Is it possible for a pelvic ct scan to not detect a tumor in the rectum? how likely is that?

CT scan: The ct scan is one thing - the radiologist, who reads the ct scan another. Radiologists typically can interpret radiographic images the better the clinical information is that they get - in your case chronic constipation, firmness and fullness on left side of rectum.

Diagnosing rectal ca: Some symptoms may be due to benign conditions and are not necessarily cancer. Pelvic ct scans are routinely used to detect rectal cancers, but small cancers may escape detection. A qualified physician should perform a physical exam and order additional tests, usually a combination of scans, blood tests (including tumor markers), plus colonoscopy.