An 8 year - old female domestic short - hair cat was presented with a chronic history of difficulty in defecation.

Radiographs were made of the caudal abdomen.

Radiographic changes

• The caudal abdomen is filled with a distended fecal - filled colon
• The intrapelvic opacity has a different appearance from the feces in the colon
• Inability to evaluate the bony pelvis, especially the sacrum
• The lucent area over the right sacrum was assumed to be air within the rectum
• The hip joints are radiographically normal
• Urinary bladder not visualized
• Contrast study of the rectum was performed to further evaluate the intrapelvic mass

Partial barium enema

Radiographic changes

• A markedly displaced rectum ventrally and to the left with a narrowed lumen
  • Apparently an extramural lesion without evidence of leakage of the contrast agent
• Sharply marginated opacity fills the pelvic inlet dorsally and on the right
  • Pelvic mass appears separated from the rectum
• With the rectum displaced to the leI, the lucent area noted in the region of the right sacroiliac joint on the first study is probably indicative of bony destruction and not the result of gas within the rectum as thought originally

Radiographic diagnosis

• Intrapelvic mass with distortion of the rectum resulting in only a minimal passage of feces possible
• Intrapelvic mass has an opacity approaching that of bone
• Urinary bladder not evaluated
• Differential diagnosis
  • Primary bone tumor originating from pelvis
  • Soft tissue tumor with mineralization

Diagnosis and comments

• The tumor was an osteosarcoma originating from the ventral aspect of the sacrum creating an intrapelvic mass effect causing displacement and narrowing of the rectum
• The tumor acted as a peripheral cortical lesion without invasion into the adjacent vertebral canal
• Primary bone tumors in the cat are uncommon and their appearance is not well known. It is thought that those originating from the axial skeleton have a different presentation from those in the appendicular skeleton. This well marginated osteosarcoma has a benign appearance both radiographically and clinically and did not cause obvious neurological signs but only expanded into the pelvic canal causing the presenting sign of difficult defecation that was chronic.
• The use of either barium sulfate or air as a contrast agent in the rectum is an easy, safe technique to evaluate intrapelvic lesions. A retrograde urethral positive contrast study is another technique to determine the relationship of the urethra/bladder neck to the lesion. If surgical correction were to be considered, a CT study would permit a much more complete evalation of the tumor.