Introduction: The Growing Challenge of Intracranial Control in Non-Small Cell Lung Cancer
Non-small cell lung cancer (NSCLC) remains a leading cause of cancer-related mortality worldwide, with a significant propensity for central nervous system (CNS) involvement. Approximately 25% of patients present with brain metastases at the time of diagnosis, and nearly 50% will develop them during the course of their illness. As systemic therapies—including targeted agents and immune checkpoint inhibitors (ICIs)—continue to improve extracranial survival, the brain has increasingly become a sanctuary site for disease progression, necessitating more effective and less toxic intracranial management strategies.
Historically, whole-brain radiation therapy (WBRT) was the standard for multiple metastases, but its association with significant neurocognitive decline led to a paradigm shift toward stereotactic radiosurgery (SRS). While SRS provides excellent local control for treated lesions, it does not address the risk of distant intracranial failure (the development of new lesions elsewhere in the brain). Consequently, there is an urgent clinical need for regional therapies that can extend the time to intracranial progression (TTIP) without the debilitating side effects of WBRT.
The METIS Trial: A New Frontier in Regional Brain Therapy
The METIS trial (NCT02831959) was designed to evaluate whether Tumor Treating Fields (TTFields) could fill this therapeutic gap. TTFields are low-intensity, intermediate-frequency (150 kHz for NSCLC) alternating electric fields delivered non-invasively via transducer arrays applied to the scalp. Unlike ionizing radiation or cytotoxic chemotherapy, TTFields exert their effects by disrupting the dipole moments of highly polar proteins—specifically tubulin and septins—during mitosis. This disruption leads to mitotic spindle misalignment, chromosomal non-disjunction, and eventual cell death, specifically targeting rapidly dividing cancer cells while sparing quiescent healthy neurons.
Study Methodology and Patient Population
This international, multicenter, randomized phase 3 trial enrolled 298 adults with 1 to 10 newly diagnosed NSCLC brain metastases. All patients were suitable for SRS and were receiving optimal therapy for their extracranial disease. Participants were randomized 1:1 to receive either SRS followed by TTFields (150 kHz) or SRS alone. The TTFields therapy was initiated within 21 days of SRS, with a recommended usage of at least 18 hours per day.
The primary endpoint was time to intracranial progression (TTIP), defined according to the Response Assessment in Neuro-Oncology Brain Metastases (RANO-BM) criteria. This was assessed by an independent radiology review committee to ensure objectivity. Secondary endpoints included overall survival (OS), time to distant intracranial progression, neurocognitive function (using the HVLT-R, TMT, and COWA tests), quality of life (QoL), and safety.
Primary Efficacy Outcomes: A Significant Delay in Progression
The final results of the METIS study demonstrate a statistically significant benefit for the addition of TTFields to the post-SRS treatment regimen. With a median follow-up of 8.6 months, the addition of TTFields significantly delayed TTIP compared to SRS alone. The hazard ratio (HR) for progression was 0.72 (95% CI, 0.53-0.98; Fine-Gray P = .044), representing a 28% reduction in the risk of intracranial progression.
Detailed longitudinal analysis revealed consistent benefits at multiple time points. The intracranial progression rates for the TTFields group versus the SRS-alone group were 13.6% vs. 22.1% at 2 months (P = .034), 33.7% vs. 46.4% at 6 months (P = .018), and 46.9% vs. 59.4% at 12 months (P = .023). By the 24-month mark, the progression rate remained lower in the TTFields arm (53.6% vs. 65.2%; P = .031). These data suggest that the regional control offered by TTFields is both early-acting and durable.
Synergy with Immune Checkpoint Inhibitors
One of the most clinically provocative findings from the METIS trial was the subgroup analysis of patients receiving immune checkpoint inhibitors (n = 118). In this cohort, the delay in TTIP was even more pronounced, with an HR of 0.63 (95% CI, 0.39-1.0; Cox P = .049). Furthermore, the time to distant intracranial progression in the ICI subgroup showed a remarkable improvement with TTFields (HR, 0.41; 95% CI, 0.21-0.81; log-rank P = .0087).
This observation aligns with emerging preclinical evidence suggesting that TTFields may enhance the anti-tumor immune response. By inducing immunogenic cell death and potentially increasing the permeability of the blood-brain barrier, TTFields may work synergistically with ICIs to overcome the immune-suppressive environment of the CNS. For clinicians, this suggests that TTFields are not only compatible with modern systemic standards of care but may actually augment their efficacy.
Safety, Neurocognition, and Quality of Life
A critical concern in any brain-directed therapy is the preservation of cognitive function and quality of life. The METIS trial results are highly reassuring in this regard. There was no significant difference between the two arms in terms of neurocognitive decline, suggesting that TTFields do not impose the same neurotoxic burden as WBRT.
Regarding safety, the majority of device-related adverse events were grade 1 or 2 skin reactions (dermatitis) beneath the transducer arrays, which were manageable with topical treatments or temporary pauses in therapy. No systemic toxicities were attributed to the device.
Importantly, TTFields did not cause a deterioration in quality of life. In fact, post hoc analyses indicated improvements in deterioration-free survival and time to deterioration in several key domains, including global health status, physical functioning, and fatigue. This indicates that by delaying the symptomatic progression of brain metastases, TTFields help maintain a higher functional status for a longer duration.
Expert Commentary: Mechanistic Insights and Clinical Implementation
The success of the METIS trial marks the third positive phase 3 study for TTFields in oncology, following the EF-14 trial in glioblastoma and the LUNAR trial in metastatic NSCLC. The ability of 150 kHz fields to specifically inhibit the progression of NSCLC cells in the brain provides a localized, non-toxic bridge between focal SRS and systemic therapy.
From a clinical standpoint, TTFields represent a “regional” approach. While SRS targets visible lesions, TTFields treat the entire supratentorial volume, potentially suppressing micrometastases that are below the threshold of MRI detection. The lack of overlap in toxicity profiles with radiation or chemotherapy makes it an attractive add-on therapy. However, the requirement for patients to wear the device for 18 hours a day requires significant patient education and commitment. The QoL data from METIS suggest that patients who adhere to the therapy find the trade-off worthwhile to maintain intracranial control.
Conclusion: A New Pillar in Multimodal Care
The final results of the METIS study establish TTFields as a valid and effective treatment option for patients with NSCLC brain metastases following SRS. By significantly prolonging the time to intracranial progression without worsening cognitive function or quality of life, TTFields address a critical unmet need in the management of CNS disease. Particularly for patients on immune checkpoint inhibitors, the addition of TTFields may offer a potent synergistic effect that redefines the standard of care for brain metastasis management.
Funding and Clinical Trial Information
The METIS study was funded by Novocure. ClinicalTrials.gov Identifier: NCT02831959.
