Adenomyosis is defined as the presence of endometrial tissue within the myometrium. The endometrial tissue should be at least 100x field (about 2.5 mm) deep from the endomyometrial junction. The surrounding myometrium is usually hyperplastic. The endometrial glands may be inactive or hormonally responsive. In some cases, the glands are scant or absent and such cases may be mistaken for low-grade endometrial stromal sarcoma.