White Daffodil is the flower for lung cancer.
I would strongly suggest that you do not get a tattoo.
I think that in a fairly short time you will regret having the tattoo.
Every time you look at the tattoo flower you will forever be reminded of your mom and and her connection with the dreadful disease of cancer and her pain and suffering, and sadness as she struggles with this, as well as your own sadness and both your fears during this time.
I've been through this for almost 5 years with someone I loved very very much, and believe me as cancer treatments and time goes on from weeks to days to months to years, your mom (and you) may shudder at even driving by the hospital or clinic where your mom went through so much. Conquer the beast and move on. As much as your mom and you will want to, you will never forget.
An alternative suggestion would be to wear a silver or gold chain with a pendant of something your mom loves. If she loves horses, a pendant of a horse, or a reminder of something that would make her smile, a favourite bird if she loves birds maybe a hummingbird pendant, if she loves music a musical note, or loves the beach a seashell, maybe something with an inscription of your mom's name and your love for her, or a gardening symbol, or a sunshine symbol. Whatever you choose will be associated with happiness because it is what makes your mom the most happy.
I hope my thoughts and our personal experiences help.