Improvements in dose accuracy delivered with static‐MLC IMRT on an integrated linear accelerator control system
Authors: Li Ji , Wiersma Rodney D. , Stepaniak Christopher J. , Farrey Karl J. , Al‐Hallaq Hania A. 
Published: April 2012

Purpose:

Dose accuracy has been shown to vary with dose per segment and dose rate when delivered with static multileaf collimator (SMLC) intensity modulated radiation therapy (IMRT) by Varian C‐series MLC controllers. The authors investigated the impact of monitor units (MUs) per segment and dose rate on the dose delivery accuracy of SMLC‐IMRT fields on a Varian TrueBeam linear accelerator (LINAC), which delivers dose and manages motion of all components using a single integrated controller.

Methods:

An SMLC sequence was created consisting of ten identical 10 × 10 cm2 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 1 to 40 and dose rates of 100–600 MU/min for a 6 MV photon beam (6X) and dose rates of 800–2400 MU/min for a 10 MV flattening‐filter free photon (10XFFF) beam. All measurements were made with a Farmer (0.6 cm3) ionization chamber placed at the isocenter in a solid–water phantom at 10 cm depth. The measurements were performed on two Varian LINACs: C‐series Trilogy and TrueBeam. Each sequence was delivered three times and the dose readings for the corresponding segments were averaged. The effects of MU/segment, dose rate, and LINAC type on the relative dose variation (Δi) were compared using F‐tests (α = 0.05).

Results:

On the Trilogy, large Δi was observed in small MU segments: at 1 MU/segment, the maximum Δi was 10.1% and 57.9% at 100 MU/min and 600 MU/min, respectively. Also, the first segment of each sequence consistently overshot (Δi > 0), while the last segment consistently undershot (Δi < 0). On the TrueBeam, at 1 MU/segment, Δi ranged from 3.0% to 4.5% at 100 and 600 MU/min; no obvious overshoot/undershoot trend was observed. F‐tests showed statistically significant difference [(1 − β) =1.0000] between the Trilogy and the TrueBeam up to 10 MU/segment, at all dose rates greater than 100 MU/min. The linear trend of decreasing dose accuracy as a function of increasing dose rate on the Trilogy is no longer apparent on TrueBeam, even for dose rates as high as 2400 MU/min. Dose inaccuracy averaged over all ten segments in each beam delivery sequence was larger for Trilogy than TrueBeam, with the largest discrepancy (0.2% vs 3%) occurring for 1 MU/segment beams at both 300 and 600 MU/min.

Conclusions:

Earlier generations of Varian LINACs exhibited large dose variations for small MU segments in SMLC‐IMRT delivery. Our results confirmed these findings. The dose delivery accuracy for SMLC‐IMRT is significantly improved on TrueBeam compared to Trilogy for every combination of low MU/segment (1–10) and high dose rate (200–600 MU/min), in part due to the faster sampling rate (100 vs 20 Hz) and enhanced electronic integration of the MLC controller with the LINAC. SMLC‐IMRT can be implemented on TrueBeam with higher dose accuracy per beam (±0.2% vs ±3%) than previous generations of Varian C‐series LINACs for 1 MU/segment delivered at 600 MU/min).