“…A crucial feature of the Greenspan model is that the contraction rate is proportional to the size of the necrotic core; McElwain & Morris (1978) showed that similar qualitative behaviour can be achieved by assuming that the only source of cell contraction is through apoptosis ('programmed' cell death, Moore, 1987) in the viable rim. Glass (1973) studied a single quasisteady reaction-diffusion equation for the inhibitor distribution in a tumour and its effects on eventual tumour saturation; this study has also spawned many subsequent investigations involving the effects of geometry and of different source functions representing tumour heterogeneity (Shymko & Glass, 1976;McElwain & Ponzo, 1977;Adam, 1986Adam, , 1987aAdam & Maggelakis, 1989;Chaplain & Britton, 1993); however, Chaplain et al (1994) showed that the same qualitative behaviour can be obtained from a spatially varying diffusion coefficient. More recently, Byrne & Chaplain (1996) extended the assumptions of Greenspan to include the effects of apoptosis, and they generalized the assumptions on the inhibitor so that it could be viewed as the application of an anticancer drug.…”