Residency · Residency · Radiation Oncology
Primary CNS Lymphoma: Whole Brain Radiation and Its Declining Role
Introduction
Primary CNS lymphoma (PCNSL) is a rare and aggressive form of extranodal non-Hodgkin lymphoma that is confined to the brain, spinal cord, leptomeninges, or eyes. The vast majority of PCNSL cases are diffuse large B-cell lymphoma (DLBCL). Historically, whole brain radiation therapy (WBRT) was the mainstay of treatment for PCNSL. However, its role has significantly declined due to the severe neurocognitive toxicity associated with WBRT and the demonstrated efficacy of high-dose methotrexate (HD-MTX)-based chemotherapy regimens.
Epidemiology and Presentation
PCNSL occurs at an incidence of approximately 0.5 cases per 100,000 person-years, with a median age at diagnosis of 65 years. The incidence is rising particularly among immunocompetent elderly individuals. Patients typically present with symptoms such as cognitive decline, focal neurological deficits, headaches, and personality changes. Multifocal disease is observed in 30-50% of cases, with a predilection for the periventricular regions and deep white matter. Diagnosis requires a stereotactic biopsy, and it is important to avoid corticosteroids before biopsy when possible because steroids can cause a "vanishing tumor" effect, complicating diagnosis.
Historical Role of WBRT
WBRT as Sole Treatment
Before the advent of effective chemotherapy, WBRT was the primary treatment modality for PCNSL. Radiation doses typically ranged from 36 to 45 Gy. Despite treatment, the median overall survival with WBRT alone was only 12 to 18 months. Recurrence was nearly universal within the irradiated volume. Moreover, WBRT was associated with severe delayed neurotoxicity, manifesting as leukoencephalopathy, dementia, ataxia, and incontinence. This neurotoxicity was particularly devastating in patients older than 60 years.
Dose and Fractionation (Historical)
Historically, WBRT was delivered to the whole brain at doses of 36 to 40 Gy, fractionated in 1.8 to 2.0 Gy increments. A boost of 10 to 14 Gy was often delivered to the tumor bed, resulting in a total tumor dose of 45 to 50 Gy.
High-Dose Methotrexate-Based Chemotherapy
Current Standard
High-dose methotrexate (HD-MTX), administered at doses of 3.5 g/m^2 or higher, forms the backbone of current PCNSL treatment. HD-MTX is effective because it crosses the blood-brain barrier at high concentrations. It is commonly combined with agents such as rituximab, procarbazine, and vincristine (the R-MPV regimen) or other chemotherapy combinations. These regimens achieve complete response rates between 50% and 80%, with median overall survival ranging from 30 to 50 months when chemotherapy is used alone.
IELSG-32 Trial
The IELSG-32 trial randomized patients to receive HD-MTX with cytarabine alone, HD-MTX with cytarabine plus rituximab, or HD-MTX with cytarabine, rituximab, and thiotepa (the MATRix regimen). The MATRix regimen achieved the highest complete response rate of 49% and improved progression-free survival, establishing it as the standard induction therapy in fit patients.
Consolidative WBRT vs. Alternatives
WBRT as Consolidation After Chemotherapy
WBRT was historically used as consolidation therapy after HD-MTX-based chemotherapy to solidify treatment response. The RTOG 93-10 trial demonstrated that combining HD-MTX with WBRT at 45 Gy resulted in a median overall survival of 36 months. However, 15% of patients experienced severe neurotoxicity, with neurocognitive decline being the major limitation, especially in elderly patients.
G-PCNSL-SG-1 Trial
The G-PCNSL-SG-1 trial randomized PCNSL patients to receive HD-MTX-based chemotherapy with or without WBRT at 45 Gy. The trial found no significant difference in overall survival between the two groups, which was the primary endpoint. Although WBRT improved progression-free survival, this benefit came at the cost of significant neurotoxicity. This landmark trial supported the omission of WBRT in patients who responded to chemotherapy.
Consolidative Autologous Stem Cell Transplant
An alternative consolidation strategy involves thiotepa-based high-dose chemotherapy followed by autologous stem cell transplant (ASCT). In the second part of the IELSG-32 trial, ASCT was compared with WBRT at 36 Gy as consolidation after MATRix induction. ASCT demonstrated equivalent progression-free and overall survival outcomes but with less neurotoxicity. Consequently, ASCT is increasingly preferred as consolidation in fit patients younger than 65 to 70 years.
Reduced-Dose WBRT
Reduced-dose WBRT, typically 23.4 Gy, is used following complete response to chemotherapy. The RTOG 1114 trial showed that reduced-dose WBRT in patients achieving complete response preserved neurocognitive function better than historical full-dose WBRT. This approach represents a compromise for patients who are ineligible for ASCT. Some centers also deliver a 12 to 14 Gy boost to the tumor bed in addition to the reduced-dose WBRT.
Current Treatment Paradigm
Fit Patients (Age < 65-70)
For fit patients under 65 to 70 years of age, the treatment paradigm begins with induction chemotherapy using MATRix or R-MPV regimens administered over 4 to 6 cycles. Response is assessed with contrast-enhanced MRI. Consolidation is preferably performed with ASCT. For patients achieving complete response who are not candidates for ASCT, reduced-dose WBRT at 23.4 Gy is an alternative. Full-dose WBRT is reserved for patients with partial response or those ineligible for ASCT.
Elderly or Unfit Patients
In elderly or unfit patients, HD-MTX-based chemotherapy is administered with dose adjustments as needed. If a complete response is achieved, observation is preferred to avoid WBRT due to its unacceptable neurotoxicity risk in this population. WBRT at 23.4 Gy or lower is considered only if chemotherapy fails, and palliative WBRT may be used for refractory disease.
Relapsed/Refractory Disease
For relapsed or refractory PCNSL, re-challenge with HD-MTX is considered if the initial response was durable. Salvage chemotherapy regimens such as temozolomide or ibrutinib may be used. WBRT can be employed if the patient has not previously received radiation. Prognosis in this setting remains poor, with median survival of only 2 to 4 months without effective salvage therapy.
WBRT Technique When Indicated
Target Volume
When WBRT is indicated, the target volume includes the entire brain parenchyma with coverage of the meninges. The inferior border extends to the C2 vertebral body, ensuring inclusion of the cribriform plate and temporal lobes. Treatment is typically delivered using opposed lateral fields or intensity-modulated radiation therapy (IMRT) to allow for hippocampal sparing.
Hippocampal-Sparing WBRT
Hippocampal-sparing WBRT is under investigation for PCNSL but is not currently standard practice. PCNSL tends to involve periventricular regions, including the hippocampi, raising concerns that hippocampal avoidance could result in undertreatment of disease in these zones. Therefore, hippocampal-sparing WBRT is not recommended outside of clinical trials for PCNSL.
Key Clinical Pearls
WBRT as a sole treatment for PCNSL results in a median survival of only 12 to 18 months and is associated with unacceptable neurotoxicity, especially in elderly patients. High-dose methotrexate-based chemotherapy has become the cornerstone of PCNSL treatment, with autologous stem cell transplant replacing WBRT as the preferred consolidation strategy in fit patients. The G-PCNSL-SG-1 trial demonstrated that WBRT can be safely omitted after chemotherapy without compromising overall survival. Reduced-dose WBRT at 23.4 Gy remains an option for patients who achieve complete response but are ineligible for ASCT. It is critical to avoid WBRT in elderly patients over 60 to 65 years whenever possible due to the devastating risk of leukoencephalopathy.
References
- Thiel E, Korfel A, Martus P, et al. High-dose methotrexate with or without whole brain radiotherapy for primary CNS lymphoma (G-PCNSL-SG-1): a phase 3, randomised, non-inferiority trial. Lancet Oncol. 2010;11(11):1036-1047.
- Ferreri AJM, Cwynarski K, Pulczynski E, et al. Whole-brain radiotherapy or autologous stem-cell transplantation as consolidation strategies after high-dose methotrexate-based chemoimmunotherapy in patients with primary CNS lymphoma: results of the second randomisation of the International Extranodal Lymphoma Study Group-32 phase 2 trial. Lancet Haematol. 2017;4(11):e510-e523.
- Morris PG, Correa DD, Yahalom J, et al. Rituximab, methotrexate, procarbazine, and vincristine followed by consolidation reduced-dose whole-brain radiotherapy and cytarabine in newly diagnosed primary CNS lymphoma: final results and long-term outcome. J Clin Oncol. 2013;31(31):3971-3979.
- Houillier C, Taillandier L, Dureau S, et al. Radiotherapy or autologous stem-cell transplantation for primary CNS lymphoma in patients 60 years of age and younger: results of the intergroup ANOCEF-GOELAMS randomized phase II PRECIS study. J Clin Oncol. 2019;37(10):823-833.