Low-grade sinonasal adenocarcinoma non-intestinal type of the maxillary sinus presents challenges for clinicopathologic diagnosis in a case report

Low-grade sinonasal adenocarcinoma, non-intestinal type (LG non-ITAC) is a rare malignant epithelial tumor of the sinonasal tract. Its clinical and radiologic manifestations are often non-specific, and its histologic features may overlap with benign sinonasal glandular lesions and other low-grade adenocarcinomas. Accurate diagnosis is clinically important because it directly affects surgical management, prognosis assessment, and long-term surveillance. LG non-ITAC arising in the maxillary sinus is uncommon and may present with symptoms that obscure its sinonasal origin. A 50-year-old man presented with a left maxillary gingival mass and tooth mobility without nasal symptoms. Computed tomography showed a destructive lesion centered in the left maxillary sinus. Histology demonstrated back-to-back glands, tubules and papillary structures with scant intervening stroma. The tumor cells were relatively uniform, with round-to-oval nuclei and eosinophilic cytoplasm. Focal necrosis was present, and 4 mitoses/10HPF. Immunohistochemistry (IHC) showed positivity for pancytokeratin (AE1/AE3) and CK7, with CK5/6 staining in occasional tumor cells and androgen receptor (AR) was focally weakly positive. HER2 IHC score of 3+. IHC negative markers are CK20, CDX2, SATB2, Villin, MUC2, Calponin, DOG1, mammaglobin, p40, p63, pan-TRK, S100, SMA and SOX10. The Ki-67 labeling index was approximately 40%, and tumor-cell p53 nuclear staining was absent. Fluorescence in situ hybridization detected no NTRK3 or EWSR1 rearrangements and no ETV6::NTRK3 fusion. The primary tumor was originally classified as low-grade non-ITAC. Treatment included left subtotal maxillectomy, adjacent-structure resection and vascularized flap reconstruction, without postoperative radiotherapy or chemotherapy. The patient remained recurrence-free for five years. A skull-base recurrence detected in the sixth postoperative year was resected at another hospital and reported as recurrent non-ITAC. A gingival mass with tooth mobility may be the presenting manifestation of maxillary sinus LG non-ITAC. Skull-base recurrence in the sixth postoperative year in this patient highlights the importance of long-term surveillance.