Biphasic Synovial Sarcoma of the Hypopharynx in a Six-Year-Old Child Diagnosed via SS18 Immunohistochemistry Without Molecular Confirmation in a Resource-Limited Setting: A Case Report

<title>Abstract</title> <p>Background Synovial sarcoma is a rare, aggressive soft-tissue malignancy accounting for approximately 5–10% of all soft-tissue sarcomas globally, with head and neck involvement reported in fewer than 10% of cases. Within the pediatric population, primary pharyngeal synovial sarcoma is exceptionally uncommon, with only a handful of cases documented in children under ten years of age. The submucosal growth pattern characteristic of this tumor renders early clinical recognition and histopathological confirmation particularly challenging, frequently resulting in non-diagnostic superficial biopsies and critical delays in definitive oncological management. In resource-limited healthcare settings, the inaccessibility of molecular cytogenetic confirmation via fluorescence in situ hybridization (FISH) or reverse transcription-polymerase chain reaction (RT-PCR) for the characteristic t(X;18)(p11.2;q11.2) translocation poses an additional diagnostic barrier, necessitating reliance on advanced immunohistochemical surrogate markers. Case Presentation We report a six-year-old female — representing one of the youngest documented presentations of hypopharyngeal synovial sarcoma globally — who presented to AKFA Medline University Hospital, Tashkent, Uzbekistan, with a one-year history of progressive dyspnea and rhinolalia secondary to critical laryngo-pharyngostenosis. Cross-sectional magnetic resonance imaging (MRI) and computed tomography (CT) demonstrated a 2.9 × 2.5 × 3.8 cm infiltrative mass within the right supraglottic larynx and adjacent hypopharynx causing near-total luminal obliteration. An initial endoscopic biopsy at a regional center yielded only non-specific ulcerative granulation tissue, failing to establish a diagnosis. Emergency tracheostomy followed by open surgical resection was performed on the day of admission. External histopathological review initially classified the lesion as Grade 3 embryonal sarcoma based on primitive round-cell morphology and high mitotic index. Subsequent comprehensive immunohistochemical (IHC) re-evaluation using an automated platform demonstrated strong diffuse positivity for SS18 — a highly sensitive and specific surrogate marker for the SS18::SSX oncogenic fusion protein — alongside BCL2, CD99, and CD56, with focal pan-cytokeratin (PanCK) expression confirming biphasic epithelial differentiation. An exhaustive lineage-exclusion panel systematically eliminated hematologic, neuroendocrine, myogenic, vascular, and squamous cell malignancies. In the absence of molecular genetic confirmation, the integrated histomorphological and immunophenotypic profile enabled definitive reclassification to Grade 2 Biphasic Synovial Sarcoma of the Hypopharynx, classified as overall Grade 2 per the French Federation of Cancer Centers Sarcoma Group (FNCLCC) grading system. Following multidisciplinary tumor board discussion, active surveillance was adopted over immediate adjuvant therapy given favorable pathological parameters including FNCLCC Grade 2 classification, absence of tumor necrosis, and complete gross surgical resection. Subsequent PET-CT demonstrated no regional nodal fluoro-2-deoxy-D-glucose (FDG) avidity and no distant metastatic disease, with post-operative site changes only. At 22 months follow-up, the patient has been successfully decannulated, demonstrates excellent functional recovery with normal swallowing and respiration, and remains clinically and radiologically disease-free under active three-monthly surveillance. Conclusions This case establishes that hypopharyngeal biphasic synovial sarcoma can present in early childhood, extending the lower age boundary previously reported in the literature. It demonstrates that superficial endoscopic biopsy is inadequate for submucosal pharyngeal sarcomas and must be followed by deep open surgical sampling when initial results are non-diagnostic; that a comprehensive SS18-anchored IHC panel can establish a definitive diagnosis of biphasic synovial sarcoma without FISH or RT-PCR confirmation in resource-limited settings, provided classic biphasic histomorphology and exhaustive lineage exclusion are demonstrated; and that extended oncological surveillance well beyond ten years is essential given this tumor's well-documented propensity for late locoregional and distant relapse. At 22 months post-operative follow-up, the patient has been successfully decannulated and remains clinically and radiologically disease-free under active surveillance without adjuvant therapy — an outcome supported by negative PET-CT staging and consistent with the favorable prognostic profile of this case — though the well-documented propensity of synovial sarcoma for late relapse necessitates structured surveillance extending well beyond ten years.</p>