I have noticed the following statement.
The authors are testing some drugs. They select some drug and treat the animals with tumor,
then look at the effect of this particular drug.
perturbed group - treated group
unpertubed group - untreated group
"Perturbed Growth Model (Treated Groups).
Whereas in the unperturbed model, all tumor cells are assumed to be proliferating,
the perturbed growth model assumes that the anticancer treatment makes some cells
nonproliferating (see Fig. 1 ⇓ ), eventually bringing them to death".
The tumor cell is proliferating infinite number of times,
the healthy cell = treated cell should die after about 40 or some limited amount of proliferation cycles, right?
I've found one more explanation in the paper:
"It was assumed that cells affected by drug action stop proliferating and pass
through n different stages (named x2, …, xn+1), characterized by progressive degrees of damage,
and, eventually, they die. The dynamics by which the cells proceed through progressive degrees
of damage is modulated via a rate constant k1 that can be interpreted in terms of kinetics of cell death.
The number n of stages of damage and the value of k1 affect the shape of the distribution of the time-to-death of
damaged tumor cells."