Abstract
Purpose: To investigate the impact of monitor units (MUs) per segment and dose rate on the dose delivery accuracy of step‐and‐shoot intensity modulated radiation (SS‐IMRT) fields on a TrueBeam LINAC.Methods: A step‐and‐shoot multi‐leaf collimator (MLC) sequence was created consisting of 10 identical 10×10cm segments with identical MUs. Beam holding between segments was achieved by moving one out‐of‐field MLC leaf pair. Measurements were repeated for various combinations of MU/segment ranging from 2–40 and dose rates of either 300 or 600MU/min. All measurements were made with a Farmer (0.6cc) ionization chamber placed at the isocenter in a SolidWater phantom at 10cm depth. The measurements were performed on two Varian LINACs: Trilogy and TrueBeam. Each sequence was delivered three times and the charge readings for the corresponding segments were averaged. The effects of MU/segment, dose rate, and LINAC type on the relative dose variation (Delta_i) was compared using F‐tests.Results: On the Trilogy, large Delta_i was observed in small MU segments: at 2MU/segment, the maximum Delta_i was 20.0%/26.2% at 300/600MU/min, respectively. Also, the first segment of each sequence consistently over‐shot(Delta_i>0), while the last segment consistently under‐shot(Delta_i<0). On the TrueBeam, at 2MU/segment, Delta_i ranged from −1.6% to +2.0% at 300 MU/min and −3.0% to +3.1% at 600MU/min; no obvious over/under‐shoot trend was observed. F‐tests showed statistically significant difference ((1—beta)=1.0000) in 2 MU segments between the Trilogy and the TrueBeam, at either dose rate. No significant difference was observed for large (>10 MU/segment) MU segments Conclusions: Earlier generations of Varian LINACs exhibited large dose variations for small MU segments in SS‐IMRT delivery. Our results appeared to confirm these findings. The dose delivery accuracy in small MU segments and at high dose rate for SS‐IMRT is significantly improved on TrueBeam compared to Trilogy, likely due to the faster sampling rate (100Hz vs. 20Hz).
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