Fig. 1.

Impediments to virotherapy. This schematic drawing represents a tumor mass, and highlighted are some of the known and putative problems oncolytic viruses are facing. Oncolysis is ongoing in the nodule to the left, whereas tumor destruction is almost complete in the nodules to the right. At the top, a new nodule has formed consisting of virus-resistant cells emerging under the selective pressure of oncolysis. Tumor cells may also avoid viral destruction by hiding within strands of connective tissue.