you are right, there is no real accurate answer to this.

but this is the thought pattern i get from your words (oh and im a smoker too btw but have another vibe about it) :

1. "my family are heavy smokers therefore, im a heavy smoker, therefore everything that ever happened to them is going to happen to me"

2."i quit and than smoke again, therefore i cant quit therefore my destiny is written"

so when it come to smoking, we see the damages of cigaretts all around us, in tv,hospitals, commercials... but when it get closer to our family the fear become even bigger. because now, beyond the fact that we know it is dangerous, we also think that it is going to happen to us!! it is only natural.and it makes us forget that there are also many old healthy smokers and they are never on the news.

so from here there are few paths:

1.keep being fearful and stressed from the outcome that "you are causing yourself by cigarettes", smoking even more out of this stress, and probably manifesting some kind of bad situation, not necessarily cancer.

2.keep being fearful and stressed from the outcome that "you are causing yourself by cigarettes", come to a place of a magor fear and and from there make a decision to quit smoking. believing and knowing (it comes from the decision moment) that this act will bring you the healthy future you wish for. that will redue dramatically the stress and probably you'll get the outcome you want.

3.stop making any connection between your family members life paths to yours. make affirmations to yourself like "they are here to teach me that i creat my own reality", "i love cigaretts and cigaretts love me" , "i love to smoke, i enjoy every minute of it", "i never smoke and regret it because i love how it makes me feel" , "cigarrets are harmful to those how are afraid of them but me and my cigaretts are best friends" and i can go on and on.... and...you will probably get the outcome you wished for.
***and you can add here: "im not addicted to cigarettes. i can quit whenever i want to. i already did it and i can do it again, i just dont want to, because i love to smoke, i love smoking so much, it makes me feel so good" "im a healthy person NOW"

im sure there are more paths i didnt think about....but hope it helps

(tip - any affirmation - say OUT LOUD to attract the correct vibrations)

Occasionally I get a recurring thought in my head that I might get cancer in the future. I really don't like it and I quickly shift it. But it resurfaces every now and then.

If you change the direction of your thought within 17 seconds (more like 16 seconds, in these fast-moving times) then it does not add to your existing vibration.

However, you'll want to check that you definitely are changing the direction of it by checking you haven't gone numb to thinking the thought. You can do this by comparing how you feel about it to a solid good-feeling benchmark.

Engaging in a practice of regular meditation will provide that comparison benchmark. Though, regular appreciation is better if you are in vibrational range of it.

Florence Scovel Shinn wrote about a metaphysician that would under no circumstances mention about an operation he had just undergone.

He kept his mind on positive things and healed quickly because he would not give his thought to negative things.

The imagination is stronger than the will, if a thought of cancer enters my mind, not only that but having it. I must imagine myself with this, suffering or dealing with it, so this has an effect of multiplying into something I really don't want.

You can work on things to place in your mind to get you imagining health. For example, I read that as soon as you quit smoking your lungs are rebuilding back to health. That is something better to imagine.

From your description, you seem to be holding a negative belief that it is possible you may get cancer.

To hold a belief is the same thing as thinking something regularly without noticing.

How you get rid of it is you bring it to the surface, in order to think it fully, once, while you are noticing you are doing so, in order to shift it once and for all.

A great way to do that is to encourage the feeling you get from it instead of shifting the thought but also repressing the feeling out of habit. Really, really allow yourself to feel the fear of possibly getting cancer.

Then, intervene: Ask yourself: What would I have to believe in order to feel this?

When you get a straight answer, allow yourself to grasp what the belief means. When you do, and it kind of clicks, you can trust it's gone.

This will drastically reduce the frequency the thought may surface. A caveat, beliefs do often come in similar-looking clusters, so you may have to do this a lot with similar-looking beliefs. Don't worry about this- if you feel relief, you're making progress, and as long as you're making progress, that's all you need to do.

materialization
feeds by thought, even if
not wanted, see health, eat,
sleep, think right instead