Looking for a confirmation on what I suspect on CT scan results?

Not sure from your question though it sounds like the endoscopist is unable to get past the presumed area of "thickening" in your sigmoid. Statistically this thickening most likely represents diverticular scarring. Your greatest concern would be a colon carcinoma(leiomyosarcoma would be way way down on the list.) If there is any way your endoscopist to get througth this area(?pediatric scope) and visualize the mucosa(inner lining) you could eliminate the risk of a colon cancer since as you know they orginate in the mucosa. Also a negative PET scan may also give you more reurance that you are dealing with a benign "thickening."