Are uterine fibroids always benign?

Uterine Fibroids Are Benign Tumors

Uterine fibroids (leiomyomas) are benign tumors that do not become malignant, with the extremely rare exception of leiomyosarcoma occurring in approximately 0.1% of cases, primarily in postmenopausal women. 1

Benign Nature of Fibroids

• Uterine fibroids represent benign sex steroid-responsive clonal expansions of individual smooth uterine muscle cells, with histology virtually indistinguishable from normal myometrium 1
• For practical purposes, these tumors should be considered benign in premenopausal women, though all specimens should undergo careful pathologic examination 1
• Fibroids are the most common benign tumors of the female genital tract and likely the most common soft-tissue tumors in the entire body 1

Prevalence and Demographics

• Fibroids occur in 20-50% of women over the age of 30 years, with prevalence increasing with age 1
• By age 50, the estimated incidence exceeds 80% in Black women and approaches 70% in White women 1
• Most fibroids (70-80%) are asymptomatic and require no intervention or further investigation 2

Rare Malignant Potential

• Leiomyosarcomas comprise only 0.1% of all uterine tumors, but the incidence increases to 1.7% in women undergoing hysterectomy for fibroids in their seventh decade of life 1
• The risk of malignancy should be considered in specific populations, particularly postmenopausal women with fibroid growth or persistent abnormal uterine bleeding 3
• Rapid increase in fibroid size in perimenopausal women should be investigated as it may indicate the presence of sarcoma 4

Diagnostic Considerations

• Transvaginal ultrasound provides high sensitivity (90-99%) and specificity (98%) for detecting fibroids 3
• MRI is superior to ultrasound for identifying and mapping fibroids and should be used when cancer is suspected 3
• Endometrial biopsy is essential prior to intervention to rule out endometrial neoplasia and, in some cases, diagnose sarcoma 3

Natural Course

• Fibroids typically grow in a spherical nodular fashion with distinct demarcation from surrounding normal myometrium 1
• Many fibroids have limited growth potential and remain static in size until menopause when sex steroid levels decline 1
• The natural regression of fibroids begins in menopause, though some may still persist in this age group 5

Clinical Implications

• When symptomatic, fibroids can cause abnormal uterine bleeding, pelvic pressure, urinary symptoms, low back pain, constipation, and dyspareunia 6
• Fibroids, especially the submucosal type, are associated with increased risks of reproductive complications including spontaneous abortion, preterm delivery, and postpartum hemorrhage 1
• The effect of fibroids on fertility remains somewhat controversial, though impaired gamete transport, distortion of the endometrial cavity, and impaired blood supply to the endometrium may reduce implantation rates 1

Warning Signs of Potential Malignancy

• Postmenopausal status with fibroid growth or persistent abnormal uterine bleeding significantly increases suspicion for malignancy 3
• Unusual appearance on imaging studies that doesn't fit typical fibroid characteristics raises suspicion for malignancy 3
• For definitive diagnosis of suspected malignancy, surgical removal and pathologic evaluation may be necessary 3

In conclusion, while uterine fibroids should be considered benign tumors in the vast majority of cases, healthcare providers should remain vigilant for the rare possibility of malignancy, particularly in postmenopausal women with new or growing fibroids or persistent abnormal bleeding.