Pulse‑level characterization of low monitor unit deliveries on a modern linear accelerator using a plastic scintillation detector

BACKGROUND: 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. PURPOSE: 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. METHODS: 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. RESULTS: 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 (< 0.33% COV). This enables highly reproducible readings (< 0.23% COV) for moderate to high MU deliveries. The variation in dose per pulse coupled with a lower number of required pulses leads to challenges when delivering a low number of MUs with FFF beams. Discrepancies in the planned versus delivered MUs were observed on both detectors and the treatment delivery system. CONCLUSIONS: Time-resolved analysis shows that dose per pulse fluctuations and the limited number of pulses comprising low MU FFF deliveries lead to measurable variability in delivered MU. These findings characterize the pulsed MU substructure on a modern linac, in which each MU is composed of a finite number of discrete radiation pulses, and quantify the achievable precision of fractional MU delivery, which is fundamentally limited by the dose per pulse (∼0.1 MU for TrueBeam FFF beams).