My cat is 6 years old. He suffered from asthma for 3 years. Also he has the tumor in lung. He is taking Flexotide 250 twice a day for 1 year already and is very well now, no cough attacks, but the tumor is growing. Could it be caused by Flexotide ( Flovent) or not? How long should we treat the cat with this inhaller?

I have read nothing that would lead me to believe that the medication is the reason for the tumor growth.
That is a great question for your vet.
Should you stop the inhaler, No, not unless instructed to do so by your vet.
Do you know that the tumor is in the lung or is it in the chest cavity?
Has your vet discussed referral to a specialist for tumor removal, or is that not an option?

Thanks for you answer, Ladysam. I live in Russia and I don't think we have very experienced vets here ( even in Moscow where I live). The tumor is in lung, all other parts of body are OK. Oncolologist proposed a surgery to remove the tumor, but I don't think they can do this successfully. The cat is active now, some small attackes appear very seldom, I'm afaraid of flexotide can be used for so long time... The tumor first discovered 3 years ago. And since that time it didn't grow so much. The fast growth was determined within this last year ( and we took Flexotode for this time). But I still don't think the surgery is an option. Do you know cats taking Flexotide for a long time without any break?

I've had many cats over the years, but not one with asthma, we only have a couple of clients that have required flovent as a treatment option.
This site has some good basic information and some links you can check out.
Feline Asthma - Treatment and Management

In the grand scheme of things, the lung tumor is problematic in that it takes up valuable space that is meant to work for oxygen movement.

I do understand your concern though regarding having the tumor removed.
There are a couple of other options for treatment, like I said I've not read anything that would lead me to believe the tumor growth is related.
I'm curious though as to whether your cat has been tested for heartworms, typically thought to be a canine disease, but cat's can get it and I don't know if it is prevalent where you are, but it may be worth checking into.
Also take into consideration his environment, smoke , pollen, dust mites, and other things can make matters worse or may trigger his condition.
If there is any way for you to reduce those irritants in the home it may help him out.
I wish you best in caring for your little friend, he's lucky to have a caring owner such as you.