Grading Brain Tumors

The sixth digit of the ICD-O-3 morphology code describes the histologic grade or differentiation of the tumor. The ICD-O-3 grade or differentiation is not always described by pathologists for CNS tumors. When it is not described, assign code 9 (not determined, not stated, or not applicable). The ICD-O-3 grade or differentiation code for non-malignant CNS tumors is always assigned code 9. This is documented in ICD-O-3, page 30, Rule G, paragraph 1.

Other grading systems used to describe CNS tumors are WHO grade, Kernohan grade, and St. Anne/Mayo grade.

• Kernohan's original description of a four grade system for astrocytomas was published in 1949. It formed the basis for the atypia, necrosis, vascularity, and other factors now used in the WHO classification, although Kernohan grades I and II became WHO grade II.
• The Ringertz classification first published in 1950 is a three grade system where grade I is the same as the WHO classification, but WHO grades II, III and IV are split between Ringertz grades II and III.
• The St Anne/Mayo system from 1988 is also known by the names of its authors, Daumas and Duport. This system is based on four criteria—atypia, necrosis, mitosis and endothelial proliferation. There is a potential score of 0 - 4 in this system.
• The WHO grading system was initially published in 1979, but was modified in 1993 and again most recently in 2000. The WHO system includes all types of central nervous system tumors. It is not criteria based.

WHO grades are not the same as the ICD-O-3 grade or differentiation and are not recorded in the sixth digit histology code data field for grade. The grade is used by the clinician to plan treatment and predict prognosis.

The most important thing for a registrar to understand about the WHO grade for central nervous system tumors is that it does not parallel the ICD-O-3 grade code in the 6th digit of the morphology code. As this graphic shows, Grade I in the WHO system is roughly equivalent to a behavior code of benign in ICD-O-3.

The following table shows both WHO histologic typing and ICD-O 6th digit Grade.