My dad says he has a tumor in his chest but he's scared to go to the doctor.what harm can it do to him?

Must be evaluated: The word tumor just means a lump of cells that isn't supposed to be there. It can be small, benign (not cancerous, so doesn't spread to other places), in an unimportant place, and slow-growing, which is not a big deal. It can be big, cancerous, growing into vital organs, and rapidly increasing, which can kill the person quickly. Of course, it can also be anywhere in between those extremes.

Ostrich : Approach. Sticking ones head in the sand does not make the danger go away, you just don't see what happening. First, need to chracterize the "tumor" by size location and what else it is doing (invading?, making fluid?, causing blood in sputum). Ct scan and biopsy; lung function tests. Does he smoke? Delay=denial. Hurry to get the work up done. Life and death difference.