Uterine fibroid or cancer? When is a fibroid cause for concern? In the muscle layer of the uterus, a single muscle cell can begin cloning itself, multiplying until it forms a benign tumor—a uterine fibroid. Why this happens is still a mystery, but it’s surprisingly common. Now, there is a rare exception. A type of cancer called leiomyosarcoma can also form from muscle tissue. It’s very rare—I’ve only seen a few cases in my entire career—but if a tumor grows rapidly, especially after menopause, t