Leiomyoma of soft tissue

Last revised by Joachim Feger on 7 Dec 2022

Soft tissue leiomyomas are benign smooth muscle neoplasms usually found in the somatic deep soft tissues, the retroperitoneum and the abdominal cavity 1.

Leiomyomas of the deep somatic tissues are rare soft tissue neoplasms that have been primarily found in middle-aged adults with no gender predominance 1-3. Leiomyomas of the abdominal cavity or retroperitoneum are most frequently found in women, commonly in a perimenopausal age 1,4.  

The diagnosis of leiomyomas is strongly founded on typical histological criteria and in selected cases on immunohistochemistry.

Diagnostic criteria according to the WHO classification of tumors: soft tissue and bone (5th edition) 1:

  • intertwining fascicles of bland smooth muscle cells

The following immunohistological criteria are desirable in selected cases 1:

Most cases are solitary growing deep soft tissue masses, but retroperitoneal lesions might be also multiple and incidentally  found on imaging studies 1.

Leiomyomas of soft tissue are characterized by interlacing fascicles of smooth muscle cells , but as per definition are distinct from uterine leiomyomas 1.

The etiology of somatic leiomyomas is unknown 1.

In retroperitoneal leiomyomas a multifocal cause is suspected 1.

Leiomyomas of the deep somatic soft tissues arise from the skeletal muscle and deep subcutaneous tissues of the extremities such as the thigh, the upper extremity and the trunk 1-3. Abdominal lesions may occur in the retroperitoneum the abdominal wall, the mesentery, the omentum and on the peritoneal surface, they might also originate from the round ligament 1.

Macroscopically leiomyomas of the deep soft tissues are circumscribed tumors with a tan-grey to white sometimes reddish-tan color and and a rubbery consistency.  They might have myxoid changes and can get quite large up to 15 cm and retroperitoneal lesions even larger 1.

Microscopically leiomyomas of soft tissue are characterized by the following histologic features 1-4:

  • smooth muscle cells with eosinophilic cytoplasm

  • uniform cigar-shaped, blunt-ended nuclei

  • arrangement in intersecting fascicles

  • degenerative or regressive changes including hyalinization calcification and fibrosis

  • no or minimal atypia

  • rare mitoses

  • no tumor necrosis

Immunohistochemistry stains are reactive for smooth muscle actin, desmin and h-caldesmon and negative for S100 and CD34 1,2. Retroperitoneal soft tissue leiomyomas are positive ER, PR and WT1 1,2,4, whereas somatic soft tissue leiomyomas do not express ER or PR 1.

Leiomyomas of the retroperitoneum and the abdominal cavity show similar genetic changes as uterine leiomyomas including HMGA2 and PLAG1 rearrangements 1,2.

Leimyomas of soft tissue are frequently calcified 1,2.

On CT cases have been described as well-marginated and are often calcified 5,6.

On MRI soft tissue leiomyomas have been described as hypointense on T2 weighted images 5 and seem to have a similar appearance to muscle in both T1 and T2 weighted images with high signal intensity streaks 7.

  • T1: isointense to slightly hyperintense to muscle 7,8

  • T2: isointense to hyperintense to muscle 7,8

The radiological report should include a description of the following:

  • form, location and size

  • tumor margins

    • relation to muscular fasciae and skeletal muscles in soft tissues

    • relation to organs and viscera in the abdominal cavity

    • relationship to local nerves and vessels

Leiomyomas are benign tumors with no risk of metastases. Patients are usually cured by wide local excision, local recurrences have been rarely reported 1,2.

A case of a deep leiomyomas of the extremity has been first described by the American pathologists Alan H. Goodman and Russel C. Briggs in 1965 2,9.  

The radiological appearance of leiomyomas of soft tissue is usually unspecific including the following 2,6:

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