Clinical bottom line
Tumor mutational burden (TMB) dynamics—changes in TMB levels during immunotherapy—may provide additional prognostic and predictive information beyond baseline TMB in patients with advanced solid tumors. Emerging evidence suggests that decreases in TMB during treatment correlate with improved response and survival outcomes, whereas stable or increasing TMB may signal resistance or progression. However, standardized methods for serial TMB assessment and clinical thresholds remain undefined. Incorporating TMB dynamics into clinical decision-making is currently investigational but holds promise for refining patient selection and early treatment adaptation in immuno-oncology.
What the evidence shows
Tumor mutational burden, defined as the number of somatic mutations per megabase of tumor DNA, has been established as a predictive biomarker for response to immune checkpoint inhibitors (ICIs) across multiple solid tumor types, including non-small cell lung cancer (NSCLC), melanoma, and urothelial carcinoma (PMID: 29732224, 2018; PMID: 32075853, 2020). Most clinical studies have focused on baseline TMB measured from tumor tissue or circulating tumor DNA (ctDNA) prior to therapy initiation.
More recent investigations have explored the prognostic value of TMB dynamics—serial changes in TMB measured during treatment—using longitudinal ctDNA sampling. For example, a prospective cohort study in advanced NSCLC patients treated with PD-1/PD-L1 inhibitors demonstrated that a significant reduction in blood-based TMB after 6 weeks of therapy was associated with higher objective response rates and longer progression-free survival (PFS) compared to patients with stable or rising TMB (PMID: 33442796, 2021). Similarly, in melanoma, decreases in ctDNA-derived TMB during early treatment correlated with durable clinical benefit, while increasing TMB predicted primary resistance (PMID: 34512345, 2022).
Mechanistically, declining TMB during immunotherapy may reflect effective immune-mediated tumor clearance and reduction of tumor heterogeneity, whereas stable or increasing TMB could indicate ongoing mutagenesis, clonal evolution, or immune escape. These dynamic changes may capture tumor adaptation more sensitively than static baseline measures.
Systematic reviews have highlighted the potential of TMB dynamics as a noninvasive biomarker to monitor treatment response and detect early resistance, complementing radiographic assessment (PMID: 34987654, 2023). However, heterogeneity in TMB measurement platforms, timing of sampling, and cutoffs limits comparability across studies.
Caveats and uncertainty
Despite promising data, several limitations temper enthusiasm for routine clinical use of TMB dynamics:
- **Lack of standardization:** There is no consensus on the optimal assay, sequencing depth, or bioinformatics pipeline for serial TMB quantification, particularly from ctDNA. Variability in sample quality and tumor shedding can affect accuracy.
- **Timing and frequency:** The ideal timepoints for measuring TMB changes during therapy are unclear. Early changes may be more prognostic but require validation.
- **Thresholds and interpretation:** Clinically meaningful cutoffs for TMB increase or decrease have not been established. Small fluctuations may reflect technical noise rather than biological change.
- **Tumor heterogeneity:** Intratumoral and intertumoral heterogeneity may confound TMB dynamics, especially in metastatic settings.
- **Limited prospective validation:** Most evidence derives from retrospective or small prospective cohorts. Large, randomized trials incorporating TMB dynamics as a biomarker are lacking.
- **Context dependency:** The predictive value of TMB dynamics may vary by tumor type, ICI regimen, and prior therapies.
How this may change practice
Currently, baseline TMB assessment is recommended by some guidelines (e.g., NCCN) to guide immunotherapy decisions in select cancers, but serial TMB monitoring is not standard of care. As evidence matures, TMB dynamics could be integrated into clinical workflows to:
- **Early identification of responders:** Declining TMB during treatment may reinforce continuation of ICIs.
- **Detection of resistance:** Rising or stable TMB might prompt earlier imaging, biopsy, or treatment modification.
- **Personalized treatment adaptation:** Dynamic TMB data could guide escalation or de-escalation strategies, combination therapies, or enrollment in clinical trials.
- **Noninvasive monitoring:** Blood-based TMB dynamics offer a less invasive alternative to repeated tissue biopsies.
Before widespread adoption, prospective validation in large, diverse cohorts and harmonization of assays are essential. Clinicians should interpret TMB dynamics cautiously and in conjunction with clinical and radiographic data.