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A sex cord-stromal tumor of the postmenopausal ovary that is yellow, large, and unilateral, composed of fascicles of lipid-rich spindle cells interspersed with collagen, reticulin fibers, and hyaline plaques. Thecomas and other estrogen-producing tumors (e.g., granulosa cell tumors) may induce adenomatous hyperplasia of the endometrium or well-differentiated endometrial carcinoma in 3%-20% of the cases. (Segen, Dictionary of Modern Medicine, 1992)