Uterine fibroids are noncancerous growths that develop in or on the muscular walls of the uterus. An estimated 20-80% of women develop fibroids by age 50. Fibroids are often detected during a routine pelvic exam or prenatal ultrasound. However, several other uterine and non-uterine conditions can have a similar appearance to fibroids on ultrasound, which can lead to an inaccurate diagnosis. Some of the most common conditions mistaken for fibroids include adenomyosis, uterine leiomyosarcomas, ovarian tumors, endometriomas, broad ligament fibroids, nabothian cysts, pregnancy, and uterine synechiae. Being aware of the differential diagnoses is important to ensure the proper identification and management of fibroids.
Adenomyosis
Adenomyosis is a common condition that involves the abnormal growth of endometrial tissue (the lining of the uterus) into the myometrium (muscular wall of the uterus). On ultrasound, adenomyosis can look similar to fibroids with a heterogeneous uterine enlargement and poorly defined margins. However, adenomyosis typically appears as diffuse thickening of the uterine wall rather than a distinct mass. The inner endometrial-myometrial border also appears indistinct, which helps differentiate it from a fibroid. Adenomyosis can be definitively diagnosed on MRI, which will demonstrate tissue high in signal intensity extending from the endometrium into the myometrium.
Uterine Leiomyosarcomas
Uterine leiomyosarcomas are rare, malignant smooth muscle tumors of the myometrium. On ultrasound, they may resemble benign fibroids as solid masses with whorled appearance. However, leiomyosarcomas tend to have more irregular borders and are often solitary masses rather than multiple. LMS may also demonstrate rapid growth on serial ultrasounds. Definitive differentiation requires MRI, where leiomyosarcomas will show high T2 signal intensity and irregular enhancement, reflecting increased vascularity. Biopsy is necessary for confirmation. Any rapidly growing uterine mass in a postmenopausal woman requires evaluation to rule out leiomyosarcoma.
Ovarian Tumors
Some ovarian masses can mimic the appearance of uterine fibroids on ultrasound. Ovarian fibromas are benign solid tumors that contain fibrous tissue, smooth muscle, and collagen. They appear as solid masses with whorled echotexture, often with calcification. Large uterine fibroids can also extend into the ovary, making the organ of origin difficult to discern. An adnexal mass in continuity with the uterus raises suspicion for a uterine source. Ovarian cancers may also resemble fibroids but often have additional features like ascites or peritoneal implants. Identifying the originating organ is key to determining appropriate management.
Endometriomas
Endometriomas are benign ovarian cysts filled with old blood resulting from endometriosis. On ultrasound, they can look similar to fibroids as solid masses with homogeneous low-level echogenicity. However, endometriomas often have small internal cystic spaces representing hemorrhagic degeneration. They may also demonstrate peripheral vascularity on Doppler. Endometriomas are typically located in the ovary, while fibroids are within or attached to the uterine wall. This helps distinguish the two entities. MRI can definitively identify endometriomas based on hypersignal T1 intensity due to blood products.
Broad Ligament Fibroids
Broad ligament fibroids arise from the smooth muscle of the broad ligament connective tissue adjacent to the uterus. On ultrasound, they may seem to originate from the uterus itself. However, broad ligament fibroids are typically located lateral to the uterus and can often be visualized as separate from the uterine wall. They tend to be more spherical or lobulated compared to the whorled appearance of intramural fibroids. MRI can help identify the surrounding tissue plane and confirm broad ligament origin. Proper identification of broad ligament fibroids is important for surgical planning.
Nabothian Cysts
Nabothian cysts are benign fluid-filled cysts that arise from the cervical glands. On ultrasound, they may mimic the appearance of uterine fibroids as heterogeneous masses protruding from the cervical wall. However, nabothian cysts have a thin wall and anechoic or hypoechoic internal fluid, differentiating them from the solid echotexture of fibroids. They are also located on the cervical surface, while fibroids arise within or adjacent to the uterine wall. Definitive diagnosis can be made with saline infusion sonography or MRI.
Pregnancy
Early pregnancy can be mistaken for fibroids on ultrasound. The gestational sac may resemble a cystic fibroid. The decidual reaction of the endometrium also appears as thickened, heterogeneous endometrial stripe, similar to adenomyosis. Doppler flow helps identify the chorionic circulation of early pregnancy. Serial ultrasounds determine if the mass is an evolving gestation. Fibroids remain stable or grow slowly over time. Pregnancy hormone production also causes fibroids to hypertrophy during early gestation, further complicating diagnosis.
Uterine Synechiae
Uterine synechiae, also called Asherman’s syndrome, occurs when scar tissue forms adhesions within the uterine cavity, often due to trauma from dilation and curettage. On ultrasound, these can appear as echogenic masses projecting into the endometrial canal, resembling submucosal fibroids. However, synechiae cause distortion of the uterine cavity rather than a focal mass. Hysteroscopy can definitively diagnose synechiae and differentiate them from fibroids based on direct visualization. Subtle grayscale and Doppler differences also help distinguish the two entities.
Conclusion
Ultrasound has limitations in its ability to differentiate uterine fibroids from other pelvic conditions with similar sonographic appearances. Awareness of potential mimics is important to avoid diagnosis pitfalls. Serial ultrasounds, Doppler assessment, MRI, and tissue sampling help characterize suspicious masses. Correct identification of fibroids ensures appropriate treatment, while ruling out cancers and avoiding unnecessary procedures. With an understanding of their typical ultrasound characteristics, most fibroid mimics can be accurately distinguished from true uterine fibroids.