This classification is based on the World Health Organization (WHO) classification of central nervous system (CNS) tumors. The WHO approach incorporates and interrelates morphology, cytogenetics, molecular genetics, and immunologic markers in an attempt to construct a cellular classification that is universally applicable and prognostically valid. Earlier attempts to develop a TNM-based classification were dropped: tumor size (T) is less relevant than tumor histology and location, nodal status (N) does not apply because the brain and spinal cord have no lymphatics, and metastatic spread (M) rarely applies because most patients with CNS neoplasms do not live long enough to develop metastatic disease.
The WHO grading of CNS tumors establishes a malignancy scale based on histologic features of the tumor. The histologic grades are as follows:
WHO grade I includes lesions with low proliferative potential, a frequently discrete nature, and the possibility of cure following surgical resection alone.
WHO grade II includes lesions that are generally infiltrating and low in mitotic activity but recur more frequently than grade I malignant tumors after local therapy. Some tumor types tend to progress to higher grades of malignancy.
WHO grade III includes lesions with histologic evidence of malignancy, including nuclear atypia and increased mitotic activity. These lesions have anaplastic histology and infiltrative capacity. They are usually treated with aggressive adjuvant therapy.
WHO grade IV includes lesions that are mitotically active, necrosis-prone, and generally associated with a rapid preoperative and postoperative progression and fatal outcomes. The lesions are usually treated with aggressive adjuvant therapy.
The following table is from the WHO Classification of Tumours of the Central Nervous System and lists the tumor types and grades. Tumors limited to the peripheral nervous system are not included. Detailed descriptions of histopathology, grading methods, incidence, and what is known about etiology specific to each tumor type can be found in the WHO classification book.