How rare is a tumor in the lung in a 20 year old causing horners? had a chest x-ray last year too does this rule out upper lung tumor or no? does a chest x-ray last year rule out a lung tumor?

Need to examine: It is not feasible to provide a meaningful opinion without examining you. Lung cancer is unlikely from the information you provided. A cause may not be discovered in some cases. Visit this site for information and talk to your doctor: https://www.mayoclinic.org/diseases-conditions/horner-syndrome/symptoms-causes/syc-20373547 Wish you good health!