Distinct H3F3A Driver Mutation Defines Giant Cell Tumor of Bone: A Necessary Adjunct in the Diagnosis of Such a Lesion in the Hyoid Bone, Presenting as a Base of Tongue Mass

Abstract

<h3>Introduction</h3> Giant cell tumor of bone (GCTB) is a rare benign tumor mostly affecting the epiphyses of long bones in skeletally mature patients that is rarely described in the head and neck. Conventionally, diagnosis is based on microscopic features, exclusion of mimics, and clinico-radiologic correlation. Morphologic overlap with numerous giant cell-containing lesions of bone and soft tissue complicates the diagnosis. GCTB is characterized by the <i>H3F3A</i> mutation, which can be detected immunohistochemically using H3.3 G34W, a monoclonal antibody specific for this mutation. <h3>Case Findings</h3> A 57-year-old male presented with dysphagia, dysarthria, and right base of tongue (BOT) mass suspicious for carcinoma. Imaging revealed a heterogeneous parapharyngeal lesion extending from BOT to the hyoid bone. A BOT biopsy demonstrated giant cell-rich granulation tissue and non-endothelial lined blood lakes, suggestive of aneurysmal bone cyst (ABC). Fluorescent in situ hybridization (FISH) analysis did not identify <i>USP6</i> rearrangement, effectively eliminating primary ABC and favoring secondary ABC. Base of tongue resection, partial pharyngectomy and hyoid bone resection followed. <h3>Results</h3> Microscopically, the resection revealed a giant cell-rich lesion emanating from the hyoid bone, suggestive of GCTB and thus unlikely soft tissue giant cell tumor. Cellular pleomorphism and atypical mitoses were absent. Stromal cells expressed H3.3 G34W, confirming the diagnosis of GCTB. Zonal features of secondary ABC were juxtaposed to the dominant GCTB. <h3>Conclusions</h3> Hyoid GCTB is exceedingly rare, and such a diagnosis presents a conundrum. H3.3 G34W nuclear staining is an indicator of <i>H3F3A</i>mutation in GCTB that can help distinguish GCTB from other mimics. Prior reports of hyoid GCTB did not employ ancillary studies to identify <i>H3F3A</i> mutation or <i>USP6</i> rearrangements, as these have only been recently available. Our case fulfills current diagnostic criteria for GCTB, and excludes other giant cell-rich lesions. Accurate diagnosis of GCTB in this atypical location allows for appropriate surveillance algorithms.