Ditzel, Ricky M

BACKGROUND Tumor-to-tumor metastasis (TTM) is an exceptionally rare phenomenon. To date, only 12 cases of the metastasis of clear cell renal cell carcinoma (ccRCC) to a meningioma have been reported in the literature. The authors present a unique case of TTM involving ccRCC metastasizing to a meningioma in a patient with a history of both primary cancers. OBSERVATIONS A 58-year-old male with a history of 2 primary cancers developed a biphasic tumor diagnosed as TTM of ccRCC to meningioma. The patient initially presented with left lower-extremity (LLE) weakness and underwent subtotal resection of his meningioma. Postsurgical imaging revealed rapid tumor progression, for which the patient completed adjuvant radiotherapy. Concurrently, the patient developed hematuria and was diagnosed with ccRCC. Subsequent recurrent LLE weakness and intracranial tumor progression prompted re-resection, revealing a TTM. LESSONS In patients with a history of multiple cancers who present with recurrent intracranial disease progression radiographically and/or clinically, it is worth considering TTM in the differential diagnosis to facilitate multidisciplinary workup, management, and treatment. https://thejns.org/doi/10.3171/CASE24670