I'm thinking of my grandmother across the world dying of lung cancer
What are you feeling right now while you are thinking of your sweet grandma?
I was just thinking of another person who is suffering.
If she's on your mind, why don't you consider visiting her?  I'm sure she would appreciate it, and since it sounds so serious the general expectation would be that family members should visit.  See if you can take time and go see her!
But I'm thinking of my grandmother dying of lung cancer across the world and how to not think about such a thing.
Well, there may be a reason that you cannot get that out of your mind. I think that a visit with her might make you feel better.