The first blood sample was collected at the time of diagnosis before the start of treatment. Circulating free DNA (cfDNA) was quantified using the QX200 Droplet Digital PCR System (Bio-Rad Laboratories, Hercules, CA). For the EGFR multiplex assays, a final PCR mix volume of 20 μL was manually loaded into wells of a DG8 cartridge (Bio-Rad Laboratories) with 70 μL of Droplet Generation Oil for probes (Bio-Rad Laboratories). After droplet generation by the QX200 Droplet Generator (Bio-Rad Laboratories), 40 μL of the sample was transferred into a 96-well PCR plate and amplified with a C1000 Touch Thermal Cycler (Bio-Rad Laboratories), using the following thermal cycling conditions: 95 °C for 10 min, 40 cycles at 94 °C for 30 s, 55 °C for 1 min (2 °C/s), and 98 °C for 10 min, and a final cooling step to 12 °C (1 °C/s). The droplets were analyzed using the QX200 Droplet Reader (Bio-Rad Laboratories), and data were analyzed using QuantaSoft software version 1.7.4.0917 and QuantaSoft Analysis Pro software version 1.0.596 (Bio-Rad Laboratories). Thresholds were placed manually, and the fractions of positive and negative droplets were used to calculate the concentration and fractional abundance of target DNA sequences with their 95% Poisson-based confidence intervals (CI). EGFRm primers and probes were based on previous studies [27 (link), 28 (link)].