From intraosseous meningioma and ossified metaplastic meningioma to osteomeningioma: a novel voxel-based, atlas-normalized MRI framework for the radiological classification of skull-involving meningiomas

Background and objectives: Skull-involving meningiomas remain ill-defined, resulting in heterogeneous classifications and terminology. Existing schemes mainly describe bone-dura relationships and often overlook the relative burden of the intracranial soft-tissue component. This study aimed to develop and apply a deterministic, atlas-normalized, MRI-based radiological framework for the standardized description of imaging-defined osteomeningiomas and to explore associations between compartmental tumor distribution, radiological phenotype, and clinical presentation. Methods: We retrospectively reviewed adults with skull-involving meningiomas at our tertiary neurosurgical center between 2000 and 2024. Tumors were segmented on contrast-enhanced T1-weighted MRI, normalized to MNI152 space, and classified using a deterministic voxel-based radiological framework across osseous, juxta-osseous/dural, and intradural compartments. Tumors were classified as primary osteomeningioma (POM; isolated osseous compartment involvement) or secondary osteomeningioma (SOM; osseous plus adjacent juxta-osseous/dural compartment involvement), with subtypes SOM-I (no intradural extension), SOM-IIA (all three compartments involved, with an osseous component equal to or greater than the intradural component), and SOM-IIB (all three compartments involved, with an intradural component greater than the osseous component). These imaging-defined categories were intended as radiological descriptors of compartmental tumor distribution rather than distinctions between microscopic osseous invasion, reactive hyperostosis, or osseous metaplasia. All analyses were performed at the tumor level, with a predefined sensitivity analysis restricted to one index tumor per patient. Exploratory multivariable logistic regression models were fitted for brain edema, epileptic seizure, raised intracranial pressure, and exophthalmos. Results: ). Epileptic seizures and signs of raised intracranial pressure were most frequent in SOM-IIB, exophthalmos in SOM-I, and subcutaneous mass in POM. In exploratory adjusted analyses, SOM-IIB remained associated with brain edema, epileptic seizure, and raised intracranial pressure, whereas SOM-I remained associated with exophthalmos. Conclusions: This voxel-based, atlas-normalized MRI framework provides a radiological standardization for the description of skull-involving meningiomas. Rather than establishing histological proof of bone or dural invasion, it standardizes compartmental tumor burden across osseous, juxta-osseous/dural, and intradural spaces. In exploratory analyses, the proposed imaging-defined subtypes were associated with distinct clinicoradiological presentation patterns, which warrant further pathological, multimodal, and external validation.