Pilomyxoid astrocytoma: neuroimaging with clinicopathologic correlates in 4 cases followed over time.

Pilomyxoid astrocytoma (PmA) is a recently identified tumor type characteristically located in the hypothalamus and occurring in young children (<2 y). PmAs were formerly included in large series of pilocytic astrocytomas (PA) until it was realized in 1999 that this differing phenotype tended to manifest a more aggressive biologic course. PmA is defined by its pathologic features of a monomorphous architectural pattern, abundant myxoid background, and absence of features seen in classic PA. We present 4 histologically definite cases of PmA seen at our institution over a minimum 5-year follow-up time: one was rapidly fatal after initial treatment and 2 recurred during therapy. No singular neuroimaging feature can reliably diagnose PmA. However, PmAs tend to be solid, are more commonly necrotic and show extension of abnormal signal intensity into adjacent structures. Cysts, calcification, and perilesional edema are more common in classic PA. Serial neuroimaging in PmA shows early progression of predominantly solid, and later progression of predominantly cystic component. Radiologists should consider this diagnosis particularly in young children with hypothalamic tumors that lack the typical cystic appearance seen in PA.