Feeling healthy

My 80 year old father has been diagnosed with growths in both lungs, one being larger than the other and also in his chest lymph nodes. We are waiting for the results of his biopsy in order to determine treatment. My father is fit and healthy and is a candidate for treatment we have been told.  He can’t understand how he can have cancer when he feels so healthy. He is continuing with his busy lifestyle, all other tests results were excellent. Is this normal please?