"looking for pain relief due to stage 4 rectal cancer that has metastasized to spinal and pelvic bone."
I would look into taking "rick simpson oil/ hash oil / FECO" as adjunctive treatment.
I would contact https://constancetherapeutics.com and discuss treatment options using the oil.
I would also use rectal cannabis suppositories. Inhaled cannabis and sublingual tinctures can also be very helpful for immediate relief.