TK1 and breast cancer: ,26,41 Nevertheless, the plausibility of this finding—that TK1 kinetics behave differently during cytotoxic chemotherapy—is bolstered by similar data in non-small-cell lung cancer showing better prognosis when an increase in TK1 activity was observed following chemotherapy,42 while this early spike in TK1 activity has also been demonstrated in metastatic BC as well.43