Frozen section discrepancy in the evaluation of central nervous system tumors.

CONTEXT: Frozen section (FS) evaluation of central nervous system (CNS) lesions provides an assessment of specimen adequacy and facilitates triage for ancillary studies. Frozen section also provides an accurate preliminary diagnosis; however, certain lesions are recognized to cause diagnostic challenges at FS.
OBJECTIVE: To identify cases in which there was a discrepancy between the FS diagnosis and final diagnosis in the case to heighten awareness of common diagnostic pitfalls in surgical neuropathology.
DESIGN: All CNS FS cases involving a tumor diagnosis at FS or permanent section (N = 2156) from September 1997 until June 2005 were retrospectively reviewed. Discrepancies between the FS and final diagnoses were identified.
RESULTS: Of the 2156 cases identified, 57 (2.7%) discrepant diagnoses were found. Twelve (21.1%) of 57 discrepancies involved errors in classification of spindle cell lesions, most commonly confusing schwannomas or meningiomas with other lesions. Twelve (21.1%) of 57 cases involved errors in differentiating oligodendrogliomas from astrocytomas. Nine (15.8%) of 57 discrepancies involved errors in the diagnosis of CNS lymphoma. Eight (14.0%) of 57 cases involved errors in differentiating reactive from neoplastic processes, most frequently gliosis versus glioma. Four (7.0%) of 57 discrepancies involved errors in the overgrading of tumors. The remaining 12 (21.1%) of 57 cases included an assortment of other discrepancies.
CONCLUSIONS: Frozen section of CNS neoplastic processes can be highly accurate. Less than 3% of FS diagnoses in 1 institution's experience were discrepant with the final diagnoses. Approximately 80% of the discrepant cases were classified into 5 categories: spindle cell lesions, astrocytoma versus oligodendroglioma, differential diagnosis of CNS lymphoma, reactive versus neoplastic process, and tumor overgrading. Awareness of these pitfalls may help in further increasing diagnostic accuracy.