Residency · Residency · Radiation Oncology
Low-Grade Glioma: Timing and Extent of Radiation
Overview
Low-grade gliomas (LGG) include WHO grade 2 diffuse astrocytomas and oligodendrogliomas. The WHO 2021 classification incorporates molecular markers such as IDH mutation status and 1p/19q codeletion, which define distinct tumor entities with differing prognoses. LGG typically affects younger adults, with a median age between 35 and 45 years, and patients often experience long survival times ranging from 7 to over 15 years depending on the molecular subtype. Treatment decisions must carefully balance effective tumor control against the risk of long-term neurocognitive toxicity, as many patients survive for decades. Key controversies in management revolve around the optimal timing of radiation therapy and the role of chemotherapy.
Classification and Molecular Subtypes
WHO 2021 Integrated Classification
The 2021 WHO classification categorizes LGGs based on molecular features. Oligodendrogliomas that are IDH-mutant and 1p/19q-codeleted have the best prognosis, with median survival exceeding 15 years when treated appropriately. Astrocytomas that are IDH-mutant but lack 1p/19q codeletion have an intermediate prognosis, with median survival between 10 and 15 years; these tumors often exhibit ATRX loss and p53 mutation. Diffuse astrocytomas that are IDH-wildtype are now reclassified as glioblastoma, IDH-wildtype, if they meet molecular criteria for glioblastoma such as TERT promoter mutation, chromosome +7/-10 copy number changes, or EGFR amplification. Despite their low-grade histology, these IDH-wildtype tumors carry a poor prognosis.
Prognostic Factors
Favorable prognostic factors include IDH mutation with 1p/19q codeletion, younger patient age, smaller tumor size, frontal lobe location, absence of contrast enhancement on imaging, and good performance status as measured by the Karnofsky Performance Scale (KPS). Unfavorable factors include IDH-wildtype status (now considered glioblastoma), age over 40 years, tumors crossing the midline, preoperative neurological deficits, and tumor size greater than 6 cm.
Role of Surgery
Maximal safe resection is the first-line treatment for accessible LGGs. The extent of resection correlates strongly with outcomes; gross total resection (GTR) is associated with longer progression-free survival (PFS) and overall survival (OS). For tumors located in eloquent brain areas, awake craniotomy with cortical and subcortical mapping is employed to maximize resection while preserving function. Biopsy alone is reserved for deep-seated, bilateral, or diffusely infiltrative tumors where resection is not feasible. Even after GTR, the diffuse infiltrative nature of LGG means that microscopic disease typically remains.
Landmark Radiation Trials
EORTC 22845: Early vs. Delayed Radiation
The EORTC 22845 trial was a phase III study comparing immediate postoperative radiation therapy (RT) at 54 Gy in 30 fractions to RT administered at tumor progression. Immediate RT improved progression-free survival, with a median of 5.3 years versus 3.4 years for delayed RT. However, there was no difference in overall survival, which was approximately 7.4 years in both groups. This indicates that radiation can be safely deferred in selected patients without compromising overall survival, although patients will eventually require treatment. These findings support observation after GTR in favorable-risk patients.
RTOG 9802: Radiation with or without PCV Chemotherapy
The RTOG 9802 trial was a phase III study comparing RT alone (54 Gy in 30 fractions) to RT combined with PCV chemotherapy (procarbazine, CCNU, vincristine) in high-risk LGG patients. High-risk was defined as age 40 or older, or subtotal resection/biopsy only. Long-term follow-up published in 2016 demonstrated a median overall survival of 13.3 years for the combined RT + PCV arm versus 7.8 years for RT alone. This dramatic survival benefit was especially pronounced in oligodendroglioma patients. The trial established RT plus chemotherapy as the standard of care for high-risk LGG. Although PCV is preferred based on this evidence, temozolomide is often substituted in clinical practice due to better tolerability.
EORTC 22033: Temozolomide versus Radiation
The EORTC 22033 trial compared temozolomide monotherapy to RT (50.4 Gy in 28 fractions) in high-risk LGG patients. Overall, there was no significant difference in progression-free survival between the two arms. However, in the subgroup of IDH-mutant, non-codeleted astrocytomas, RT was superior to temozolomide for PFS. These results support radiation as the backbone of treatment for IDH-mutant astrocytomas and do not endorse temozolomide monotherapy as a substitute for radiation.
CODEL Trial
The CODEL trial was initially designed to compare RT + PCV, RT + temozolomide, and temozolomide alone in patients with 1p/19q-codeleted tumors. The temozolomide alone arm was closed early due to inferior progression-free survival. The trial continues to compare RT + PCV versus RT + temozolomide for codeleted oligodendrogliomas. Results are awaited to determine whether temozolomide can effectively substitute for PCV in this molecular subtype.
| Trial | Population | Arms | Key Result |
|---|---|---|---|
| EORTC 22845 | All LGG | Early RT (54 Gy) vs. RT at progression | PFS improved with early RT; no OS difference (7.4 yr both) |
| RTOG 9802 | High-risk LGG (age ≥ 40 or STR) | RT (54 Gy) vs. RT + PCV | Median OS 13.3 yr (RT+PCV) vs. 7.8 yr (RT alone) |
| EORTC 22033 | High-risk LGG | RT (50.4 Gy) vs. TMZ alone | No PFS difference overall; RT superior for IDH-mut non-codel |
| CODEL | 1p/19q-codeleted | RT+PCV vs. RT+TMZ (TMZ alone closed) | Pending; TMZ alone inferior |
Radiation Therapy Technique
Dose and Fractionation
The standard radiation dose for LGG is 50.4 to 54 Gy delivered in 28 to 30 fractions at 1.8 Gy per fraction. Trials such as EORTC 22844 and NCCTG/RTOG/ECOG have shown no benefit to higher doses (59.4 to 64.8 Gy) compared to lower doses (45 to 50.4 Gy) in LGG. Higher doses increase neurotoxicity without improving outcomes. A dose of 45 Gy in 25 fractions is also acceptable based on data from the low-dose arms of these studies.
Target Volume Delineation
The gross tumor volume (GTV) is defined by the T2/FLAIR abnormality on MRI, as LGGs are typically non-enhancing. The clinical target volume (CTV) includes the GTV plus a 1 to 2 cm margin, respecting anatomic barriers such as the falx, tentorium, bone, and ventricles. The planning target volume (PTV) adds an additional 3 to 5 mm margin to the CTV. MRI fusion is mandatory for accurate delineation, with FLAIR sequences providing the best visualization of tumor extent. It is important not to include the entire T2 signal if it represents edema beyond the tumor; clinical judgment is required to distinguish tumor from edema.
Organs at Risk (OAR) Considerations for Long-Term Survivors
Given the long life expectancy of LGG patients, sparing critical structures is essential. Hippocampal sparing is employed to reduce radiation dose to the contralateral hippocampus and, when feasible, the ipsilateral hippocampus to preserve memory function. The optic pathways require strict adherence to dose constraints to prevent vision loss. The cochlea should receive a mean dose below 45 Gy to preserve hearing. Minimizing dose to the pituitary and hypothalamus reduces the risk of endocrine dysfunction. Intensity-modulated radiation therapy (IMRT) or volumetric modulated arc therapy (VMAT) is preferred over three-dimensional conformal radiation therapy (3D-CRT) for superior sparing of organs at risk.
<image>Axial T2/FLAIR MRI of a left frontal low-grade glioma showing a homogeneously hyperintense, non-enhancing mass. Overlaid contours demonstrate the GTV (red, following the FLAIR abnormality boundary), CTV (blue, 1.5 cm expansion respecting the falx and ventricular boundary), and PTV (green, 5 mm uniform expansion). The contralateral hippocampus is contoured separately as an avoidance structure (yellow).</image>
Risk-Stratified Treatment Approach
Low-Risk (Favorable)
Low-risk patients are typically younger than 40 years, have undergone gross total resection, harbor IDH-mutant and 1p/19q-codeleted tumors, present with non-enhancing, small tumors. These patients are managed with observation and serial MRI scans every 3 to 6 months initially, then annually. Radiation and chemotherapy are reserved for progression.
High-Risk
High-risk patients are aged 40 or older, have subtotal resection or biopsy only, large tumors, or neurological deficits. The standard treatment for these patients is radiation therapy at 50.4 to 54 Gy combined with chemotherapy, preferably six cycles of PCV. Temozolomide is an acceptable alternative. Treatment should commence within 3 to 12 weeks after surgery, with urgency depending on the clinical status.
<image>Kaplan-Meier overall survival curves from RTOG 9802 showing RT alone versus RT + PCV for high-risk low-grade glioma. The curves separate after 5 years and diverge dramatically by 10 years, with median OS of 13.3 years for RT + PCV versus 7.8 years for RT alone. The hazard ratio and p-value are annotated. A subgroup analysis panel shows the survival benefit is most pronounced in oligodendroglioma (1p/19q-codeleted) patients.</image>
Long-Term Neurocognitive Effects
Radiation-induced cognitive decline is a significant concern in LGG survivors. Risk factors include higher total radiation dose, larger irradiated volumes, older age, and concurrent chemotherapy. Cognitive domains affected include attention, processing speed, executive function, and memory. The hippocampal dose is a key determinant of memory decline. These effects may be progressive and can appear years after treatment. Cognitive rehabilitation, physical exercise, and potentially pharmacologic interventions such as memantine and donepezil may help mitigate cognitive decline. Proton therapy may reduce the integral brain dose and cognitive impact, although no randomized comparisons with photon therapy are available.
Key Clinical Pearls
Molecular classification has revolutionized the management of LGG, with IDH mutation status and 1p/19q codeletion now more important than histologic grade for prognosis and treatment selection. The term "low-grade" does not imply low priority; high-risk LGGs require definitive treatment with radiation and chemotherapy. The survival benefit of combined therapy demonstrated in RTOG 9802, with more than a five-year improvement in overall survival, is among the largest seen in neuro-oncology. Observation is appropriate after gross total resection in truly favorable patients who are young, IDH-mutant, codeleted, and have minimal residual disease, but this approach requires commitment to long-term MRI surveillance and patient education about the eventual need for treatment. Radiation dose considerations differ from glioblastoma; unlike GBM where 60 Gy is standard, LGG should not receive more than 50.4 to 54 Gy, as higher doses increase neurotoxicity without improving outcomes. Discussions about neurocognitive late effects and fertility preservation must occur before starting treatment in young LGG patients and should be addressed at diagnosis rather than as an afterthought. PCV remains the evidence-based chemotherapy regimen for LGG based on RTOG 9802, but temozolomide is widely substituted due to better tolerability. The ongoing CODEL trial will clarify whether temozolomide can effectively replace PCV in codeleted tumors.
References
- van den Bent MJ et al. "Long-term efficacy of early versus delayed radiotherapy for low-grade astrocytoma and oligodendroglioma in adults: the EORTC 22845 randomised trial." Lancet. 2005;366(9490):985-990.
- Buckner JC et al. "Radiation plus procarbazine, CCNU, and vincristine in low-grade glioma." N Engl J Med. 2016;374(14):1344-1355.
- Baumert BG et al. "Temozolomide chemotherapy versus radiotherapy in high-risk low-grade glioma (EORTC 22033-26033): a randomised, open-label, phase 3 intergroup study." Lancet Oncol. 2016;17(11):1521-1532.
- Shaw EG et al. "Prospective randomized trial of low- versus high-dose radiation therapy in adults with supratentorial low-grade glioma (NCCTG/RTOG/ECOG)." J Clin Oncol. 2002;20(9):2267-2276.
- Louis DN et al. "The 2021 WHO Classification of Tumors of the Central Nervous System." Neuro Oncol. 2021;23(8):1231-1251.
