CHICAGO — Imaging characteristics can help physicians distinguish the more aggressive pilomyxoid astrocytoma from a pilocytic astrocytoma and thereby guide treatment decisions, according to Dr. Luke Linscott, who presented his findings at the annual meeting of the American Society of Neuroradiology.
“If you see a patient with a presumed pilocytic astrocytoma [PCA] who is either very young or an adult, with a large, bulky tumor in an atypical location for PCA and, especially if that tumor demonstrates hemorrhage, the most likely diagnosis is aggressive pilomyxoid astrocytoma [PMA],” said Dr. Linscott of the University of Utah, Salt Lake City.
In what is the largest case series to date on PMA, Dr. Linscott and colleagues reviewed the images, pathology reports, and clinical information of 21 patients with pathology-confirmed PMA. The average age was 5 years, with a range from 9 months to 46 years, and there was a slight predominance of males. Researchers contributed cases from the United States, Canada, Norway, and South Africa.
The tumor's anatomic location is one diagnostic clue. Although PCAs are most commonly found in the posterior fossa and less commonly in the hypothalamus and optic chiasm, PMAs are more likely to be located in the hypothalamic/optic chiasm region, according to Dr. Linscott. However, in this series, more than 40% of PMAs were found in atypical locations such as the cerebral cortex (4 of 21), cerebellum (2 of 21), basal ganglia (2 of 21), and area around the fourth ventricle (1 of 21). “These atypical locations are more common than previously reported and are more common in older patients,” said Dr. Linscott.
Intratumoral hemorrhage is another important distinguishing feature. Although evidence of hemorrhage was noted in 20% of PMAs, it is extremely rare in PCAs, according to Dr. Linscott.
Rim enhancement on contrast-enhanced CT is also characteristic of PMA. In this series, 48% of cases showed heterogeneous rim enhancement, 43% showed uniform enhancement, and 9% showed no enhancement at all. Two cases showed cerebrospinal fluid dissemination.
Calcification is a characteristic finding of PCA. In this series, calcification was noted in only one case of PMA, making the diagnosis of PMA less likely, Dr. Linscott said.
Identifying a PMA has important clinical consequences. Because PMA is a clinically more aggressive tumor than PCA, distinguishing between these two tumor types may change the surgical and medical management of the patient, including more aggressive adjuvant chemotherapy and radiotherapy.
Cerebellar vermis tumor in 17-year-old boy on contrast-enhanced axial T1WI.
Axial T1WI shows nonenhancing posterior parietal tumor in a 2-year-old boy.
CT shows fluid-fluid levels in an acute intratumoral hemorrhage of a PMA.
T1 image with contrast shows teen's rim-enhancing lesion of the posterior fossa. Images courtesy Dr. Luke Linscott