Synchronous tumor-to-tumor metastasis of breast carcinoma to intracranial meningioma: illustrative case.

backgroundTumor-to-tumor metastasis (TTM) is a rare pathological phenomenon that most frequently involves intracranial meningiomas as recipient tumors and breast carcinoma as donor malignancy. Despite this known association, synchronous diagnosis of both tumors during the same diagnostic workup is exceptionally uncommon. OBSERVATIONS: A 70-year-old woman presented with progressive left lower extremity weakness and loss of smell. Neuroimaging revealed a right frontal extra-axial mass, initially interpreted as a metastatic lesion. Concurrent physical examination revealed an ulcerated mass in the right breast. Given her rapid neurological deterioration, surgical intervention was planned, and the patient underwent craniotomy with gross-total resection, along with a same-day biopsy of the breast lesion. Histopathological and immunohistochemical analyses revealed a meningioma harboring metastatic invasive ductal carcinoma of the breast, confirming TTM. Postoperative systemic staging revealed widespread metastatic disease, which prompted the initiation of systemic oncological therapy. LESSONS: TTM should not be regarded merely as a pathological curiosity but as a clinically relevant entity. Synchronous presentation, particularly in the setting of aggressive systemic disease, necessitates integrated surgical, pathological, and molecularly guided multidisciplinary decision-making to ensure accurate diagnosis and appropriate treatment planning. https://thejns.org/doi/10.3171/CASE26157.