Cancerous cells are formed when normal cell undergoes mutation and starts dividing uncontrollably. It is the oncogene that undergoes mutation that initiates the uncontrolled growth of the cancerous cells. It is theoretically possible to design specific RNAi that interferes with the mutated version of the oncogene. Thus, prevents the effect of the cancer.
At the same time, it should be noted that the RNAi should not silence the prototype or wild type of the oncogene, as the activity of the prototype is highly essential, for the proper functioning of the cell cycle in the human system. Instead, the most harmful chemotherapy would kill the fast growing hair and stomach cells, the usage of silencing genes are highly recommended.
Thus, the only factor that controls this process is that, we do not know where exactly mutation would occur in the oncogene. If we know the sequence of the mutated oncogene, we would be able to put end to the cancerous cells.