Abstract

Spindle cell rhabdomyosarcoma is an uncommon distinct variant of rhabdomyosarcoma. Spindle cell rhabdomyosarcoma falls in a morphologic spectrum with sclerosing rhabdomyosarcoma. The head and neck region is the most common anatomic site in adults, follows by the extremities and the trunk. Herein, we describe a 69- year-old female who presented with a neck lesion. CT imaging of the neck soft tissue showed a large mass with involvement of prevertebral soft tissues, right common carotid and proximal right internal carotid artery. The mass was biopsied and was sent for pathologic examination. Histologic sections showed a spindle cell neoplasm composed of long intersecting cellular fascicles of relatively uniform spindle cells with oval to elongated nuclei, vesicular chromatin, pale indistinct cytoplasm and frequent mitotic figures. Scattered within the neoplasm, there were spindled to polygonal rhabdomyoblasts with abundant bright eosinophilic cytoplasm. By immunohistochemistry, myogenin and MYO-D1 showed strong nuclear positivity. Desmin and muscle specific actin (MSA) were diffusely positive, as well as multifocal positivity for SMA. Pan-cytokeratin, p63, caldesmon, CD34, HMB45, SOX10 and GFAP staining were negative. Loss of histone trimethylation H3K27me3 was observed in majority of the neoplastic cells. The overall morphologic and immunohistochemical features was consistent with spindle cell rhabdomyosarcoma. The main entities to be considered in the differential diagnosis of spindle cell rhabdomyosarcoma, particularly in the head and neck region, are spindle cell (sarcomatoid) squamous cell carcinoma, spindle cell melanoma, leiomyosarcoma, malignant peripheral nerve sheath tumor with heterologous myoblastic differentiation and monophasic synovial sarcoma. Immunohistochemistry is very helpful to exclude most of these diagnostic considerations. Awareness of this uncommon entity is important to avoid misdiagnosis and for appropriate diagnosis and treatment.

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