Abstract:

Background: Spindle cell/Sclerosing rhabdomyosarcoma is the rarest variant of rhabdomyosarcoma, accounting for 5-10% of all rhabdomyosarcoma cases. Pathologically, these tumors can provide a variety of features and can resemble other tumors, thus often difficult to diagnose. We report a case of sclerosing rhabdomyosarcoma of the gingiva with histomorphology mimicking a malignant peripheral nerve sheath tumor. Case Presentation: The patient was a 39-years old Balinese male who noticed a lump on the left gum since 9 months ago. Head CT scan revealed a solid mass with soft tissue density at the left lower gingiva that extend superiorly and laterally, involving the left parapharyngeal and masticator spaces, infiltrated into the left mylohyoid muscle and left medial and lateral pterygoid muscles, and caused destruction of the left mandibular ramus and alveolar process of the left maxilla. The initial biopsy showed the proliferation of atypical spindle cells with a pattern resembling peripheral nerve sheath tumor and concluded as a low-grade malignant peripheral nerve sheath tumor. Wide excision specimen consists of proliferation of low-grade atypical spindle cells arranged in a fascicular pattern with foci of strap cells embedded in glassy eosinophilic sclerosis matrix. Some parts of the tumor showed palisading nuclei. The tumor cells were diffusely positive for desmin and negative for S100. This case was finally diagnosed as sclerosing rhabdomyosarcoma. Conclusion: Sclerosing rhabdomyosarcoma should be considered when atypical spindle lesions occur between the hyalinized stromal matrix of the head and neck in adults. Because of wide differential diagnosis, an adequate sampling, careful microscopic examination, and immunohistochemistry staining can avoid misdiagnosis.

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