Carcinogenesis is a multistep process that may involve several genetic and epigenetic alterations of transformation-associated genes, as well as changes of gene expression patterns. The type and timing of these alterations appear to be both tissue-specific and cell-type-specific. Histologically recognised phenotypes reflect such changes at the genetic and expression levels.
Without clearly defined clinical and histopathological diagnostic criteria, and more recently genetic and expression profiles, epidemiological studies and clinical trials are difficult to conduct.
