- BACKGROUND: The term "tumor-to-lesion metastasis" is an extension of "tumor-to-tumor metastasis," which is a rare but well-documented phenomenon. Tumor metastasis to the meninges and/or central nervous system (CNS) is rare in patents with multiple sclerosis (MS), although MS lesions bear many similarities to the primary tumor microenvironment and metastatic niche. We present the first case of malignant tumor metastasis to MS lesions with immunophenotyping of inflammatory cells in the metastatic foci. CASE DESCRIPTION: A 45-year-old male patient with a 6-year history of MS and newly diagnosed lung carcinoma developed carcinoma metastases in the meninges and CNS, as well as into mixed active/inactive MS lesions. The carcinoma-hosting MS lesions exhibited abundant macrophages/microglia with ongoing demyelination but rare T cells. In comparison, a 46-year-old female patient with a 21-month history of MS and newly diagnosed gastric carcinoma was found to have leptomeningeal carcinomatosis and separate active MS lesions containing not only frequent macrophages/microglia but also T cells. CONCLUSIONS: The carcinoma-hosting MS lesions are unlike typical active lesions but recapitulate the CNS metastatic niche. Our observations suggest that metastasis-hosting MS lesions might require a distinct immune microenvironment to be permissive to the seeding and growth of metastatic tumors.