Early Dynamics of Circulating Tumor DNA Following Curative Hypofractionated Radiotherapy Related to Disease Control in Lung Cancer

<b>Background/objectives:</b> We aimed to characterize the dynamic pattern of circulating tumor DNA (ctDNA) during hypofractionated radiation therapy (RT) in patients with lung cancer and assess its clinical relevance. <b>Metholds:</b> Prospectively, 24 patients diagnosed with early-stage lung cancer underwent curative RT with 60-64 Gy in 4-20 fractions. Blood samples were collected at baseline (D0) and on post-RT days 1-3 and 7 (D1-3 and D7). The ctDNA was longitudinally analyzed using LiquidSCAN. To find a feasible index associated with outcome, total VAF(%), max VAF(%), total GE (hGE/mL) and max GE (hGE/mL), were evaluated. <b>Results:</b> Thirteen patients with available samples were analyzed with a median 22.2-month follow-up (range, 5.2-34.3 months). Four patients experienced progression between 7.9 and 16.6 months after RT (PD group), and the nine presented no evidence of disease (NED group). The Dmax, the day with the highest ctDNA level among D0-7, was significantly different between the groups with total GE and max GE (<i>p</i> = 0.035 and 0.021, respectively). According to the ROC curves, the max GE showed the best AUC (86.1%) and the cut-off value of the Dmax was 1.5 (sensitivity: 66.7%, specificity: 100%, positive-predictive value: 100%, and negative-predictive value: 57.1%). Tumor size ≥ 3 cm, squamous histology, and a daily dose 3-4 Gy were correlated with the Dmax = D2-3. The Dmax showed better disease control rate with marginal significance (<i>p</i> = 0.081). <b>Conclusions:</b> The timing of early ctDNA elevation may have the potential to predict RT response. The max GE may be an index to verify the ctDNA levels after RT.