The genetically altered cells have, over time, reproduced unchecked, crowding
out the surrounding normal cells. After a period of ten years, the growth may
contain one million cells and be the size of a pinhead. At this point the cells
continue to look the same as the surrounding healthy cells.
After about a million divisions, there's a good chance that one of the new
cells will have mutated further. This cell, now carrying two mutant genes,
could have an altered appearance and be even more prone to reproduce
unchecked.
