To quote Kitano 2003: "Cancer is an extremely complex and heterogeneous disease that exhibits a high level of robustness against a range of therapeutic efforts.  Robustness enables a system to maintain functionality in the face of various external and internal perturbations.  Robustness is not an accidental property, but follows certain principles.  It allows tumours to promote growth and survival in several ways.  For example, heterogeneity among tumour cells provides a high level of redundancy, and hence increased chances of survival and growth; these benefits are further enhanced by feedback controls at the cellular level.  Viewing cancer as a robust system may provide insight for the development of new drugs and therapies."

For an introduction to tumor robustness, see Kitano 2003.  For a more in-depth discussion, see Kitano 2004.  For a discussion of robustness in a broader biological context, see Kitano 2004.  For a discussion of robustness applied to drug design, see Kitano 2007.

For two related PowerPoints by Sriram (2008?), see Modeling cancer as a robust system for therapeutic intervention and Robustness of molecular networks in normal and cancer cells.