Quantitative stability of MGMT promoter methylation in recurrent glioma and implications for temozolomide rechallenge.

PURPOSE: O^6^-methylguanine-DNA methyltransferase (MGMT) promoter methylation is a key predictive biomarker for temozolomide (TMZ) response in glioma. MGMT status is routinely assessed at diagnosis; however, its utility at recurrence remains incompletely characterized. We aimed to quantify MGMT methylation stability and assess the clinical relevance of reported status changes in recurrent glioma. METHODS: We analyzed paired tumor samples from 426 patients with recurrent glioma in a large commercial molecular database containing quantitative methylation percentage and binary methylation status at two sample time points. A separate multi-institutional clinical cohort (n = 27) with detailed treatment annotation was analyzed to explore associations between MGMT status at recurrence and treatment decision-making. Statistical analyses assessed quantitative methylation dynamics and time between samples as a surrogate for disease course. RESULTS: MGMT status remained unchanged in 84% of tumors across recurrence. 16% demonstrated a status change, commonly among tumors with low baseline methylation near classification thresholds. Longitudinal quantitative methylation changes were modest overall. Tumors that remained, gained, or lost hypermethylation demonstrated longer intervals between samples compared to consistently unmethylated tumors (p < 0.01). In the clinical cohort, TMZ rechallenge was more frequently pursued in patients retaining hypermethylation at recurrence. CONCLUSION: MGMT promoter methylation is quantitatively stable in the majority of recurrent gliomas. Status changes predominantly occur in tumors with borderline methylation, highlighting the limitations of binary classification. These findings support routine reassessment of MGMT at recurrence and underscore the value of quantitative reporting to better inform therapeutic decision-making, particularly regarding TMZ rechallenge.