Invasive Fungal Granuloma of the Skull Base Mimicking Spindle Cell Neoplasm in a Young Immunocompetent Patient: A Diagnostic Challenge

Skull base lesions presenting with proptosis and sinonasal involvement in young patients are commonly attributed to vascular neoplasms such as juvenile nasopharyngeal angiofibroma. However, rare infectious etiologies such as invasive fungal granulomas may present with overlapping radiological features, leading to diagnostic challenges. We report a rare case of a 19-year-old immunocompetent male presenting with progressive bilateral proptosis and a large skull base mass involving the clivus, nasal cavity, and orbits. Initial imaging suggested a vascular neoplasm, and biopsy revealed a spindle cell lesion with inconclusive immunohistochemistry. Fungal cultures were negative. The patient was lost to follow-up but returned four months later with worsening symptoms. Repeat biopsy demonstrated necrotizing granulomatous inflammation with septate, acute-angle branching fungal hyphae, consistent with <i>Aspergillus</i> species on GMS (Grocott methenamine silver) and PAS (periodic acid-Schiff) staining. The final diagnosis was invasive fungal sinusitis mimicking a skull base neoplasm. The patient underwent surgical debridement followed by antifungal therapy with voriconazole and showed clinical improvement. This case underscores the importance of considering fungal granulomas in the differential diagnosis of skull base lesions, even in immunocompetent individuals. It highlights the critical role of repeat biopsy, fungal stains, and a multidisciplinary approach in arriving at an accurate diagnosis when imaging and histology are inconclusive.