Epidemiology and Etiology of Prostate Cancer

In 2018 in the United States, there will be an estimated 164,690 new cases of prostate cancer (19% of all male cancer incident cases, 1st) and an estimated 29,430 prostate cancer mortalities (9% of all male cancer deaths, 2nd only to lung/bronchus cancer).1 Over the last four decades, there was a spike in prostate cancer incidence in the late 1980’s/early 1990’s secondary to the widespread adoption of prostate-specific antigen (PSA) testing for the asymptomatic detection of prostate cancer.2 Since 1991, prostate cancer mortality has decreased by more than 40% due to a combination of increased PSA screening and improvement in treatment.3 This article will discuss the epidemiology of prostate cancer, as well as focus on several important etiologic risk factors associated with the disease.

Epidemiology

Incidence
For the last 30+ years, prostate cancer has been the most common noncutaneous malignancy among men in the United States, with 1 in 7 men being diagnosed with the disease.4 Similar to the United States, prostate cancer is the second most commonly diagnosed malignancy among men worldwide, with 1.1 million new cases diagnosed per year.5 In developed countries, the age-standardized rate (ASR) of prostate cancer incidence is 69.5 per 100,000 compared to 14.5 per 100,000 in developing countries.5 Crude differences in incidence between developed and developing countries are likely secondary to poor use of screening6 and lower life expectancies in developing countries.

Mortality
Among all men in the United States, 1 in 38 men will die from prostate cancer.4 Prostate cancer ranks as the leading cause of cancer death globally, with the highest mortality rates noted in the Caribbean and Southern/Middle Africa.5 Furthermore, the ASR for mortality in developed countries is 10.0 per 100,000 compared to 6.6 per 100,000 in developing countries.5 Many hypotheses have been considered as to the decline in mortality seen in the United States after 1991, with the most commonly accepted (in addition to utilization of screening) being an increase in aggressive curative treatment of prostate cancer diagnosed in the 1980s.7,8

A Global Perspective
The incidence and epidemiology from a global perspective is much different than what may typically be seen in the United States and Europe. For example, in India the incidence of prostate cancer is a fraction (1.7-5.3 per 100,000) of other first world countries (North America: 83.2-173.7 per 100,000),6 but secondary to the sheer size of the population (1.2 billion) the crude prevalence of prostate cancer is on par with countries such as the United States, UK, France, and Italy. Despite greater awareness of prostate cancer screening, data from India suggest that 4% of patients <50 years of age present with metastatic disease. Among worldwide prostate cancer incidence, 14% of cases are diagnosed within the Asia-Pacific region, with a wide variation of incidence rates across the Asian-Pacific countries. In Latin American countries, prostate cancer represents 28% of all incidence cancers (highest) and 13% of all cancer mortalities (second after lung cancer). Specifically, in Trinidad and Tobago, Guyana and Barbados, the incidence is 3-4x that of the United States. Particularly in Cuba, the mortality rates continue to increase, despite greater adoption of screening.  The incidence of prostate cancer in the Middle East is higher than the Asian countries, specifically in Lebanon (37.2 per 100,000), Jordan (15.3 per 100,000), and Palestine (15.2 per 100,000). A closer look at the demographics at presentation among Middle Eastern men shows that 25% present with metastatic disease, with rates as high as 58% among men from Iraq. There are several hypotheses for the shift in incidence and prevalence of prostate cancer among non-North America/European countries, including (1) an increased awareness of prostate cancer leading to greater utilization of PSA testing, and (2) adoption of a more “Western” diet/lifestyle and less the traditional Indian/Asian/Mediterranean diet, particularly in the urban centers.

Unfortunately, a greater burden of disease among non-North American/European regions has presented several problems:

  1. A wide variation in incidence/prevalence across countries in these regions, particularly in Asia-Pacific and Latin America
  2. Huge discrepancies in quality and access to care between the private and public sector
  3. Poor access to newer standards of care, leading to high rates of surgical castration and limited access to radiation therapy (ie. for treatment of bone metastases)
  4. Limited organized cancer registries, thus grossly underestimating the true incidence and prevalence of prostate cancer
Age and Family History
Age is an established risk factor for prostate cancer, as men <40 years of age are highly unlikely to be diagnosed with prostate cancer, whereas men >70 years of age have a 1 in 8 chance of prostate cancer diagnosis.1 In a population-based analysis of more than 200,000 patients, increasing age was associated with higher 15-year cancer-specific mortality (CSM) rates: 2.3% for men diagnosed ≤50 years of age, 3.4% for men 51-60 years of age, 4.6% for men 61-70 years of age, and 6.3% for men ≥71 years of age.9

A family history of prostate cancer is also strongly predictive of a prostate cancer diagnosis. To be considered hereditary prostate cancer, a family must have three affected generations, three first-degree relatives affected, or two relatives diagnosed prior to age 55.10 Men with one first-degree relative previously diagnosed with prostate cancer have a risk of a prostate cancer diagnosis that is 2-3x that of individuals with no family history.11 Data on 65,179 white men from the PLCO cancer screening trial showed that men with a family history of prostate cancer had a significantly higher incidence of prostate cancer (16.9% vs 10.8%, p<0.01) and higher cancer-specific mortality (0.56% vs 0.37%, p<0.01).12

Race
Both prostate cancer incidence and mortality have been shown to be significantly related to race. African-Americans and Jamaicans of African descent have the highest incidence of prostate cancer in the world.1 Despite decreases in mortality since the 1990s among all races, death rates of African Americans are still 2.4x higher than Caucasian patients.13 Several studies have assessed possible reasons for this discrepancy. In an ad hoc analysis of the REDUCE trial, African American men had greater non-compliance with study-mandated2 and 4-year prostate biopsies, despite having greater prostate cancer risk (at 2-year biopsy),14 suggesting that population-level estimates of the excess prostate cancer burden in African American men may underestimate the degree of prostate cancer disparity. Gene expression assessment of prostate cancer specimens has noted numerous differentially expressed genes between African American and white patients, suggesting that there may be racial differences in androgen biosynthesis and metabolism.15 However, studies in mCRPC patients assessing clinical response to the potent anti-androgen abiraterone have not demonstrated racial differences when prospectively evaluated.16

The incidence of prostate cancer among races other than African-American and Caucasians is lower, including men of Asian descent living in the United States, although their incidence of prostate cancer is higher than those living within continental Asia.17 Interestingly, men moving from developing countries to high prostate cancer incidence countries demonstrate a shift in prostate cancer risk similar to that of their new country of residence.18 Ultimately, the relative components of genetics, socioeconomic factors, cultural, and environmental factors associated with racial differences observed are poorly understood.   

Trends
There has been a significant drop in prostate cancer incidence in the last decade (~10% annually per year from 2010-2014), likely secondary to a decrease in PSA testing after the US Preventative Services Task Force (USPSTF) Grade D recommendation for screening of men older than 75 years of age (2008) and subsequently all men (2012) due to concern for overdiagnosis and overtreatment.13,19 Following the USPSTF recommendations, an analysis of the National Cancer Database suggested that in the month after the recommendation, incident prostate cancer diagnoses decreased by 1,363 cases, followed by a drop of 164 cases per month thereafter for the first year (28% decrease in incident cases).20 There has been considerable debate as to whether patients present with the more advanced disease since the USPSTF recommendations with no general consensus.21 Recently, the USPSTF changed their recommendation for PSA screening among men aged 55-69 years of age to a Grade C, suggesting that men in this age bracket should undergo periodic PSA-based screening for prostate cancer based on a decision after discussion regarding the potential benefits and harms of screening with their clinician.22

Etiology and Risk Factors

Diet and Obesity
Initial evidence that diet and lifestyle may have a role in prostate cancer epidemiological outcomes was provided by ecological studies which demonstrated that men in Western countries had higher rates of prostate cancer than developing/non-Western countries.6 To strengthen this possible association, subsequent studies demonstrated that men from non-Western countries migrating to Western countries adopted similar lifestyle and prostate cancer risk as those in Western countries.23 Nonetheless, several prospective studies since these ecologic descriptions assessing self-reported dietary patterns of healthy foods and the risk of prostate cancer have failed to show an association with diet and risk of prostate cancer.24,25 Epidemiological evidence suggesting that statins reduce the risk of advanced stage prostate cancer suggests a possible role of cholesterol and prostate cancer risk.26 Regardless, the complexity of the Western diet and the association/interaction with healthier lifestyles are limitations to understanding how diet influences prostate cancer risk.  

Obesity has become an epidemic in the United States, and observational studies have suggested a modest increase in the risk of prostate cancer among obese individuals.27,28 The pathophysiology between obesity and prostate cancer is likely secondary to higher levels of estradiol, insulin, and free IGF-1 levels, as well as lower free testosterone and adiponectin levels.29 However, a clear pathophysiological explanation between obesity and prostate cancer is still uncertain and may be associated with lower serum PSA and larger prostates leading to fewer prostate biopsies.30   

Inflammation
Chronic inflammation has been implicated in the development of several cancers and may also be associated with prostate cancer. Possible etiologic factors suggested include: infectious agents, dietary carcinogens, hormonal imbalances, as well as physical and chronic trauma.31 As a result, intra-prostatic inflammation may lead to DNA damage, epithelial proliferation, cellular turnover, and angiogenesis.31 In men part of the placebo arm of the Prostate Cancer Prevention Trial (PCPT), those with at least one biopsy core of inflammation had an odds ratio (OR) of 1.78 (95%CI 1.04-3.06) for prostate cancer compared to men with no cores of inflammation.32 Furthermore, this association was even higher when considering a high-grade prostate cancer diagnosis (OR 2.24, 95%CI 1.06-4.71).32

Medications
As mentioned, there has been emerging evidence that HMG-CoA reductase inhibitors (statins) may be associated with a lower risk of prostate cancer mortality following diagnosis.33 Although there have been disparate results for the beneficial nature of statins and prostate cancer, a recent meta-analysis from observational studies of nearly 1 million patients noted that both post- and pre-diagnosis use of statins are beneficial for both overall survival (HR 0.81, 95%CI 0.72-0.91) and cancer-specific survival (HR 0.77, 95%CI 0.66-0.85).34 Nonetheless, the exact role statins play in prostate carcinogenesis/protection is still widely debated.

Similar optimism with statins has been associated with metformin use and prostate cancer outcomes. Among patients with diabetes, metformin has been associated with a significant dose-dependent benefit for both prostate cancer-specific (HR 0.76, 95%CI 0.64-0.89 for each additional six months of metformin use) and all-cause mortality.35 In a systematic review and meta-analysis of observational studies assessing clinical outcomes of patients with prostate cancer and metformin, medication use was marginally associated with a reduction in risk of biochemical recurrence (HR 0.82, 95%CI 0.67-1.01), but not associated with metastasis, prostate-cancer mortality or all-cause mortality.36

Genetics
Prostate cancer is known to have an extraordinarily complex genetic makeup, including somatic copy number alterations, point mutations, structural rearrangements, and changes in chromosomal number.37 It is estimated that 5-10% of all prostate cancers may be caused by dominantly inherited genetic factors.11 These include, but are not limited to HPC1, HPC2, HPC20, HPCX, PCAP, and CAPB.38 More famously are the mutations associated with BRCA1 and BRCA2 and the associated increase in the risk of clinically significant prostate cancer, and prostate cancer-specific mortality among men with screen-detected prostate cancer.39-41 Recent research has evaluated epigenetic markers for prostate cancer such as miRNA. The first report of miRNA and prostate cancer was reported in 2007,42 and since then many reports have implicated over 30 unique miRNAs and prostate carcinogenesis.

Conclusions

The epidemiology and etiology of prostate cancer are complex and multi-factorial. Prostate cancer remains a malignancy spanning the spectrum of localized/indolent disease to de novo advanced disease that is ultimately fatal. Although there are accepted differences in race and geography, the ultimate interplay between sociodemographic factors, environmental/lifestyle factors, and genetic differences remains to be fully elucidated.
Written by: Zachary Klaassen, MD, MSc
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