It could mean cancer but talk with your vet about it before you jump to any conclusions
What Does Spots On A Cat,s Lungs Mean?
My cat also has spots on his lungs. He has labored breathing, which is an effect of it. It could be cancer, or sometimes pneumonia or other diseases. The best thing would be to check with the veterinarian. My kitty is on strong antibiotics right now, which seem to be somewhat helping. Hope this helps!