It’s certainly not an irrational concern. On the other hand, having no concern at all isn’t necessarily irrational, is it?
So, you know of cancer and how it can affect people’s lives, and you know people close to you that have been so affected directly. It makes perfect sense to study this and see “what are the risk factors?” as well as you can tell, and how can I live my life to avoid those?
The problem is that unless you’re doing research constantly, it’s hard to know for certain what the risk factors really are. And some identified risk factors (as far as we know by the state of the art) are nearly impossible to avoid. I’m reminded in a way of the way science regarded some illnesses of the Middle Ages and even up to the twentieth century: “Night vapors” were a concern; tomatoes were thought to be poisonous; black cats represented dire things, and so on.
Well, you could live by those rules and avoid the night air, eat a very bland diet of “exactly what everyone else eats, and nothing new or tasty for me, thanks”, kill or banish various household pets and other things of that nature… and still get the diseases, because no one really knew the causes: bacterial and fungal infections, viruses and other pathogens. Dirty hands and fouled food and water killed more people than “night vapors” ever could. (The “poison” of tomatoes was the lead seals used in the canning process, which leached into the contents of the jar through the acidity of the tomato itself.)
So, where is all of this going?
I try to keep abreast of “some” research into the modern diseases that can hurt us (because something is going to kill us eventually, no matter how perfectly we try to live; I just want to avoid hurt in the meantime, and live well for as long as possible). And based on the research that I read about that seems plausible to me – not all of it does – I make choices to “moderate” my behavior in some areas.
My family has a history of heart disease, for example. So for me, the things I need to be aware of and modify are diet, exercise, blood pressure, etc. But I still eat things that are bad for me – in moderation, because I like them, dammit! – don’t exercise enough, but take care to be moderate in that when I do, and try to maintain blood pressure within the guidelines given by my doctor. But I don’t stress about things when I don’t eat right, when I feel lazy and sluggish, and when my blood pressure moves higher than it should.
I suggest that you attempt to maintain a balance between “being aware” and “control when you can”, and always tip the scale towards, “fuck it, you’re going to die eventually anyway; enjoy your life while you have it.”