What is the best way to treat lower back pain from sciatica due to a tumor in the lumbar area? On tarceva (erlotinib) for the cancer but how to stop the pain?

Get doctor advice. If your pain is due to metastatic cancer to the lumbar spine, radiation therapy should probably be administered. A combination of pain medications is often effective, and should include a narcotic analgesic and a non-steriodal anti-infllammatory agent. If the pain is radiating down your buttock and leg, addition of a medication for neuropathic pain could also be added. Please see your oncologist.
Ask your Doctor. If epidural steroid injection is OK to do. Sometimes it is sometimes its too risky. Depends on your situation.
Cancer pain. Treatment in this context is much more difficult than usual sciatica. The best option is to consult with a pain medicine specialist who can guide you through the options of cortisone injections, nerve blocks, radiation, acupuncture, biofeedback, meditation, or long-acting narcotic medications.