Which pathology findings (e.g., microsatellite instability‑high, loss of mismatch repair proteins confirming mismatch repair deficiency, or high tumor mutational burden) are required for a patient with IDH‑wildtype glioblastoma to qualify for checkpoint inhibitor immunotherapy?

Pathology Requirements for Immunotherapy Eligibility in IDH-Wildtype Glioblastoma

For patients with IDH-wildtype glioblastoma, testing for mismatch repair (MMR) deficiency or high tumor mutational burden (TMB-H) is only recommended in specific clinical contexts, but even when present, these findings do NOT justify routine immunotherapy outside of clinical trials, as prospective data show poor response rates. 1

When to Test for MMR Deficiency/TMB-H

The 2025 EANO guidelines specify testing should be considered in newly diagnosed tumors when patients meet ANY of these criteria 1:

• Age <50 years
• Unusual histological features:
  • High-grade glioma IDH-wildtype with ATRX loss
  • Severe pleomorphism and/or giant cell features
  • Tumors not fitting classic molecular subtypes
• DNA methylation patterns suggestive of MMR deficiency (PMMRDIA, "Diffuse pediatric-type high-grade glioma, RTK1 subtype, subclass A," or "Adult-type diffuse high-grade glioma, IDH-wildtype, subtype E")
• Personal or family history suggesting germline DNA polymerase or MMR deficiency

Critical Caveat: Testing Does NOT Equal Treatment Eligibility

Even when MMR deficiency or TMB-H is identified, checkpoint inhibitor immunotherapy should preferably be given only within prospective registries or clinical trials (ESCAT IIIA evidence level), despite tumor-agnostic FDA approval for TMB-high tumors in some countries. 1 This recommendation stems from prospective and retrospective cohorts showing no significant response rates in adult glioblastoma patients with de novo TMB-H or MMR deficiency treated with immune checkpoint blockade.

Recent 2026 research reinforces this caution, demonstrating that MMR deficiency in glioblastoma promotes immune suppression through a myeloid-dominant, T cell-suppressed tumor microenvironment rather than creating tumor immunogenicity 2. This fundamentally differs from other solid tumors where MMR deficiency predicts immunotherapy response.

Post-Treatment (Acquired) TMB-H

For recurrent glioblastoma after alkylating agents, testing for TMB/MMR deficiency is relevant only in these specific contexts 1:

• Available clinical trials for patients with:
  • IDH-mutant gliomas
  • MGMT promoter-methylated IDH-wildtype glioblastoma
  • Patients who initially responded to alkylating agents

The current evidence does NOT justify performing a biopsy solely to obtain tissue for TMB/MMR analysis (ESCAT IIIB evidence level for post-treatment TMB-H). 1

Testing Methods

When testing is indicated, options include 1:

• Immunohistochemistry (IHC) for MMR proteins (MLH1, MSH2, MSH6, PMS2)
• Next-generation sequencing (NGS) for TMB quantification
• Microsatellite instability testing

IHC can serve as a prescreening strategy before genomic confirmation if required for trial participation.

Bottom Line Algorithm

1. Do NOT routinely test all IDH-wildtype glioblastomas for MMR/TMB-H
2. Test only if: Age <50 years, unusual histology, suggestive methylation pattern, or family history
3. If MMR deficiency/TMB-H found: Enroll in clinical trial or prospective registry—do NOT use checkpoint inhibitors as standard therapy
4. At recurrence: Test only if clinical trial available AND patient meets specific criteria (IDH-mutant, MGMT-methylated, or prior alkylator response)

The key distinction from other cancers: MMR deficiency in glioblastoma drives immune suppression, not immunogenicity, fundamentally changing the therapeutic implications. 2