I am so sorry to hear that you have breast cancer metastasis, but I hope that with much advanced treatments we have nowadays, you will be able to beat this disease successfully.
I wish I could answer your question with certainty regarding air traveling safety due to your breathing problem...But I think the best thing to do,is to consult your healthcare provider who is the best source of information for questions and concerns related to your medical situation.
Wishing you all the best..
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