Hi Charlotte.. welcome..
The stress of caring for a loved one especially where O2 levels drop so low is more than stressful for both of you. Have you tried finding another doctor?
Your mom is 85 years old. Are you able to communicate with her on a deeper more soulful level where you speak with one another from the heart? If she is alert you might want to ask her what she truly wants. Maybe she doesn't want all the radiation etc which destroys the quality of her life and time that is left.
I know when I cared for my husband, mother, father and pets, I would become afraid of their dying alone with me... What I began to understand as I reached out more and more to heal myself as well as love them unconditionally as I cared for them was that our spirits or souls do not die.. these shells we call our bodies are an illusion.. we just transform. Unfortunately, we think we are human beings having a spiritual experience but our true essence is spirit having a human experience.
Throughout our lives we have been conditioned by our parents, families and societies that death is so horrible.. while in all indigenous cultures they celebrate the passage rather than mourn.
Caregiving is an opportunity to look within at our beliefs around mortality, death, etc. It is also an opportunity when your mother is lucid to share precious moments where perhaps she can she her fears, you can share yours, and you can connect on a deeper level so that their are quality moments of connectivity between both of you.
As for pet scans and radiation... ask yourself and then ask your mom.. if it might be better to forego all the invasive harmful treatments that aren't working.. and make the choice to keep love surrounding both of you... keep her breathing treatments up.. help with her pain to stabilize her on her journey. You might even consider hospice which provides nurses and aids in your home.. as well as loving support for both of you.
Let me know what your thoughts are.
Richest blessings in all you are doing... breathe deep angel.. you are a gift to your mother... and many heartfelt lessons can be experienced for both of you.