The size of the tumor likely makes a big difference too: a centimeter-sized lump contains 1000x more cells than a millimeter-sized one. It will contain 1000x more drug-resistant cells (given identical mutation rates). For example, a human brain tumor is likely to already contain hundreds of cells carrying a mutation for every amino acid in every protein. Thus for any inhibitor drug you try, hundreds of cells will already be resistant, and they will grow back the tumor in just a few months. That’s much less likely to be the case in a mouse tumor.
