At the fundamental level the cells lining the oesophagus have mutated and become uncontrolled in the way in which they divide to form new cells. This occurs at the DNA level inside a cell. The DNA may originally have had a propensity to mutate (such as when cancer ‘runs in families’) and/or suffer damage to them over time. In Western societies like in Australia the most common type of cancer is adenocarcinoma which can be caused by damage from Gastro-oesophageal reflux (GORD). It also has a higher association in males, caucasians and those who are overweight or obese. These may all be inter-related (such as when obesity leads to reflux which can cause oesophageal cancer), or it can just occur independently. In some people that develop this cancer there may not always be a significant history of reflux or other risk factor.
The other main type of oesophageal cancer that occurs is squamous cell carcinoma. It has a strong association with smoking. It is also the most common type in East Asia and it is thought to be due in part to high consumption of chemical carcinogens that can be present in certain foods like heavily smoked or pickled foods.