Your reply is very interesting, I never heard of oncolytic viruses before (shame on me). It seems to be quite promising, especially for affordable treatments

I worked on gene therapy to elicit an immune response against specific cancer cells. This is done on an individual bases, thus very expensive (and often not very effective), and probably will not be available for many people for a long time.

I don't think the other answers are wrong, but they describe a very different mechanism.

The oncolytic viruses you describe are targeting cancer cells and kill them.

The other answered describe gene therapy using viruses. The virus there does not kill them, but are used as a vector for a DNA vaccine that tells the immune system to target specific cancer cells. The cancer cells themselves are not infected.

The oncolytic viruses answer is probably more what OP meant.

It is original idea of tumor immune gene therapy. You isolate the cancer cell, put part of it in a virus (usually Adenovirus) and infect the humane cells. It is a fairly easy process. the difficulty is to select the best part of the DNA used as antigen

This is done since the mid 1990s, with mixed results.

This is also what the Astra Zeneca Covid vaccines is using.