If a parent and child develop the same cancer, is it any more likely that the same (mRNA) drug will work for both of them? Or do cancer cells mutate so much tha

If a parent and child develop the same cancer, is it any more likely that the same (mRNA) drug will work for both of them? Or do cancer cells mutate so much that they're unrecognizable between patients? It would have interesting implications if in the future a family could invest in a bank of mrna molecules for their future generations. A new form of wealth.