How exactly do bio markers detect gastric cancer?

I basically need a short summary on how the process actually works, because (I'm not a biologist) I'm not able to understand all the scientific vocabulary that is being used.

"Cancer precursor is a term used to describe lesions or systemic states with a high probability of invasive cancer occurrence ..."

"The precancerous process for the intestinal type of gastric carcinoma consists of the following steps...: chronic gastritis, atrophy, intestinal metaplasia and dysplasia."

1) "Chronic gastritis: ... the most frequent cause of chronic inflammation of the gastric mucosa is infection with Helicobacter pylori."

2) Metaplasia is "an adaptive substitution of one type of adult tissue for another type of adult tissue". Intestinal metaplasia most frequently resembles the small intestine in the precancerous process leading to gastric carcinoma.

3) "Dysplasia ... more advanced stages in the precancerous process are characterized by partial or complete loss of differentiation..."

So, first the stomach lining/mucosa becomes inflamed and this destroys normal cells. Second, as an adaptive response to chronic inflammation, metaplasia develops; that is, the cells lining the stomach transform into cells resembling cells in the small intestine. Finally, the cells lose their ability to differentiate (i.e. the ability to form any specific type of cell) and this is called dysplasia - the last stage before cancer.

These three states of gastric change (chronic gastritis, metaplasia and dysplasia) may be considered bio-markers or precursors of gastric cancer. These conditions are usually detected by biopsy.

An additional biomarker that can be measured in the blood and may be of potential value is Pepsinogen I or the Pepsinogen I to Pepsinogen II ratio. Pepsinogen I is secreted by the chief cells in the stomach and this secretion decreases with the loss of these cells that occurs with atrophic gastritis. So low levels of Pepsinogen I or of the I/II ratio 'are good indicators of extensive atrophy and intestinal metaplasia".

Hope this helps you. Best wishes and good luck.