There are several approaches to imaging fibroids but no imaging that can reliably differentiate between benign leiomyomas and uterine sarcomas. Both leiomyomas and uterine sarcomas look similar; both present with well delineated uterine masses and may have a necrotic core. Pelvic ultrasound followed by MRI is the most useful imaging strategy.
First-line imaging is often pelvic ultrasound. Pelvic ultrasound is the first-line study used to evaluate for uterine fibroids. Transvaginal ultrasound has high sensitivity (95 to 100 percent) for detecting myomas in uteri less than 10 gestational weeks' size . Characteristics suggestive of a sarcoma include a necrotic core, Doppler irregular vessel distribution with low flow impedance and high peak systolic velocity and mixed or poor echogenic parts. Benign leiomyomas may also have these characteristics . If an intracavitary leiomyoma exists (submucosal or intramural that protrudes into the uterine cavity), and if the percent of the fibroid within the endometrial cavity is not clearly ascertained (and could alter care) then saline infusion sonography or hysteroscopy may be used to evaluate the uterine cavity.
MRI may be used if ultrasound findings are not sufficient for surgical planning or if the diagnosis is uncertain, (suspicion of uterine sarcoma or adenomyosis). MRI may provide more information if sarcoma is suspected though a definitive diagnosis will require a pathology specimen. Absence of calcifications appear to be a consistent finding in leiomyosarcomas . Calcifications suggest that the mass has undergone necrosis, seen most commonly in fibroids that have outgrown their blood supply (postpartum or post-uterine artery embolization) or postmenopausal women with decreased uterine blood flow. Other studies suggest that ill-defined margins are consistent with a sarcoma . Two studies using MRI with gadolinium contrast reported specificities of 93 to 100 percent and positive predictive values of 53 to 100 percent [5, 6].
Diffusion weighted MRI is another investigational modality that appears to differentiate ordinary and degenerated leiomyomas from sarcomas and cellular leiomyomas .
Finally, intralesional hemorrhage appears suggest sarcoma also [8, 9]. Further study of use of MRI for this purpose is needed.
Computed tomography (CT) does not reliably differentiate between leiomyomas and uterine sarcomas .
Positron emission tomography/CT with fluorodeoxyglucose (FDG) does not appear to be useful to distinguish between leiomyomas and uterine sarcomas .
We suggest a newly diagnosed pelvic mass in a woman or one that has markedly changed in mobility or size should undergo a pelvic ultrasound as an initial imaging study. This will help to exclude other possible causes of uterine enlargement.
If uterine sarcoma is suspected based upon clinical characteristics or Ultrasound findings, MRI with gadolinium contrast is the next best step for investigations.
1 - http://www.ajog.org/article/S0002-9378(02)69122-7/fulltext
8 - http://www.fertstert.org/article/S0015-0282(07)03162-7/fulltext
9 - Stewart EA, Marsh EE, Spies JB. Minimally invasive treatments for fibroids. ACOG Update 2014. http://www.acogupdate.com/?gp_page=p_onecourse&gp_skey=359 (Accessed on August 03, 2016).
10 - https://www.ajronline.org/doi/abs/10.2214/ajr.181.5.1811369