What would happen if you injected a virus into a malignant tumour?

I understand that viruses enter cells and can destroy them, so what would happen if we purposely injected a virus, such as the adenovirus (the virus that causes the common cold) to destroy the tumour. I understand that our boides could become immune, but during the time in between? Surely this could cure cancer. Or is it possible to modify viruses to only attack cancer cells. I know that viruses are being modified for genetic modification i.e. cure genetic diseases by inserting correct alleles into faulty one. Could they do this for cancer?