SMARCB1 (INI1)-deficient sinonasal carcinoma with yolk sac differentiation, a case of long-term clinical remission after multiple rounds of radiotherapy-a case report and literature review.

Rationale: SMARCB1 (INI1) deficient sinonasal carcinoma is a subtype of Switch/Sucrose nonfermentable (SWI/SNF) complex deficient sinonasal carcinoma, which is distinct from sinonasal undifferentiated carcinoma (SNUC) in 5th edition of the WHO classification of head and neck tumors. It commonly shows basaloid, eosinophilic, oncocytoid or rhabdoid morphology. However, it can exhibit yolk sac like differentiation in very rare cases, with associated SALL4, GPC-3 and CDX2 and AFP expression, which can lead to the misdiagnosis of primary nasopharyngeal yolk sac tumor (YST).

Patient Concerns: A 58-year-old male patient with right nasal cavity mass, he complained for persistent right-sided nasal congestion for 3 months, accompanied by decreased sense of smell and protrusion of the right eyeball.

Diagnosis: Histology showed tumor cells with glandular, large cystic, and microcystic architectural arrangement. Immunohistochemically, the tumor cells expressed SALL-4 and GPC-3. The findings supported obvious yolk sac tumor like features. However, the absence of INI-1 expression confirmed the diagnosis of INI-1 deficient sinonasal carcinoma.

Interventions: The patient underwent 4 rounds of clinical tumor volume (CTV) radiotherapy.

Outcomes: The patient was followed up for 22 months with interval nasopharyngeal MRI and lung CT scan, with no sign of tumor recurrence or metastasis.

Lessons: Our case suggests that INI1-deficient sinonasal carcinoma with yolk sac differentiation is an important differential diagnosis of primary nasopharyngeal yolk sac tumor, which may have favorable disease-free survival with adjuvant radiotherapy alone.