Publication date: Jul 01, 2026
Variability in low monitor unit (MU) deliveries has been previously documented, but the underlying pulse-level behavior has not been fully described due to the integrating nature of most conventional detectors. A plastic scintillation detector (PSD) enables direct visualization of MU microstructure by isolating individual linac pulses. To use a high temporal resolution PSD to characterize pulse-level MU substructure on a modern linac and quantify how variation in integral pulse magnitude and pulse count influence the reproducibility of low MU and fractional MU deliveries. A Blue Physics Model 11 PSD was used to measure response from individual linac pulses of a Varian TrueBeam operating at energies of 6 MV, 6 MV FFF, 10 MV, and 10 MV FFF. Initial measurements benchmarked reproducibility in pulse count and dose per pulse by delivering 100 MU at a constant dose rate of 400 MU/min. Subsequent measurements focused on low MU cases, delivering 1-3 MU at dose rates of 5-2400 MU/min. The dose per pulse is variable during a single pulse train with a measured coefficient of variation (COV) of up to 13. 3%. However, the average dose per pulse across multiple deliveries is stable (
