FINAL DIAGNOSIS: Pleomorphic xanthoastrocytoma
Intracranial lesions presenting on imaging as a cystic mass with enhancing mural nodule are typically associated with the differential of juvenile pilocytic astrocytoma (JPA), pleomorphic xanthoastrocytoma (PXA) and ganglioglioma (although other lesions may also have this presentation). As this tumor did not have areas of ganglion-appearing cells, ganglioglioma became low on the differential. However, the two remaining entities, JPA and PXA, have a significant amount of overlap, creating somewhat of a diagnostic dilemma. While neither lesion is considered a high-grade astrocytic neoplasms, there is still a far better prognosis for JPA (with an 80% 20-year survival) versus PXA (15 to 20% of these will undergo malignant transformation).
Features which may overlap between the two entities include the presence of eosinophilic granular bodies (EGBs) and Rosenthal fibers; both lesions may also consist predominantly of spindled cells. JPAs characteristically are biphasic, with regions which are more cellular than other areas; however, only one component may be present. In our case a monomorphic JPA was a consideration, as the majority of the specimen appeared to be more cellular. EGBs were abundant, as is demonstrated in the images, but Rosenthal fibers were not identified. While neither feature is individually diagnostic, Rosenthal fibers are generally a common finding in JPAs, and a rare finding in PXA.
Pleomorphism is, as suggested by the name, a common finding in PXA, and this case demonstrates many large, bizarre cells (see image). This level of pleomorphism is rare in JPAs. Mitoses were easily identified; again, while not specific for either lesion, these are more common in PXAs than JPAs. However, the presence of mitoses may portend a more high-grade tumor. The two final features which helped support the diagnosis of PXA include areas with clear xanthomatous changes in the astrocytic cells, and the reticulin stain. PXAs are characterized by increased reticulin deposition around cells and groups of cells; a reticulin stain helps highlight this finding.
In summary, while no single feature is diagnostic for a single entity, the combination of the findings discussed above (EGBs, xanthomatous change, predominance of spindled cells, lack of Rosenthal fibers, mitoses and reticulin deposition) strongly favor PXA.
Contributed by Alicia F. Liang, MD and Geoffrey Murdoch, MD, PhD