Severe pain is never easy to manage and I am sure your oncology docs can refer you to a pain management team. In our hospital our pain management team mucked about with Alex's meds until everyone was happy.

For us it was Fentanyl patches (don't remember the dose but it was relatively high), Oxycodone long acting and oxycodone short acting. He took the first two come hell or high water and the oxycodone short acting if he had breakthrough pain. This took him all the way through radiation and few weeks beyond.

The one piece of advice I would give is to take your meds even if you think you will be ok. Once the pain starts it is so much more difficult to get it back under control. My macho Alex took quite a while to learn this.