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Currently the management of intracranial astrocytic tumors relies heavily on histologic grade. Although histologic grading is the current gold standard for tumor grading, it is a limited method which can be effected by both sampling error and interobserver agreement. The heterogeneous nature of astrocytomas makes the identification of necrosis difficult histologically. MR imaging provides important adjunct information regarding astrocytomas that may not be appreciated histologically and can potentially limit histological undergrading.
The detection of necrosis on MR imaging has significant prognostic implications. In this study, necrosis was defined by complete enhancement surrounding a central non-enhancing area on post-contrast MR imaging. The authors of this article defined MR Grade II as tumors with wispy or no enhancement and no necrosis, MR Grade III as tumors with enhancement without convincing necrosis and MR Grade IV as tumors with imaging findings suggesting necrosis.
In this study, the presence of necrosis on MR imaging had a significant impact on patient survival. Histologic Grade II and III tumors with MR evidence suggesting necrosis had survival more statistically similar to histologic Grade IV tumors rather than their respective histologic Grade II or III counterparts. Additionally, five of these patients that had histologic Grade II or Grade III tumors but MR evidence of necrosis went on to have further surgery and all were subsequently found to be histologic Grade IV tumors. Conversely, five patients with histologic Grade IV tumors but without necrosis or enhancement on MR imaging had longer survival than those with Grade IV histology and necrosis.
Based on the survival data in astrocytomas with and without necrosis, the authors propose the introducing MR imaging into the formal grading after a histologic grade has been obtained. In the purposed algorithm, if a patient has histologic Grade II or III astrocytoma, preoperative MR imaging should be reviewed. If there is evidence for necrosis, the tumor can either be directly managed as a Grade IV astrocytoma, histology can be reviewed or biopsy could be reconsidered. If necrosis is not present on imaging, the tumor can be managed histologically. In summary, the detection of necrosis in a Grade II or III tumor should raise suspicion for possible histologic undergrading.
