Oncologist fires Stage 4 Cancer Patient

My wife is a stage 4 cancer patient in active chemo treatments. We see our oncologist regularly and have had no lapse of insurance. Today we received a certified letter in the mail from the cancer center stating we are no longer able to see any doctors at this clinic. We have 30 days to find a new clinic and doctor. Is this legal? Some quick research shows that as long as she is in active treatment this is illegal. Any insights, please?