In some patients, a diagnosed non-malignant tumor will transform into a malignant tumor, which is a rare occurrence.

Malignant transformation of a non-malignant tumor or progression of a non-malignant primary CNS tumor to a malignant tumor can be determined from physician statements in the patient medical record or by pathological review.

Pathologists develop a final diagnosis of malignant transformation or progression from a non-malignant tumor by review of old slides from previous biopsies, excisions of the non-malignant tumor, or by review of the newly-biopsied or -resected malignant brain tumor.

Malignant transformation and progression to malignancy mean the same thing. A change in morphology in a tumor from WHO grade I to WHO grade II, III, or IV indicates malignant transformation. When malignant transformation occurs, the tumors are considered separate primaries and two abstracts are completed. Recording these as separate primaries will allow researchers investigating these specific conditions to identify cases.

Use the following table as a reference for determining whether a new abstraction is needed.