Iirc, some cancers can be very, very slow growing (some leukaemias and blood cancers are like this too), and with time, they can become more aggressive.  So treatment is usually left until such a point that the patient gets the most benefit from whatever that treatment may be. I think if they were to treat it too soon, there's a chance the cancer can adapt to the chemo etc and it would be useless when he would have benefited from it the most. i also think chemo isn't really effective on slow growing cancers.  it targets fast growing cells. I could have remembered this wrongly, but it's how i remember someone telling me that, that was going through something similar.

Some people can live for years and years without needing treatment because the cancer remains in its current state. A guy (that someone i mentioned previously) i worked with had a leukaemia that acted in this way.  He never received treatment for it. He died 11 years later from something totally unrelated.