An endometrial stromal tumor with osteoclast-like giant cells.

Endometrial stromal tumors (ESTs) of the uterine corpus have a striking propensity to display diverse morphological variations, including sex cord-like, smooth muscle, or skeletal muscle differentiation; fibrous change; myxoid change; or bland endometrioid-type glands. They may also contain rhabdoid, foam, clear, or epithelioid/granular cells among others. Recently, we have encountered an EST showing smooth muscle differentiation and osteoclast-like giant cells that were predominantly concentrated in the areas showing smooth muscle differentiation. Osteoclastlike giant cells have not been previously reported in EST to our knowledge; thus, this finding expands the morphological spectrum of these tumors. In addition, although the level of infiltration at the peripheries of the tumor exceeded that allowable under the Tavassoli and Norris criteria for stromal nodules, it did not reach the classic permeative infiltration generally associated with endometrial stromal sarcomas. Historical, prognostic, and diagnostic aspects of margins in EST, especially in those borderline cases such as ours, are also discussed.