It's not a particularly reliable source, but according to Wiki"

"Gliomas are further categorized according to their grade, which is determined by pathologic evaluation of the tumor.

- Low-grade gliomas are well-differentiated (not anaplastic); these are benign and portend a better prognosis for the patient.
- High-grade gliomas are undifferentiated or anaplastic; these are malignant and carry a worse prognosis."

. . . "low-grade gliomas grow slowly, often over many years, and can be followed without treatment unless they grow and cause symptoms."

http://en.wikipedia.org/wiki/Glioma

How do they "evaluate" the tumor, Rex?

Cherie