Cancer is considered a progressive multi-step disease, why is this? Tumor development arises from the accumulation of 4-6 different mutations in cells over the course of many years O Tumors development spreads to other tissue with time. O Tumor development requires mutations in the affected cells, as well as defects in the immune system. Tumor development happens in a step-wise fashion as telomeres shorten with each cell division. O Tumor development gets worse with time.