Solution: Uterine leiomyosarcoma:

Though to be a de novo process independent of fibroid formation the majority of the time, although a minority of these represent sarcomatous transformation of a previously existing fibroid. It is a rare sarcoma arising from the smooth muscle cells found within the myometrium and have a poor prognosis with a tendency to invade aggressively into surrounding tissues and for early hematogenous dissemination.

Imaging findings in this case are relatively obvious with a large heterogeneous mass essentially replacing the uterus. It is irregular along its margin and shows heterogeneous enhancement. Although this could also represent an endometrial carcinoma, those tumors tend to present earlier and are generally not as locally aggressive.