This is Dr. David
yes, this could be dangerous
you should call up your radiation oncology doctor or chemotherapy medical oncology doctor and let them know you are coughing up blood
you could have a lung infection
or the esophageal cancer could be spreading into the lungs.
your doctors need to check for where the blood is coming from. your lungs or your esophagus or stomach.
hopefully it is only a small amount of blood.
blood from your esophagus and stomach can also go down your digestive system and can turn your stools tar black. this is called melena.
let me know if you have questions.