Inflammatory leiomyosarcoma

Last revised by Joachim Feger on 23 Nov 2022

Inflammatory leiomyosarcomas are malignant tumors with smooth muscle differentiation and a prominent inflammatory infiltrate that were just recently recognized as a distinct entity by the WHO in 2020 1-3.

Inflammatory leiomyosarcomas are very rare lesions with most cases seen in adults with a peak in the 3rd and 4th decade of life and some cases reported in adolescents 1,2. There is a predilection for the male gender 1.

The diagnosis of inflammatory leiomyosarcomas is based on histology and immunohistochemistry 1.

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

  • variably atypical eosinophilic spindle cells with 
  • variable mitotic activity
  • lymphoid or histiocytic-xanthomatous inflammatory infiltrate with variable composition
  • immunoreactivity for smooth muscle antigens

Most cases present as growing deep soft tissue mass of inconstant duration. Symptoms suggesting an inflammatory origin are present in selected cases 1.

Pathologically inflammatory leiomyosarcomas are characterized by low-grade neoplastic smooth muscle cells associated with lymphoplasmacytic infiltrates 1-3.

The etiology of inflammatory leiomyosarcomas is unknown 1.

Inflammatory leiomyosarcomas are seen in the deep soft tissues most often in the lower limb, the trunk and retroperitoneum 1-5. Rare cases have been found in visceral locations 1.

Macroscopically inflammatory leiomyosarcomas are well-delineated with a yellow-tannish or red and fleshy surface 1. Tumor size is variable with examples ranging from 3-12 cm 1.

Microscopically inflammatory leiomyosarcomas are characterized by the following histologic features 1,4-6:

  • eosinophilic spindle cells with
  • fascicular or storiform growth pattern
  • variable nuclear atypia, pleomorphism
  • variable mitotic activity
  • diffuse inflammatory infiltrate of small lymphocytes, some plasma cells and often prominent histiocytes
  • psammomatous calcifications in some cases

Immunohistochemistry stains are reactive for one or the other combination of smooth muscle actin, desmin and/or caldesmon 1,4.

Inflammatory leiomyosarcomas are characterized by a near-haploid genotype that is considered pathogenetically important 1,4.

At the time of writing, there are only very few reports of the radiological features of this entity 5.

On CT a case report of an inflammatory leiomyosarcoma of the head and neck region described the lesion as a well-circumscribed homogenously enhancing tumor 5.

On MRI a case of a reported lesion has been described as lobulated 5.

  • T1: mildly hyperintense
  • T2: markedly hyperintense
  • DWI: moderately restricted
  • T1 C+ (Gd): diffuse enhancement

The radiological report should include a description of the following:

  • form, location and size
  • tumor margins
  • relation to organs and viscera
  • relation to muscular fasciae and skeletal muscles in soft tissues
  • relationship to local nerves and vessels

Due to their rarity information about follow-up and prognosis of inflammatory leiomyosarcomas is limited. Metastases occur in some cases. The cases associated with near-haploidization seem to be less aggressive than other leiomyosarcomas and appear to have a very good prognosis 1-3.

Inflammatory leiomyosarcomas have been first described by the British pathologists William Merchant, Eduardo Calonje and Christopher DM Fletcher in 1995 5,6. The tumors were recognized by the WHO as distinct lesions in 2020.

Conditions that can mimic the presentation and/or the appearance of inflammatory leiomyosarcoma include many benign and malignant soft tissue tumors affecting the deep soft tissues and the retroperitoneum including the following:

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