thickening of esophagus on the outside

New here so just looking for any thoughts. My husband is a 3 year survivor of colon cancer and on the past few ct scans has shown thickening of the esophagus, he has had 2 endoscopies and the last one the doc biopsied the inside area of esophagus where thickening shows up on the ct scan all came back negative which he thought would happen, because the inside of esophagus is clean no lesions or masses. he said the abnormality is on the outside. That was back in october. his oncologist just ran another ct scan and it shows the same thing no real change, no masses in lungs or on liver. He said he would run a petscan if we want but he doesn't think that would show anything. He said he cannot guarantee that it is not cancer, but he feels it is a real low chance of it being anything. He did go on to say that it is our choice and if we decide in a month or whenever before we see him again he would order the petscan. Just wanted to know if anyone has had or heard of cancer on the outside of esophagus. My dad did have cancer of the esophagus back in the 90's so I am not too unfamiliar as to what it entails. Thank you in advance for any insight anyone might have. Take care and God Bless. Margaret