Very sorry to hear about your mother. Pain management is a very specialized branch and there are may interventions which can be tried but you need to consult a pain management expert for that.
There is a pain ladder which is to be followed and considering the pain you are describing your mother would require round the clock analgesics under supervision.
If she has bony pain, then radiotherapy and steroids
can also be helpful. Steroids are also helpful in cases of cerebral metastasis