Myometrial vessels are filled with benign endometrium.
Intravascular endometrium is the presence of endometrial tissue within myometrial vessels. This phenomenon is more common that expected. In a study of 277 hysterectomy specimens, intravascular endometrium was identified in 14 cases -- all 14 cases had concurrent adenomyosis (Sahin).
Finding small dark stromal cells with associated glands inside parametrial blood vessels can be alarming -- as one begins to think of carcinoma, sarcoma or lymphoma with intravascular invasion. However, intravascular endometrium is a benign entity that ought to be considered, especially if the clinical picture is fitting (Banks).
Sahin AA, Silva EG, Landon G. Endometrial tissue in myometrial vessels not associated with menstruation. Int J Gynecol Pathol 1989;8(2):139-46.
Banks ER, Mills SE, Frierson HF Jr. Uterine intravascular menstrual endometrium simulating malignancy. Am J Surg Pathol. 1991 Apr;15(4):407-12.