Leiomyoma is the most common uterine tumor, with an approximate prevalence of 30%-40% in reproductive-age women.

These well-circumscribed, benign tumors are composed primarily of smooth muscle and various amounts of fibrous connective tissue. They are surrounded by a pseudocapsule of areolar tissue from which the blood supply is derived, usually through one or two large vessels that enter the mass together.In most cases, the mass itself is relatively avascular.The three major types of leiomyoma are classified on the basis of their location- intramural, subserosal and submucosal.

Submucosal lesions project into the endometrial cavity, whereas subserosal lesions project into the abdomen and/or pelvis. Intramural lesions, the most common type, are located within the substance of the myometrium. The location of the leiomyoma is of major clinical importance, since the symptoms and treatment differ among the types. Submucosal lesions predispose to recurrent abortion and debilitate patients through hypermenorrhea and anemia.These tumors are amenable to hysteroscopic removal, sparing patients abdominal surgery. An intramural lesion located near the ostium of the oviduct may cause infertility, whereas a lesion near the internal os of the cervix may obstruct labor. Subserosal lesions are less symptomatic but may simulate an ovarian mass, thereby precipitating exploratory surgery. Although large leiomyomas may cause symptoms regardless of their location, the intramural and submucosal varieties have the greatest impact on fertility.

The diagnosis of leiomyoma can usually be made by means of pelvic examination; however, other causes of uterine enlargement, such as adenomyosis, should be ruled out.

Although a leiomyomatous uterus can be identified with ultrasound (US), discrete localization of individual leiomyomas is usually not possible. The most common sonographic finding is an abnormal uterine contour .Uterine enlargement, with an inhomogeneous texture, is frequently seen. Leiomyomas exhibit a wide variety of appearances but classically are hypoechoic, with poor sound transmission; the sonogram may be falsely normal.

MR imaging has proved particularly useful in depicting whether a mass is ovarian or adnexal in origin, as demonstrated in a study of adnexal masses by Mitchell et al in which MR imaging provided additional information or increased diagnostic confidence in patients who had undergone US or computed tomography. Additionally, determination of uterine volume with MR imaging is possible in all patients, but with US it is not possible in uteri larger than 140 cm3. MR imaging is clearly the modality of choice in the evaluation of leiomyomas, as demonstrated by Hricak .MR imaging accurately depicted tumor number, size, and location and the presence and extent of degeneration.

Adenomyosis on T2-weighted images appears as diffuse, ill-defined lesions adjacent to the endometrium and isointense with the junctional zone .Small, high-signal-intensity foci are noted within this low-intensity lesion.On T1-weighted images, the lesion is isointense with surrounding myometrium,but sometimes high-intensity foci are seen.

Leiomyomas, in contradistinction, appear as well-circumscribed masses sharply marginated from the myometrium. Small, nondegenerated leiomyomas usually had low signal intensity, whereas a range of signal intensity is seen with degeneration. Large vessels,often seen with leiomyoma, are not seen with adenomyosis.Interestingly,the endometrium in adenomyosis is of the basalis type and therefore does not usually undergo the cyclic changes of the functionalis layer. This is in contrast to endometriosis (thepresence of endometrium outside the uterus), in which the zona functionalis is hormonally responsive, accounting for the characteristic hemorrhagic and reparative changes of this disease. In adenomyosis, fresh hemorrhage and hemosiderin-laden macrophages in the stroma are uncommon but can be more frequently seen during pregnancy and progestational therapy .

The histologic and MR imaging findings of adenomyosis and !eiomyoma explain the different surgical approaches required. Adenomyosis is treated by means of hysterectomy, whereas leiomyoma can be treated with myomectomy. Adenomyosis is nonresectable, since this abnormality interdigitates with myometrium, whereas leiomyomas compress myometrium and create a pseudocapsule that enables enucleation at surgery.