Increased colorectal cancer incidence in Iran: a systematic review and meta-analysis

Iran is the second-largest nation in the Middle East. It is a large and diverse country,
comprising several different religious and ethnic groups. It is divided into five
main regions, with 31 provinces. Recently, Iran was ranked as an upper middle-income
economy country by the World Bank. According to the GLOBOCAN 2012, colorectal cancer
is the fourth most common cancer among men and the second among women in Iran 4]. The rising incidence and mortality rates of colorectal cancer demand a systematic
approach toward a specific and national research program in Iran. We included in this
meta-analysis the studies reporting different measures of incidence, age-standardized
incidence rate (ASR), and crude incidence rate (CR). The data sources were primarily
based on the Iranian National Cancer Registry System (INCRS) database results, in
which more than 85 % of the expected numbers of cancers are being registered.

Globally, colorectal cancer is the second most common cancer in women and third in
men. Although rates differ significantly in different parts of the world, incidence
rates vary ten-fold in both genders worldwide. North America, Western Europe, Australia
and New Zealand (with an ASR of 44.8 and 32.2 per 100,000 in males and females, respectively),
have the highest estimated incidence rates. The lowest ASR was reported from Western
Africa, with 4.5 and 3.8 per 100,000 for men and women, respectively 4].

Colorectal cancer is the third leading cause of cancer incidence and mortality in
North America (North America includes Canada) in both genders and about 245,000 new
cases occur annually in United States. The American Cancer Society(ACS) has reported
the colorectal cancer ASR as 57.2 and 42.5/100,000 in men and women, respectively,
during 2003–2007 25]. According to the latest reports, these rates decreased to 50.0 and 37.8 per 100,000
in males and females, respectively, by 2011 26]. The incidence and mortality rates have been decreasing slightly but steadily every
year, and in March 2014, ACS released data showing that colon cancer incidence rates
had dropped by 30 % in the last decade 27].

Colorectal cancer is the second most common cancer as reported in most European epidemiological
studies. Its incidence rates have been steadily increasing from 1984 to 2007 according
to some reports 28], 29]. Mistry et al. from United Kingdom have stated that although the ASRs for males and
females were 56.2 and 37.3, respectively, in 2007, the incidence rate is projected
to decrease by ?7 % in males. Also, ASR for 2013 was projected to decrease to 52.3for
men, but this theory is not applicable for women, according to this projection 28].

In this systematic review and meta-analysis, we included 39 studies from 2000 to 2014,
from different provinces and areas of Iran. The northern part of the country is well-recognized
to have the highest incidence of gastrointestinal tract cancers. The Golestan province,
in northern Iran, is located at the western end of Asia.

Historically, esophageal cancer(EC) incidence is very high in this region 30]. Latest reports have shown a decrease in EC incidence rates, but an increasing trend
in the incidence rates of CRC, and the highest overall ASR for colorectal cancer was
reported from this province 31]. In Tehran city, the capital of Iran and the largest city in Iran and Western Asia,
the incidence of CRC is higher than the WHO estimate for Iran. Colorectal cancer ASR
has the highest rate (16.4) in males, and for women it is 12.2. It was higher than
neighboring countries 12], 23], 32]. In an ecological study conducted in 2013 in all the 22 districts of Tehran, an interesting
correlation between colorectal cancer incidence and different food groups, socioeconomic
position scores, and other risk factors such as smoking was established. Colorectal
cancer was found to be more common among the higher socioeconomic position groups,
with a higher prevalence of risk factors, such as western dietary models, smoking,
obesity, and low physical activity 23].

Several reports on the ASR and CR of colorectal cancer in East Azerbaijan province,
in the North West of Iran, are also included in this review 8], 14], 15], 33]–35]. It is located in Iranian Azerbaijan, bordering West Azerbaijan and Turkey, with
Turkish culture. There is an increasing trend in CRC incidence, especially in females.
According to our data, the highest CR for colorectal cancer was reported from this
province, with 11.50 cases for males, and 9.22 cases for females. These rates are
comparable with other regions of Iran and some neighboring countries, and should be
considered in more comprehensive studies 12], 15]. Turkey is located in Western Asia with the portion of Eastern Thrace in southeastern
Europe, bordering Iran from the East. Based on data retrieved from the Turkey Cancer
Registry System, Yilmaz et al. estimated ASRs and CRs of all cancers from 2000. Colorectal
cancer was the seventh most frequent cancer in both genders with lowest ASRs and CRs
as compared to our results for the same Turkish cultural provinces of Iran. The overall
ASR was about 7, and overall crude rate was 7.51 36], 37]. In Turkey, cancer control programs aim to reinforce healthy nutritional habits by
education and raising public awareness. Colorectal cancer screening programs were
also introduced in more restricted parts of Turkey, which seem to be successful. Comprehensive
screening programs could be useful for future planning of colorectal cancer control
in Iran. Timely diagnosis and removal of adenomatous polyps may help in the prevention
of this disease. Early diagnosis of local tumors may also improve the survival rate
of patients. Educational programs on colorectal cancer screening may increase the
overall knowledge about risk factors and screening modalities.

Several reports have shown that the ASRs and CRs of colorectal cancer in southern
Iran are lower than in the north 2], 21], 38], 39]. The CRs in both genders were the lowest in Hormozgan province in the southeast of
Iran, with 1.10 and 1.2, for males and females, respectively 21]. Nutritional habits (high consumption of vegetables and fruits), and genetic factors
may play an important role. Also because of climatic conditions, date fruit is extensively
grown and consumed in this part of Iran. Some studies suggested that dates have a
strong antioxidant, which can potentially decrease the incidence and mortality rates
of most gastrointestinal tract cancers.

Recently several reports revealed an increasing incidence and crude rates of colorectal
cancer in Asian countries, which was comparable with other developed countries, especially
in the younger population. Westernized lifestyle and diet, specifically high consumption
of red and processed meat, obesity, and low physical activity have been established
as the main cause of increasing risk of colorectal cancer in most Asian countries.
However, how westernized diet increases the risk of cancer remains unknown, and could
be due to both genetic and epigenetic polymorphisms 40].

While colorectal cancer was the fifth most common cancer among males, and the third
in females in China 41], it was the fourth most frequent invasive tumors according to the data quality analysis
on cancer incidence in six cancer registries in different parts of China, including
urban and rural areas. The overall ASRs and CRs were respectively 14.61 and 21.33,
in urban areas, and 11.09 and 12.43 in rural areas. This suggests other potential
causes such as public education, promotion of healthy lifestyles and socioeconomic
factors 42] . Hong Kong had a higher ASR and CR of colorectal cancer in recent years, and an
increasing trend from 1983 to 2006. The overall crude rate was 57.1, and ASR was markedly
higher in men than in women (68.2 and 47.1, respectively) 40].

In Lebanon, a small middle-income country on the Eastern Mediterranean shore, the
ASRs for colon cancer in both genders are slightly higher than our and other results.
Colon cancer was the fourth most common cancer among males, with ASR of 15.3, while
it was the second in females with ASR of 14.1. The ASRs for rectal cancer were 4.2
and 4.8 in men and women, respectively. The higher ASRs for both colon and rectal
cancers may be attributed to the highest elderly population in Lebanon among other
Arab countries 43]. According to Saudi Cancer Registry in recent years, colorectal cancer was the most
common cancer among men and the third in women. The overall ASR was 9.6, with 9.9
for men and 9.2 for women, which were similar to our rates in Iran 44].

Limitation

The data sources for this study were primarily based on the INCRS database results,
in which more than 85 % of the expected numbers of cancers are being registered. However,
we did not have access to data from all cities and rural areas of the country.