Neuropathology Division
   Brain Tumor Research

What are Pilocytic/Pilomyxoid Astrocytomas?

Pilocytic/pilomyxoid astrocytomas are pediatric low-grade gliomas. Astrocytes are a type of "glial" cell that provide support for the neurons in the brain. Tumors resembling astrocytes, called astrocytomas, make up the largest group of brain cancers. The World Health Organization (WHO) classification scheme divides astrocytomas into four grades, and pilocytic astrocytomas fall into the least aggressive group (grade I). Pilomyxoid astrocytomas are a microscopically distinctive type of grade II tumor.

Pilocytic Astrocytomas

Pilocytic smear
Figure 1 – A "smear" of pilocytic tumor cells highlights the long, hair-like processes extending out from blue-colored oval nuclei. (click for larger)

Figure 2: Polocytic Astrocytomas
Figure 2 – In this pilocytic astrocytoma, some cell processes form thick, pink, lumpy Rosenthal fibers. (click for larger)

Pilocytic astrocytomas are slow-growing tumors that are the most common gliomas in children. They can arise at any site in the brain and spinal cord; preferred locations include the optic nerves, hypothalamus, cerebellum and brain stem. Some pilocytic astrocytomas occur in patients suffering from an inherited condition known as Neurofibromatosis type I. Unlike most grade II, III and IV astrocytomas (also known as diffuse astrocytoma, anaplastic astrocytoma and glioblastoma), pilocytic astrocytomas grow as solid masses and tend to not invade diffusely through the brain. Clinical features vary depending on where the tumor is located (e.g. visual loss for those in the optic nerve). Seizures are relatively uncommon because the cerebral cortex is not often involved. Microscopically, the tumor cells usually have numerous long, thin processes (Figure 1), giving rise to the lesion’s name ("pil" means hair in latin). Pathology reports may also mention Rosenthal fibers (red, corkscrew-shaped masses; Figure 2) and eosinophilic granular bodies (red globular aggregates), both microscopic features common in pilocytic astrocytomas. Initial therapy generally consists of surgical resection. While pilocytic astrocytomas can recur, they almost never progress to more malignant, higher grade tumors.

Pilomyxoid Astrocytomas

Pilomyxoid astrocytoma slide
Figure 3 – Pilomyxoid astrocytomas lack Rosenthal fibers, and the tumor cells usually cluster around blood vessels, as is seen in the upper right of this image. (click for larger)

Pilomyxoid astrocytomas were first described by Dr. Burger and his colleagues in 1999. They recognized them as relatively rare tumors with some features of pilocytic astrocytoma, but a distinctive microscopic growth pattern, as well as a higher recurrence rate and chance of spreading within the brain. Pilomyxoid astrocytomas are distinctive because of their monomorphous appearance, “myxoid” background and perivascular orientation (Figure 3). The majority of pilomyxoid tumors occur in infants and young children, and involve the hypothalamic/chiasmatic region. As discussed in the Research section of this Website, we are expanding our analysis of pilomyxoid lesions in order to get a better sense of their microscopic appearance, clinical behavior, and molecular features.

Other Pediatric Low Grade Gliomas

Some children suffer from pediatric low grade gliomas that are more infiltrative that pilocytic/pilomyxoid ones. The most common type of these are WHO grade II diffuse astrocytomas. These can occur throughout the brain and spinal cord, and can be difficult or impossible to completely remove surgically depending on their location. It is still not entirely clear what molecular events drive the formation and growth of grade II diffuse astrocytomas, but they seem to be somewhat different from those in tumors of the same name found in adults.