|Dr. Alexandra Golby, of the Harvard Medical School and the Brigham and Women’s Hospital, reads images.
Although various classification systems may be used to grade meningiomas, the current version of the World Health Organization (WHO), published in 2000, is the most commonly utilized.
Even though the WHO classification system was revised in 2007, the two publications are nearly identical, and the 2000 edition is still used. The WHO’s classification scheme recognizes 15 variations of meningioma according to their cell type as seen under the microscope. These variations are called meningioma subtypes, and because they can only be seen and therefore identified under a microscope, the technical term for these cell variations is histological subtypes. WHO graded each of these meningioma subtypes into one of three categories based primarily upon the likelihood of recurrence and the rate of growth exhibited by each. The overall classifications are benign (Grade I), atypical (Grade II) and malignant (Grade III).
| WHO Grade I – Benign
||WHO Grade II – Atypical
||WHO Grade III – Malignant
- Fibrous (fibroblastic)
- Transitional (mixed)
- Clear Cell
A higher tumor grade is associated with a greater likelihood of the tumor’s recurrence and/or faster growth rate and increased potential of brain invasion (often referred to as more aggressive behavior). Characteristics influencing the grading of a tumor include the cell subtype (as described above), the rate at which the tumor cells are multiplying and the tumor is growing (cell proliferation and mitotic index are terms that express this concept) and the degree of brain invasion by the tumor.
Approximately 90% of all meningiomas fall into the benign category. These tumors exhibit slow growth and very little multiplication of cells and very rarely invade the brain tissue. Overall, benign meningiomas are less likely to recur than the atypical and malignant grades.
Atypical meningiomas represent approximately 7-8% of meningiomas and exhibit increased tissue and cell abnormalities. These tumors exhibit a faster growth rate than benign meningiomas and, on occasion, some degree of brain invasion. Atypical meningiomas have a higher likelihood of recurrence than benign.
Malignant meningiomas account for approximately 2–3% of all meningiomas. These tumors show increased cellular abnormalities as well as a faster growth rate compared to benign and atypical meningiomas. Malignant meningiomas are the most likely to invade the brain and spread (metastasize) to other organs in the body. They are the most likely of the three types to recur.
Some level of variation may exist when a pathologist is trying to categorize the particular subtype of a meningioma, as some subtypes have similarities in appearance. One pathologist might look at the tissue and decide that it is a type of meningioma that falls into the atypical, Grade II category, while a different pathologist may look at the same tissue and decide that the meningioma should be classified as a subtype that falls into the benign, Grade I category.
The WHO 2000 classification scheme was significantly revised from the version published in 1993 and is more rigid in the characterizations and distinctions made between the subtypes of meningiomas and their corresponding grades. The WHO 2000 system will hopefully lead to a more objective and precise classification of meningioma subtypes and their grades, thereby laying the basis for studies that will allow increased accuracy in determining such issues as predicted survival times, expected incidence of tumor recurrence, and the efficacy of different treatment methodologies.