Can Fibroids Be Cancerous?
It is perfectly normal for healthcare providers and patients to be concerned about malignancy (cancer) when a pelvic tumor - symptomatic or asymptomatic - is encountered. A malignant tumor of the smooth muscle is called a leiomyosarcoma.
Fibroids Cancer Studies
Thankfully, a number of studies have looked at the rate of leiomyosarcoma findings in women with pelvic tumors, and the review consensus is that about 1 in 400 women (0.25%) undergoing surgery for fibroids is at risk of having a leiomyosarcoma.
In a review of 6815 patients who underwent myomectomy between 1950 and 1993, only 18 patients (0.26%) had leiomyosarcomas. In the subpopulation of women whose masses had grown rapidly, the prevalence was the same at 0.27%.¹⁻² Based on this evidence, rapid growth of a fibroid does not seem to be a predictor of leiomyosarcoma. However postmenopausal growth or onset of symptoms should carry a higher suspicion for malignancy. Other case series have estimated the incidence of leiomyosarcoma at 0.22% to 0.49%, although in women in their 50's it may rise to 1% of hysterectomy specimens.¹⁻²
The general consensus in the most up-to-date literature is that the incidence of sarcoma among women with presumed fibroids is 0.05% (1/2000) to 0.29% (1/340).³ To better contextualize these stats, this means that cancer is quite rare compared to uterine fibroids, and the vast majority of uterine growths are benign.
 George A Vilos et al. The management of uterine leiomyomas. SOGC Clinical Practice Guideline. No. 318, February 2015.
 Parker WH, Fu YS, Berek JS. Uterine sarcoma in patients operated on for presumed leiomyoma and rapidly growing leiomyoma. Obstet Gynecol 1994;83:414–8.
 Seagle, B. L. L., Alexander, A. L., Strohl, A. E., & Shahabi, S. (2018). Discussing sarcoma risks during informed consent for nonhysterectomy management of fibroids: an unmet need. American Journal of Obstetrics and Gynecology, 218(1), 103.e1-103.e5.