Computed tomography in the differential diagnosis of pelvic and extrapelvic disease

Abstract

Displacement of various landmarks within the pelvis by enlarging mass lesions can be used advantageously to determine the nature and origin of a particular mass. The noncompliant pelvic osseous ring accentuates the effect of displacement by forcing the vector inwards. This usually results in cephalad extension of a pelvic process into the abdomen. Lesions originating in the sidewall force the neurovascular bundle medially while central lesions will displace these structures against the noncompliant osseous pelvis. The differentiation of supralevator from infralevator lesions by CT aids in planning the appropriate approach to a lesion preoperatively. The various peritoneal spaces can become enlarged by intraperitoneal abscesses, fluid collections or tumor. These processes will also displace sidewall structures laterally against the osseous rim. It is extremely rare for an intraperitoneal process to extend below the level of the mid-femoral head. Extraperitoneal disease arising out of the pelvis may involve areas caudad to the femoral heads as there is no peritoneum to limit the distal spread. The differentiation of intraperitoneal and large extraperitoneal masses arising out of the pelvis can be difficult. A mass which incorporates bowel loops suggests intraperitoneal disease, whereas one which abuts them smoothly suggests an extraperitoneal lesion.