Firstly, I'm sorry to hear about your Dad, it's a very unfortunate scenario.
When you administer radiotherapy you are relying upon the fact that cancer cells divide very rapidly and are therefore rapidly turning over their DNA, making it more susceptible than healthy tissue to damage by the radiation.
But tumours are a heterogeneous mix of cells, all dividing at different rates and different times, and some cells are more abnormal than others.
This means that the radiotherapy can often miss some cells which can then subsequently 'seed' recurrent tumours at the same site, or elsewhere.
So why not administer more radiotherapy then, to take them all out ? Well, unfortunately, whilst cancer cells are more sensitive than healthy tissue to the effects of radiation, eventually healthy tissue is damaged too, particularly the lungs - which can develop fibrosis - and the heart. So it's all about treading a fine line between hurting the tumour as much as you can, and hurting the patient as little as possible.
The evidence is that radiotherapy is very good for pain relief and control of local symptoms produced by metastatic disease. There are some conditions in which it can also be curative, but, in the context of disseminated cancer, its main application is in zapping troublesome mets.
Surgery is often not the best option because, with metastatic disease, it is associated with considerable morbidity (and mortality) for the patient, yet has no impact on survival.
I wish I could offer you a more positive response, but I regret that there are no magic cures for this condition. The emphasis must therefore be placed on achieving the best quality of life possible, and ensuring that your dad remains comfortable and pain free. Luckily we're pretty good at that these days, and there are some excellent support services on offer which can make a huge difference.
"I never forget a face, but in your case I'll make an exception"
- Groucho Marx