Working with NCI statistician Richard Simon, Norton came up with a new model of tumor growth based on the
work of the 19th-century mathematician Benjamin Gompertz. The concept (which other researchers proposed in the
1960s) holds that tumor growth generally follows an S-shape curve. Microscopic tumors below a certain threshold
barely grow at all. Small tumors grow exponentially, but the rate of growth slows dramatically as tumors get bigger,
until it reaches a plateau. A corollary of this: The faster you shrink a tumor with chemo, the quicker it will grow
back if you haven't killed it all.