I would say that having a tumour growth protruding on the forehead is very rare. If its brain cancer, the tumour would not grow till it protrudes out of the head because the cranium, which is the bone covering the brain, is too hard. Also, the bump seems to be very centralised. Tumours would not be able to choose a centralised location to grow... their growth is random. Unless you are a strong believer in coincidence.
From what I see, the highest possibilty of cause of this bump is bone deformity. This is fairly common and can happen during the time when the cat was a foetus. Ask the old man who feeds the cat how long he has been feeding it and from when did the cat get this bump. If its been there all along, this is probably it. You don't have to worry if its bone deformity. If it has survived till now, it will probably continue to survive till something else kills it.
It could also be an abscess/ inflammation. Although no wound is visible, this does not rule out the possibility. This is because it only takes a very small puncture for bacterial infection to enter and cause an inflammatory reaction. Or, the would might have closed externally but the abscess remains inside because it isn't drained. Scarring might not be obvious because of the fur covering. Sometimes, the immune system will kill off the abscess after some time. But sometimes, the immune system cannot kill it and it needs to be drained physically and use antibiotics to kill it.
Well, the way to get the closest guess is to touch the thing. Press it, feel it. If it feels like a water sac and it hurts, it could be an abscess. If it feels like there's a ball inside, it could be a tumour. If it is kinda irregular and very firm (cannot move at all), it could be bone deformity.
Btw, I am not a Vet and I did not study very hard when in school so pls do not take my words as professional advice or the ultimate truth. I'm just sharing what I know/ think. Hope that helps.
