Poorly differentiated spindle cell carcinoma with rhabdomyoblastic differentiation: highlights on immunohistochemistry interpretation

Introduction

Tumors of the head and neck that exhibit rhabdomyoblastic differentiation include rhabdomyosarcoma, leiomyosarcoma, synovial sarcoma, spindle cell carcinoma (SpCC), malignant peripheral nerve sheath tumor, or metastatic disease. Spindle cell carcinoma (SpCC) is a rare subtype of squamous cell carcinoma (SCC) typically seen in the oral cavity and larynx. Cases with ulcerated surface epithelium completely devoid of surface dysplastic epithelium that present entirely of a spindle cell proliferation can pose a diagnostic challenge. Additionally, immunohistochemistry (IHC) can demonstrate evidence of differentiation to variable mesenchymal components. Here we present a poorly differentiated SpCC with rhabdomyoblastic differentiation as evidenced by positivity for the rhabdomyoblastic biomarker MyoD-1.

Case Presentation

A 54-year-old female presented to her dentist with a nodular mass on the anterior mandibular gingiva mimicking a pyogenic granuloma. Upon histopathological examination, H&E tissue sections showed an ulcerated mass devoted of any surface epithelium with malignant spindle cells invading the connective tissue. The spindle cells showed several malignant criteria including pleomorphism, hyperchromatism, increased nuclear-to-cytoplasmic ratio, and atypical mitotic figures. They were arranged in short interlacing fascicles. Abundant eosinophilic cytoplasm with rhabdomyoblastic differentiation was featured. Initial IHC analysis showed diffuse positivity for Myo-D1, focal positivity for Desmin, and negativity for Myogenin, SMA, h-Caldesmon. However, Pancytokeratin, was diffusely positive confirming the diagnosis of SpCC. Other cytokeratins, p53, p63, p40, TLE-1, CD99, and neural markers were all negative. Surgical management at the University of Maryland Medical Center included composite mandibulectomy and bilateral neck dissection, followed by adjuvant radiotherapy and chemotherapy. Surgical pathology findings were positive for perineural invasion, negative for all lymph nodes and tissue margins.

Conclusion

This case highlights the challenge in diagnosing cases of SpCC devoid of surface epithelium. Pathologists should also be aware of the possibility of rhabdomyoblastic differentiation in SpCC and cautiously interpret skeletal muscle biomarkers given the variable expression patterns.