[Lani]

Ellie F--clonal selection induced by treatment refers to the idea that tumors are heterogeneous and have different kinds of cells in them, say some cells that her her2+ER+pr+ and others that are her2+ er- pr- and others still that are her2-ER+pr+ just for an example. Let's say you treat the tumor with antiestrogens--that would be effective against the her2-ER+PR+ clones within the tumor, but give a competitive advantage to the her2+er-pr- parts of the tumor (and the her2+er+pr+ parts as well, although to a lesser extent as her2+ tumors are less sensitive to antiestrogen treatments). Conversely, if you treated that same original tumor conglomeration with herceptin, it would affect the two her2+ types of clones, but not the her2-er+pr+ parts of the tumor, which, again, would have a competitive advantage and be able to thrive and grow despite the treatment.

It used to be thought that the tumors were made out of only one kind of cell and that a mutation took place which changed the cell from her2- to her2+ or er+ to er- in order to elude treatment effectiveness.

Now the clonal concept is more popular ...it may actually be that the cancer stem cell , whether a true stem cell or a progenitor cell, is responsible for the recurrence of the tumor, but the bulk of the tumor is made up of more differentiated "offspring" of the stem or progenitor cell and if there are many types of these "offspring" each can start its own family using these disparate clones.

Hope this helped!