High-Grade Acinic Cell Carcinoma of the Parotid with SYN2::PPARG Fusion.

CASE PRESENTATION: A 60-year-old man presented with a 3.4 cm, slow-growing, hypermetabolic, right parotid mass of 1-year duration with associated intermittent pain, facial swelling, and transient facial nerve paresthesia, together with FDG avid osseous lesions of the right mandibular ramus, lateral scapula, and iliac crest. Subsequent FNA of the parotid mass showed findings consistent with acinic cell carcinoma (AciCC). The patient underwent a right total parotidectomy with ipsilateral neck dissection. Histologic examination revealed a diffusely-infiltrative, lobulated, malignant neoplasm composed of large polygonal cells with overt serous acinar differentiation. Lesional cells featured variably-sized nuclei with finely granular to vesicular chromatin, occasional acidophilic macronucleoli, and abundant, granular, basophilic-to-amphiphilic cytoplasm. Necrosis, perineural and lymphovascular invasion were also present. The supporting stroma was fibrous to densely hyalinized. By immunohistochemistry, lesional cells showed patchy positivity for AE1/AE3 and CK7, diffuse SOX10 immunoreactivity, and apical-luminal membranous staining for DOG1. PAS-D highlighted the diastase-resistant zymogen granules. A diagnosis of high-grade AciCC, stage pT4aN0, was rendered. NGS disclosed an underlying SYN2::PPARG fusion. Biopsies of the osteolytic lesions of the right mandibular ramus and left iliac crest confirmed AciCC metastases. The patient received adjuvant chemoradiation therapy but showed rapid disease progression with multifocal pulmonary and T10–T12 vertebral metastatic lesions. The patient was alive with disease 10 months post-surgery.

CONCLUSION

Here, we report the clinical, radiologic, cytologic, and histopathologic characteristics of an example of high-grade AciCC with a rare SYN2::PPARG fusion and aggressive biologic behavior.