Primary central nervous system lymphoma of the third ventricle: Diagnostic pitfalls, the “Ghost Tumor” phenomenon, and steroid-induced complications

Background: Primary central nervous system lymphoma (PCNSL) is an aggressive extranodal non-Hodgkin’s lymphoma confined to the brain, leptomeninges, spinal cord, or eyes. It accounts for approximately 1–5% of primary brain tumors. Intraventricular localization is uncommon, and tumors arising predominantly within the third ventricle represent an exceptionally rare clinical presentation with only a few cases described in the literature. Because of its unusual location and nonspecific imaging features, third ventricular PCNSL may mimic other suprasellar and intraventricular tumors. Case Description: A 26-year-old immunocompetent female presented with rapidly progressive cognitive impairment, behavioral changes, headaches, and visual deterioration. Magnetic resonance imaging revealed a homogeneously enhancing lesion located predominantly within the floor of the third ventricle associated with extensive diencephalic edema. Cerebrospinal fluid analysis demonstrated lymphocytic pleocytosis and elevated protein levels, while tumor markers were negative. High-dose corticosteroid therapy resulted in partial radiological regression of the lesion, demonstrating the characteristic “ghost tumor” phenomenon associated with PCNSL. Histological verification was obtained using an endoscopic endonasal transtuberculum–transplanum biopsy approach. Pathological examination confirmed diffuse large B-cell PCNSL. The postoperative course was complicated by systemic Candida albicans infection related to prolonged corticosteroid therapy. Conclusion: PCNSL arising within the third ventricle is an extremely rare entity that may mimic other intraventricular tumors. Steroid-induced tumor regression may provide an important diagnostic clue but may also complicate histological verification. The endoscopic endonasal approach represents a safe and minimally invasive option for obtaining diagnostic tissue from lesions located at the floor of the third ventricle.