Urogenital ImagingLeiomyoma, uterine
benign tumour of smooth muscle cell origin. It is typically a well-circumscribed
tumour containing smooth muscle and a variable amount of fibrous tissue. The latter is believed to arise by degeneration of the smooth muscle component so that the popular synonym, fibroids, is semantically incorrect, if one is referring to the cell of origin. Leiomyomas are surrounded by a pseudocapsule of areolar tissue.
Leiomyomas are classified by their position in relation to the uterine wall as submucosal, intramural (or
interstitial) or subserosal. Uterine leiomyomas are the most common female pelvic
tumour, occurring in up to 40% of women by the fifth decade of life. Leiomyomas shrink and may calcify after the menopause. New leiomyomas are rare in the postmenopausal female. Sometimes leiomyomas undergo a rapid increase in size during pregnancy. These features suggest the
tumour is oestrogen dependent, but the exact aetiology is unknown. They may be single, but are usually multiple. Size is also variable; leiomyomas can be microscopic or result in gross uterine enlargement.
Symptoms usually arise from pressure effects due to the large pelvic mass. Leiomyomas may also present with pelvic pain, dysmenorrhoea and abnormal uterine bleeding. Submucosal leiomyomas project into the uterine cavity and may cause infertility, recurrent abortion, or hypermenorrhoea. Intramural leiomyomas may also be associated with infertility. Depending on their size and location, both submucosal and intramural leiomyomas may obstruct labour. Subserosal leiomyomas are exophytic from the uterus and may undergo torsion and present with acute abdominal pain. Occasionally, leiomyomas are the only identifiable abnormality after a detailed infertility work-up, and in such cases, myomectomy has been reported to result in a successful term pregnancy rate of 40 - 50%. Rarely, patients present with features of polycythaemia. This paraneoplastic syndrome is related to elevated circulating levels of erythroporitin and regresses after hysterectomy.
CT features of uterine leiomyomas include a focal solid mass causing lobulation or protrusion from the outer margin of the uterus, or distorting or obliterating the uterine cavity. Focal calcifications and irregular low-density areas in the mass are also suggestive. At MRI, leiomyomas appear as well-circumscribed masses of similar or slightly low T1 signal intensity and homogeneously low T2 signal intensity, relative to the adjacent myometrium. Slight slow diffuse homogeneous enhancement may occur after intravenous gadolinium chelates. Degenerative or vascular leiomyomas can be recognized on MRI because they are inhomogeneous on T2-weighted images, often with foci of T2 signal hyperintensity. They may also demonstrate variable degrees of contrast enhancement.
MRI is indicated when ultrasound results are limited or inconclusive. Sonography has been reported to result in false negative results in 22% of cases. Ultrasound (US) is particularly limited when the tumour is small (less than 2 cm in diameter), or very large when the uterus is retroverted or duplicated and when there is coexisting adnexal or pelvic pathology. MRI is especially useful in the search for leiomyomas in the infertile patient. Furthermore, it is possible to differentiate between leiomyoma and adenomyosis with MR imaging. MRI may also be used prior to myomectomy when the location and precise size of leiomyomas need to be known. Serial MR images can also facilitate an assessment of response to hormone analogue treatment.
MR imaging is considered superior to US in the evaluation of uterine leiomyoma with a reported sensitivity of 92%. (Fig.1) (Fig.2) (Fig.3) (Fig.4) (Fig.5) (Fig.6) (Fig.7). Tumours as small as 5 mm are consistently depicted, and tumour location, either submucosal, intramural or subsero hypoechoic and demonstrate relatively poor through-transmission of sound. Some produce focal areas of high attenuation with distal acoustic shadowing, which can give rise to the so-called "venetian blind" sign of diverging rays of acoustic shadowing. Acoustic shadowing due to tumoural calcification may also be recognized. Central areas of cystic degeneration may be noted. In general, transvaginal US is better than transabdominal in the demonstration of internal architecture. Very large masses may result in bilateral hydroureter or hydronephrosis, due to compression of the distal ureters. Therefore, a full examination should always include an evaluation of the kidneys.
At hysterosalpingography, leiomyomas may produce a variety of findings (Fig.8). Intramural leiomyomas may result in a large capacious endometrial cavity, which may have an irregular asymmetric outline. Subserosal leiomyomas appear as smooth polypoid filling defects, typically with obtuse margins.
Treatment of leiomyomas depends on their size and location. Small submucosal lesions may be removed hysteroscopically. Intramural tumours are removed by myomectomy, particularly in women desiring uterine preservation or pregnancy. Treatment with gonadotropin-releasing hormone analogues causes a hypo-oestrogenic state and promotes atrophy of the leiomyoma.
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