Pleomorphic rhabdomyosarcoma

Last revised by Joshua Yap on 13 Oct 2022

Pleomorphic rhabdomyosarcoma is a type of rhabdomyosarcoma, and is the least common, accounting for only 5% of all rhabdomyosarcomas. Unlike embryonal and alveolar types, these tumors occur in adults over the age of 40 years 1 and are difficult to distinguish from other pleomorphic sarcomas such as malignant fibrous histiocytomas 2.

Some publications divide into three specific types 3:

  • classic: predominantly atypical pleomorphic polygonal rhabdomyoblasts (PRMB) in sheets

  • round cell: clusters of PRMB throughout the tumor with a background of slightly atypical, medium-sized, round, blue RMB

  • spindle cell: scattered PRMB in a predominance of atypical spindled RMB arranged in a storiform growth pattern

They typically occur in the limbs, with a predilection for the thigh 2.

Immunohistochemistry can reveal the following to varying proportions ref:

  • myoglobin: common

  • MyoD1

  • skeletal muscle myogenin (myf4)

  • fast skeletal muscle myosin

  • desmin

  • muscle-specific actin

  • smooth muscle actin (SMA)

  • muscle-specific myogenin (myf3)

The features are non-specific (see radiographic features in the parent article: rhabdomyosarcoma), although in general pleomorphic rhabdomyosarcomas have multiple areas of necrosis. In some instances, these areas are surrounded by pronounced ring-like enhancement 1.

They are considered a high-grade sarcoma with an aggressive clinical course 3.

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Cases and figures

  • Case 1a: on ultrasound
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  • Case 1b: on MRI
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