California Cancer Reporting System Standards Volume I: Abstracting and Coding Procedures for Hospitals
The five digit histology field consists of two parts:
The morphology, or cell type, of the primary tumor (first four digits).
The tumor's behavior - that is, the degree of malignancy or how the tumor can be expected to eventually behave.
A separate one digit differentiation code represents the grade, or degree of differentiation, of neoplastic tissue-that is, the extent to which cells have the specialized characteristics of a particular tissue or organ.
In general, the less differentiated the cells, the more aggressive the tumor.