short-term modelling projection followed by the long-term constant rates projection (method ADa + AVG, see Chapter 2 for the definition). Consequently, British Columbia and the Prairies are likely to have a drop in prostate cancer incidence rates, Ontario and the Atlantic region will experience an increase, and Quebec and Canada will remain stable in the first 5 prediction years (Figure 4.14.1). The rates for each region will remain unchanged thereafter.

From 2003–2007 to 2028–2032, the ASIRs for prostate cancer in Canada are expected to be stable at about 123.3 per 100 000 (Table 4.14.2). Despite this trend in ASIRs, the aging and growth of the population mean that the annual number of new cases is projected to increase by 97%, from 21 460 to 42 225 (Table 4.14.1).

Comments

The lower prostate cancer incidence rates observed in Quebec are likely artefactual, as a result of possible underreporting of cases (see details in Chapter 5: Data quality issues).

A range of medical investigations, including digital rectal examination, transrectal ultrasonography, PSA level, fine-needle aspiration biopsy (FNAB), and magnetic resonance imaging have been considered for the early detection of prostate cancer. ^{Endnote 195} Part of the rise in incidence in the 2 decades prior to 1990 has been attributed to detection of cancers following transurethral resection of the prostate for benign prostatic hypertrophy.^{Endnote 196} The 1993 and 2001 peaks in incidence mirror the 2 waves of increased PSA screening activity.^{Endnote 1} In 2003, the percentage of men reporting a screening PSA test in the past 12 months was highest in the 60–69 age group at just over 35%.^{Endnote 197}

When a slowly developing cancer is detected through screening, over-diagnosis may occur. This is concurrent with the fact that these cancers are most frequent at older ages when competing causes of death are more frequent.^{Endnote 198} The observed increase in prostate cancer incidence rates may be biased by such over-diagnosis from PSA screening. The benefits and harms of prostate cancer screening by PSA test is still being debated,^{Endnote 199} and it is important that men know the arguments in order to decide whether to screen for the disease.

Similar to England, recent incidence rates of prostate cancer in Canada increased in the younger age groups (<75 years) because of increasing uptake of the PSA test, and decreased in the older age groups (75+) from increasing pharmacological treatment for obstructive uropathy caused by benign prostatic hypertrophy and the resulting decrease in use of transurethral resection, which had previously led to detection of many prostate cancers (Figure 4.14.2).^{Endnote 28, Endnote 200} This pattern resulted in relatively stable crude rates in the last 2 observation periods. Therefore, the recent trends could be used for a model of short term projections. However, such a model is less suitable for long-term projections because it would be expected that the prevalence of screening would plateau in the future. Figure 4.14.3 shows the projected ASIRs in the first 10 projection years derived from using the current 2-step approach (ADa + AVG), and using the age-specific trend model (ADa) only to project for all the 10 years. There appears to be no substantial advantage to a 10-year projection for using model ADa only. Similarity of the estimated national ASIRs between using ADa + AVG and ADa may be because the decreases in rates in older age groups were partly cancelled out by the increases in younger ages, and may also be a result of offsetting provincial differences. The projected increases in the 10-year incidence rates in the Atlantic region and Ontario, from using ADa only, seem less likely. Thus current trends were only extended to the first 5 projection years through ADa based on yearly data. The age-specific average rates of the predicted 5-year incidence data were then assumed to remain constant in the future years. Current projections of prostate cancer rates will be an overestimate if recent decreases in the rates (based on the more recent observed data, which were not available when present study was undertaken) continue.^{Endnote 1}

The established risk factors, which are all non-modifiable, are age, family history and ethnicity. Having a first-degree relative with prostate cancer more than doubles a man's risk of developing this tumour.^{Endnote 201, Endnote 202, Endnote 203, Endnote 204} Risks to sons appeared to be lower than in brothers. Risk is further increased by early age at onset in relatives and multiple relatives with the disease.^{Endnote 201} A number of studies may suggest an overall contribution of inherited genes or a shared environment in the development of this disease.^{Endnote 201, Endnote 202, Endnote 203, Endnote 204} Genes and family history account for about 5% to 9% of all prostate cancers.^{Endnote 205} White males have a lower risk of prostate cancer than Black males worldwide but have a higher risk than males of Asian ethnicity.^{Endnote 56, Endnote 206, Endnote 207} However, the risk for Asian Americans is higher than that for males of a similar background living in Asia.^{Endnote 208} Ethnicity may be a surrogate for genetic, environmental or socioeconomic factors.^{Endnote 208}

Although a definite modifiable risk factor has not been identified, a number have been implicated in prostate cancer initiation. Potential preventative factors include physical activity and frequent intake of soy and foods containing lycopene.Endnote 209 Lycopene may reduce prostate cancer risk by preventing oxidative DNA damage in prostate tissue by mitigating exposure to cellular free radicals.Endnote 209, Endnote 210 A meta-analysis shows that high soy consumption reduces the risk by 26%; however, this inverse association appears to be confined to the Asian population.Endnote 211 Possible explanations for this observation include different types or amounts of soy products consumed in Asian and Western societies.

Factors that may increase prostate cancer risk include high intakes of dairy products and meat.^{Endnote 209} Higher intake of calcium has been associated with a 39% increased risk,^{Endnote 212} possibly because of the suppression of 1,25-dihydrovitamin D, which may inhibit cancer cell invasion.^{Endnote 213} IARC stated that evidence of an increased risk of prostate cancer in relation to exposure to thorium-232 and its decay products is limited, as is exposure to X-radiation and gamma-radiation, use of anabolic androgenic steroids, exposure to cadmium and cadmium compounds, or exposure to arsenic and inorganic arsenic compounds and the rubber production industry.^{Endnote 47}