Improvements in dose accuracy delivered with static-MLC IMRT on an integrated linear accelerator control system

• Published in: Medical physics (Lancaster) 2012-05, Vol.39 (5), p.2456-2462
• Main Authors: Li, Ji, Wiersma, Rodney D., Stepaniak, Christopher J., Farrey, Karl J., Al-Hallaq, Hania A.
• Format: Article
• Language: English
• Subjects: Accelerators; ACCURACY; COLLIMATORS; CONTROL SYSTEMS; DELIVERY; Devices sensitive to very short wavelength, e.g. x‐rays, gamma‐rays or corpuscular radiation; dose rate dependence; DOSE RATES; Dose‐volume analysis; DOSIMETRY; Dosimetry/exposure assessment; Drug delivery; Electric measurements; FILTERS; Humans; Intensity modulated radiation therapy; intensity modulated radiotherapy; ionisation chambers; IONIZATION CHAMBERS; LINEAR ACCELERATORS; Multileaf collimators; over shoot; Particle Accelerators; phantoms; PHOTON BEAMS; Photons; Radiation Dosage; RADIATION DOSES; Radiation monitoring; RADIATION PROTECTION AND DOSIMETRY; radiation therapy; RADIOLOGY AND NUCLEAR MEDICINE; Radiometry; RADIOTHERAPY; Radiotherapy Dosage; Radiotherapy, Intensity-Modulated - instrumentation; step-and-shoot; Transforming x‐rays; TrueBeam; X‐ray technique
• ISSN: 0094-2405; 2473-4209
• DOI: 10.1118/1.3701778

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 (Δ i ) were compared using F-tests (α = 0.05). Results: On the Trilogy, large Δ i was observed in small MU segments: at 1 MU/segment, the maximum Δ i was 10.1% and 57.9% at 100 MU/min and 600 MU/min, respectively. Also, the first segment of each sequence consistently overshot (Δ i  > 0), while the last segment consistently undershot (Δ i