These fast spin echo T2 weighted MRI scans of the pelvis in sagittal view above and axial view below reveal a large mass arising in the region of the uterus. The rectum, the vagina, and the bladder are also seen. A leiomyosarcoma is most likely to be a large, solitary mass, but they are far less common than leiomyomas. Leiomyosarcomas do not arise from leiomyomas. The biologic behavior of leiomyosarcomas can be unpredictable.