Is it possible a pelvic ct scan could miss a small tumor in rectum? gi felt a fullness is left rectum but only tilted pelvis was seen on ct scan.

Small tumour: Yes it is possible. But such a small tumour would be very unlikely to cause the symptoms you describe.

Can miss: A pelvic CT scan is designed to define structures in the abdomen pelvis such as the small bowel and colon but not to define their content within the lumen of the bowel. As such when such a luminal (phenobarbital) lesion is noted it is large and bulges within the bowel to distort the structure of that segment of bowel. Small intraluminal (phenobarbital) lesions are easily missed and require colonoscopy to be recognized.