Discordant Dynamics Between Absolute ctDNA and Tumor Fraction During Treatment Monitoring in Metastatic Colorectal Cancer: Evidence of a Dilution Phenomenon.

INTRODUCTION

Circulating tumor DNA (ctDNA) is a promising biomarker for monitoring metastatic colorectal cancer. Tumor fraction, defined as the proportion of tumor-derived DNA within total circulating cell-free DNA (cfDNA), has been used as a quantitative measure of tumor-derived DNA burden. However, because tumor fraction is a relative metric, its relationship with absolute ctDNA dynamics remains unclear.

PATIENTS AND

METHODS

We conducted a retrospective longitudinal study of patients with metastatic colorectal cancer harboring RAS or BRAF mutations who underwent systemic therapy with serial ctDNA monitoring. Plasma samples were analyzed using droplet digital PCR to quantify absolute ctDNA, expressed as mutant copies/mL of plasma, and tumor fraction. Longitudinal dynamics were assessed by comparing each sample with the preceding measurement. Dilution was defined as increased absolute ctDNA, with fold change ≥ 1.5 and increase ≥ 313 mutant copies/mL of plasma, accompanied by decreased tumor fraction.

RESULTS

Fifty-five patients with 246 evaluable plasma samples were included. Among 120 samples with preceding measurements, 7 dilution events were identified in 5 patients, occurring predominantly during progressive disease. Although absolute ctDNA and tumor fraction increased concordantly at the population level during progression, individual-level analyses revealed discordant dynamics in which absolute ctDNA increased while tumor fraction decreased. Higher absolute ctDNA levels were associated with dilution in mixed-effects logistic regression analysis (odds ratio, 3.25; 95% CI, 1.16-9.15; P = 0.025).

CONCLUSION

Tumor fraction may decrease despite increasing tumor-derived ctDNA during treatment monitoring. Simultaneous evaluation of absolute ctDNA and tumor fraction may improve interpretation of ctDNA dynamics in metastatic colorectal cancer.