An invasive and diffuse cranial actinomycosis with a dura-based mass mimicking a brain tumor: illustrative case.

backgroundActinomycosis is a chronic suppurative infection caused by non-spore-forming, anaerobic, and filamentous gram-positive bacteria. Primary central nervous system involvement is rare, with no specific clinical features, causing a clinical diagnostic dilemma. Imaging can help in localizing and characterizing the lesion; however, a definitive diagnosis relies on culture and/or histopathology. OBSERVATIONS: The authors describe a 29-year-old male farmer with a rare case of invasive and diffuse cranial actinomycosis with a dura-based mass mimicking a brain tumor. Brain magnetic resonance imaging showed a moderately enhanced right frontoparietal infiltrative dura-based mass with marked thickening of the skull and multiple scalp actinomycotic abscesses. He underwent microsurgical excision of the mass, orbital decompression, and debridement of the scalp abscess. Histopathology confirmed actinomycosis, and his postoperative course was uneventful. LESSONS: Invasive and diffuse cranial actinomycosis with a dura-based actinomycetoma is a rare presentation that poses a diagnostic challenge due to its nonspecific manifestations. Imaging is helpful in localizing and characterizing the lesion; however, histopathology remains the gold standard for diagnosing actinomycosis. A high index of suspicion is also warranted in patients with predisposing factors to promote an early diagnosis and the initiation of appropriate treatments to improve functional recovery and limit residual deficits. https://thejns.org/doi/10.3171/CASE24210.