It depends on the cancer. Well-differentiated (meaning the cell is "well-differentiated" from its stem-cell plenotype and thus a lot like a normal cell) cancers can be difficult to distinguish tumor from normal cells, but then these also tend to be the slow-growing kinds. Undifferentiated cells are easy to detect under electron microscopy - and these are of the fast, aggressive cancers. They are also the easiest to kill with systemic treatment (chemo) but the most likely to recur.
I think the author, though, is referring to targeting rather than observation.