LOW-GRADE GLIOMA (LGG)
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LGG is a generic term for WHO grade II lesions that may be of either astrocytic or oligodendroglial origin. They typically affect young adults, and are biologically characterized by a high degree of cellular differentiation, slow growth and diffuse infiltration of surround brain.
LGG represents 10-15% of all astrocytic brain tumors, with an incidence of 1.4/million. The peak incidence is in young adults between age 30 and 40, with a mean age of occurrence at 34 years. There is a male predominance (1.8:1).
LGG occurs most often in the frontal and temporal lobes. The brainstem and spinal cord are the next most affected sites. LGG is uncommon in the cerebellum.
Patients may present with seizure or signs and symptoms attributable to mass effect or brain invasion.
On MRI, LGG is usually hypointense on T1-weighted imaging and hyperintense on FLAIR and T2-weighted imaging. These lesions rarely enhance. Cystic changes are not uncommon.
Because of their infiltrative nature, these lesions show blurring of the gross anatomical boundary with normal brain. There is usually enlargement or distortion, rather than overt destruction, of surrounding anatomical structures.
LGG is composed of well differentiated fibrillary or gemistocytic neoplastic astrocytes on a loosely structured, often microcytic tumor matrix. These lesions have moderately increased cellularity and occasional nuclear atypia. Mitotic activity is absent or rare.
fibrillary astrocytoma, gemistocytic astrocytoma, protoplasmic astrocytoma
TP53 mutations are present in nearly 60% of these lesions. Increased expression of the PDGFR-alpha mRNA is also common. Finally, gain of chromosome 7q and amplification of 8q are frequent. LOH on chromosome 10q occurs in a subset of LGG, as does LOH on 22q.
Mean survival time following surgical intervention is 6-8 years. Progression to GBM occurs with a mean interval time of 4-5 years. Young age and gross total tumor resection are considered predictive of delayed recurrence and progression.