Why do we think chronic inflammation can cause cancer? I know the pathway is not fully understood, but what makes scientists believe that inflammation causes cancer?
There are good epidemiological data for this.
Links between cancer and inflammation were first made in the nineteenth century, on the basis of observations that tumours often arose at sites of chronic inflammation and that inflammatory cells were present in biopsied samples from tumours.
There are many triggers of chronic inflammation that increase the risk of developing cancer. Such triggers include microbial infections (for example, infection with Helicobacter pylori is associated with gastric cancer and gastric mucosal lymphoma), autoimmune diseases (for example, inflammatory bowel disease is associated with colon cancer) and inflammatory conditions of unknown origin (for example, prostatitis is associated with prostate cancer). Accordingly, treatment with non-steroidal anti-inflammatory agents decreases the incidence of, and the mortality that results from, several tumour types.
Cited from Mantovani, Alberto, et al. "Cancer-related inflammation." Nature 454.7203 (2008): 436-444. http://www.sld.cu/sitios/reumatologia/temas.php?idv=21500
We don't think inflammation causes cancer - we know inflammation causes cancer. The mechanisms are still being identified, but chronic inflammation causes cellular turnover and proliferation that appears to lead to the development of cellular damage and subsequent mutations.
Chronic inflammatory states are clearly associated with the development of cancer in a number of diseases. One of the most common that comes to mind is hepatitis, which literally means "inflammation of the liver". Chronic hepatitis B and hepatitis C infections are two of the most common causes of liver cancer (hepatocellular carcinoma) world wide. This is why we vaccinate people for hepatitis B - it has greatly decreased the incidence of liver cancer.
There are a number of review articles that discusses potential mechanisms available on Pubmed that are freely available and worth taking a look at for more information.
Inflammation and the inflammatory response can lead to tissue damage, and these stresses at the cellular and DNA level can lead to changes in the cell's programming. One example I can think of is neutrophils and macrophages--inflammatory cells--releasing reactive oxygen species (ROS)--reactive oxygen radicals--at the site of inflammation, which can damage normal tissue and their DNA. With enough changes and damages--in particular, mutations and translocations to genes that regulate cell growth--the cells can undergo anaplasia and ultimately dysplasia and neoplasia (cancer).
Check our wikipedia for these term, "reactive oxygen species" and the section on caner (https://en.wikipedia.org/wiki/Reactive_oxygen_species) as well as the textbook "Robbins and Cotran Pathologic Basis of Disease" if you can get your hands on it.
Chronic inflammation also appears to mediate epigenetic changes that serve as a fertile ground for the evolution and origins of fully transformed, neoplastic cells. In fact, there are inflammatory conditions such as chronic periodontitis where this is pretty well established. Paper here
There is also evidence for genetic and transcriptional disruption induced by inflammation in model organisms. Paper Here