Fig. 1

The main mechanisms by which SPT occurs. Normal cells receive one (or more) genetic hits and produce a daughter cell with genetic alterations that gradually replace normal epithelial cells to form a tumor field. When the tumor field expands at the expense of normal epithelial cells, additional genetic alterations occur, prompting the tumor field to develop into obvious cancer and metastasize or the cancer cells to form a second tumor by submucosal spread or intraepithelial migration or to form a polyclone with additional induction