2. Epidemiology of esophageal cancer

Esophageal cancer is the ninth most common malignancy in the world. Most of the cases are either squamous cell carcinoma (ESCC) or adenocarcinoma (EAC). The former is the predominant one, accounting for ~90% of the cases. ESCC occurs in the squamous cell lining of the middle section of the esophagus and is more often found in Asia and Africa. China alone is responsible for more than 50% of the patient population. EAC, on the other hand, takes place in the cuboidal cells of the esophageal glands near the gastroesophageal junction and has been growing rapidly in western countries in recent years. Both types of esophageal cancer happen more often in males than in females, and the overall ratios of male to female are approximately 2.5 for ESCC and 4.4 for EAC. At the first look, the incidence of esophageal cancer seems to be geographic-related, as ESCC is more seen in Asia and Africa while EAC more common in Europe and North America. However, if we analyze the data further, we notice that the issue is actually more ethnic rather than geographic. Take a look at the cases in the United States, where ESCC incidence is found 4.8 times higher in Asian- and African Americans than in Caucasians, while EAC is just the opposite, 5 times higher in Caucasians than in other Americans [2]. Apparently, these two diseases selectively adhere to certain races of people regardless of where they live. This notion is also supported by the data from China, where ESCC patient population is 77 times greater than that of EAC [4]. Apparently, after a long history of sharing residential resources, each ethnic group has formed its unique life habits. For this reason, they tend to develop common health problems.

As far as we know today, smoking is the No. 1 risk factor for ESCC, particularly when it is in conjunction with drinking. A study found that ESCC incidence increased 12-fold in males and 19-fold in females in the population who use tobacco and alcohol together, compared to those who have one of the hobbies alone [5]. This connection can be easily seen in China, where the tobacco consumption is the highest in the world, higher than all other developing countries combined [6]. The Chinese also consume a lot of alcohol, particularly in northern and central provinces, where the ESCC incidence can reach 0.8% of the local residential population [1]. Here is the east end of the so-called "esophageal cancer belt." This association is also reflected by the data on the American males of Asian and African origins, who tend to smoke and drink abreast, thus making up for 90% of the ESCC patient population in the United States [7].

While the use of tobacco and alcohol together has been the main risk factor for ESCC, obesity and low vegetable consumption increase the chances to develop EAC. In the obese community, the excessive body weight puts constant pressure on the stomach and causes frequent acid reflux. These highly acidic fluids regurgitated from the stomach or even from the duodenum induce inflammation in the esophagus. As the episodes continue, the epithelial lining of the esophagus gradually transforms from stratified squamous epithelium to intestinal columnar phenotype for adaptive protection, as the latter is more endurable to acidic insults. Unfortunately, however, this metaplasia confers a greater danger to become malignant. Studies have shown that people with this kind of esophageal adaptation could have 400 times more likelihood to develop EAC than the general population [8]. Insufficient uptake of fresh fruits and vegetables can also create this type of drama.

Although both ESCC and EAC take place in this short organ, they are very different cancers. From an epidemiological point of view, there is only one common feature between ESCC and EAC, and that is both preferring men over women, while differences are a lot greater.
