Differential Diagnosis of Uterine Fibroids: When Ultrasound Alone May Be Insufficient
Fibroids are the most prevalent source of uterine masses, but share clinical features with other uterine masses such as adenomyosis and ovarian cysts. Ultrasound is usually sufficient to detect uterine fibroids and rule out other differentials, but some masses are difficult to differentiate with ultrasound. In these cases, an MRI may be especially useful in confirming uterine fibroids and planning for appropriate treatment. Establishing an accurate differential has important implications on prognosis and treatment, especially as it pertains to uterine fibroids and other conditions of the uterus.
Clinical Features of Uterine Fibroids
Uterine fibroids (also called ‘myomas’ or ‘leiomyomas’) are non-cancerous masses that develop in the muscular tissue of the uterus. Fibroids can vary in size from small, undetectable seedlings to massive tumors that cause the uterus to expand to the size of a four- or five-month pregnancy.  Although the majority of women with uterine fibroids are not symptomatic, those who are typically suffering from excessive menstrual bleeding or pelvic pain. Larger fibroids can cause weight gain, an enlarged uterus, urgency, and other abnormalities of the urinary and gastrointestinal systems. 
Fibroids are extremely common, especially as women progress through the latter half of their reproductive years. One study in the United States observed that 60% of African American women had developed fibroids by age 35, with that figure increasing to 80% by age 50. Similarly, 40% of Caucasian women had developed fibroids by age 35, and almost 70% by age 50.  Indeed, uterine fibroids are one of the most common causes of abnormal uterine bleeding in reproductive-aged women,  and the number one reason that women have their uterus removed.  With these figures in mind, it’s tempting to default to a uterine fibroid diagnosis when the symptoms align, but on presentation alone, there are a number of other conditions that share the clinical features of fibroids.
Ruling out possible differentials is a critical step in uterine fibroid care, especially when uterine fibroid embolization (UFE) or other fibroid-specific treatments are being considered that won’t have any effect on other types of uterine masses. Beyond the scope of fibroid treatment, differentiation is clinically important because of differences in treatment and prognostic implications between different conditions of the uterus.
Uterine fibroids share clinical features with other conditions that involve masses in the uterus, cervix, ovaries, fallopian tubes, or the connecting tissue around the uterus.  These include:
- Focal Adenomyosis
- Cervical Polyps
- Endometrial Polyps
- Ovarian Cysts
- Ovarian Cancer
- Benign Ovarian Tumors
- Ectopic Pregnancy
- Uterine Leiomyosarcoma
While some of these masses are straightforward to differentiate via ultrasonography or other imaging modalities due to their unique anatomical locations or defining features, others require an MRI to reliably differentiate from uterine fibroids. 
Detection and Differentiation of Uterine Masses
If uterine fibroids are suspected, ultrasound and MRI imaging are the two most commonly employed imaging methods. In combination with the patient’s clinical findings, these imaging methods are usually sufficient to confirm a diagnosis, and potentially avoid unnecessary laparoscopy and/or exploratory surgery. 
Ultrasound imaging tends to be the first-line detection method for uterine fibroids due to how cost-effective and accessible it is. When a uterine mass is suspected, both transabdominal and transvaginal scans should be performed. In skilled hands, ultrasonography can detect fibroids as small as 5 mm on transvaginal ultrasound.  In most cases, the diagnosis of fibroids and ruling out differentials with ultrasound is relatively straightforward, but there are cases in which focal adenomyosis, another type of adnexal mass, or uterine leiomyosarcoma can be mistaken for a benign uterine fibroid. When there is doubt about the origin of a pelvic mass on ultrasound, further evaluation with MRI should be performed.
Magnetic Resonance Imaging (MRI)
MRI is the most sensitive imaging modality for evaluating uterine fibroids, with a demonstrated sensitivity of 88% to 93% and specificity of 66% to 91%. One critique of ultrasound is it’s lack of reproducibility due to operator-dependence. Although more costly, MRI overcomes this shortcoming of ultrasound and can more reliably detect smaller fibroids. 
In addition to more reliable fibroid detection, MRI is useful in the differentiation of uterine fibroids from focal adenomyosis and adnexal masses. [6,7,8]