Müllerian adenosarcoma of the uterus with low-grade sarcomatous overgrowth characterized by prominent hydropic change resulting in mimicry of a smooth muscle tumor.

A 28-y-old woman was found to have a large subserosal uterine mass that was excised and interpreted as a "clear cell leiomyoma." Five years later, the tumor recurred as serosal-based ileal and uterine masses; they were treated by partial ileal resection and hysterectomy. All 3 masses were predominantly characterized by conspicuous edema separating bland cells growing in cords and clusters, with scant to moderately conspicuous clear cytoplasm. The edema was indistinguishable from the hydropic change commonly seen in benign smooth muscle tumors and the cords similar to those often present in them. However, the mass from the hysterectomy specimen had a small, grossly recognizable cystic region, which on microscopic examination was a typical low-grade müllerian adenosarcoma. The stroma of the latter ranged from cellular endometrial-type to edematous and hypocellular similar to that dominating the other specimens. The cellular and edematous regions focally had cords and tubules of sex cord-like type confirmed by inhibin and calretinin positivity. Smooth muscle differentiation was also seen as a "starburst" pattern. This case of adenosarcoma is unusual due to its (1) serosal location, (2) overgrowth of stroma, which differed from typical adenosarcoma with sarcomatous overgrowth by its low-grade nature, (3) hydropic change associated with cords and nests of cells with clear cytoplasm, which prompted the initial specimen to be considered an epithelioid leiomyoma, and (4) prominent smooth muscle metaplasia mostly with a "starburst" morphology. All these features have only rarely been documented in prior müllerian adenosarcomas.