KEY POINTS
- This technical study used a high-temporal-resolution plastic scintillation detector to characterize individual pulses from a Varian TrueBeam delivering 6 megavolt, 6 megavolt flattening filter-free, 10 megavolt, and 10 megavolt flattening filter-free photon beams.
- Across 100-monitor-unit deliveries, total detector response was highly reproducible, with coefficients of variation below 0.23%. Mean dose per pulse was also stable between repeated deliveries, with coefficients of variation below 0.33%.
- Within individual pulse trains, dose per pulse varied substantially, reaching a coefficient of variation of 13.3%. This variability had little effect at higher monitor units because flattened-beam deliveries contained more than 3,400 pulses.
- For low monitor unit flattening filter-free deliveries, precision was constrained by the small number of discrete pulses. One monitor unit of 10 megavolt flattening filter-free radiation comprised approximately eight pulses, and occasional additional pulses produced recorded deliveries of 1.1, 2.1, or 3.1 monitor units.
- The overshoot pattern was independent of field size and gantry angle. Even for the highest-dose-per-pulse clinical beam, one additional pulse contributed less than 2 milligray, limiting its immediate clinical significance.
CLINICAL TAKEAWAY
Low monitor unit flattening filter-free delivery is fundamentally quantized by the finite number of linac pulses composing each monitor unit. The measured dose effect was small for conventional photon treatments, but the findings are relevant to small-segment quality assurance, fractional monitor unit commissioning, and future high-dose-per-pulse systems; validation was limited to one TrueBeam platform.