Selective cancer treatment regimes based on assaying for the presence or absence of a mutation in PI3K in a blood or serum sample obtained from a patient having cancer. The cancer is treated with 5-(2,6-di-mor-pholin-4-yl-pyrimidin-4-yl)-4-trifluoromethyl-pyridin-2-ylamine or its hydrochloride salt, or (S)-Pyrrolidine-1,2-dicarboxylic acid 2-amide 1-({4-methyl-5-[2-(2,2,2-trifluoro-1,1-dimethyl-ethyl)-pyridin-4-yl]-thiazol-2-yl}-amide) on the basis that the patient is determined to have in their ctDNA a PIK3CA mutation.
