Maximum tolerated dose chemotherapy assumes the problem is tumor volume: hit the cancer as hard as possible, kill as many cells as possible, repeat. The strategy treats the tumor as a fixed target. But a delay-differential equation model of tumor-immune-healthy cell interactions reveals the target moves.
Ansarizadeh and Zhang's four-equation model — tumor cells, healthy cells, immune cells, and drug concentration, with time delays capturing immune response latency — shows that tumor cell dynamics are intrinsically oscillatory under treatment. The population drops after a dose, partially recovers during the interval, drops again, and the oscillation envelope determines long-term outcome. Under maximum tolerated dose protocols, the oscillations are large: deep troughs that damage healthy tissue, high peaks where the tumor regrows between cycles. The immune system is collateral damage, suppressed by the same drug that targets the cancer.
Metronomic chemotherapy — lower doses administered more frequently — reduces oscillation amplitude. The tumor never drops as far, but it also never recovers as far. More importantly, the immune system is less damaged, so its ongoing contribution to tumor control is preserved. The model shows this combination — moderate direct cytotoxicity plus sustained immune surveillance — outperforms maximum cytotoxicity alone.
The structural point: the obstacle to curing cancer is not the tumor's resistance to chemotherapy. It is the oscillation. The same treatment cycle that kills cancer cells also creates the conditions for regrowth — immune suppression, healthy tissue damage, and a recovery window during which surviving tumor cells proliferate. The dose is the perturbation; the oscillation is the response; and a smaller perturbation can produce a better trajectory because it leaves the immune system intact to do the sustained work. Less damage per cycle, more control per lifetime.