I can't quote numbers for untreated brain mets, because we don't just watch people to see what happens. I would anticipate a survival of weeks to a couple of months at most without treatment, and it can be all over the map with treatment. You see some people with a survival of just 3-6 months, but I see many patients going beyond a year, and sometimes many years. WBR can allow the brain metastases to be treated effectively enough to have it take them not drive the prognosis -- instead, the treatment may be effective enough to make the cancer outside of the brain the driver of prognosis.
As for what WBR will do for quality of life, the real answer is "nobody knows". But not doing it is consigning to a very high probability of increasing headaches, nausea, possible seizures, balance problems, vision changes, and other neurologic problems.
There are many controversial areas in lung cancer, but whether to recommend WBR for someone with lung cancer and multiple brain metastases isn't one of them. Like the vast majority of oncologists, I don't hesitate to recommend WBR for such patients.
I don't know of any effective way to treat existing neuropathy. Some people try B12, or drugs for neuropathic pain, such as lyrica or neurontin, but I'm not aware of anything that has been shown to be particularly effective.
-Dr. West