Characterization of the effect of a new commercial transmission detector on radiation therapy beams

Purpose: To evaluate the influence of a new commercial transmission detector on radiation therapy beams. Methods and materials: A transmission detector designed for online treatment monitoring was characterized on a TrueBeam STx linear accelerator with 6-MV, 6-flattening filter free, 10-MV, and 10-flattening filter free beams. Measurements of percentage depth doses, in-plane and cross-plane off-axis profiles at different depths, transmission factors, and skin dose were acquired with 3 x 3, 5 x 5, 10 x 10, 20 x 20, and 40 x 40 cm(2) field sizes at 100 cm and 80 cm source-to-surface distance (SSD). A CC04 chamber was used for all profile and transmission factor measurements. Skin dose was assessed at 100, 90, and 80 cm SSD using a variety of detectors (Roos and Markus parallel-plate chambers and optically stimulated luminescent dosimeters [OSLDs]). Skin dose was also assessed for various patient sample plans with OSLDs. Results: The percentage depth doses showed small differences between the unperturbed and perturbed beams for 100 cm SSD (<= 4 mm depth of maximum dose difference, <1.2% average profile difference) for all field sizes. At 80 cm SSD, the differences were larger (<= 8 mm depth of maximum dose difference, <3% average profile difference). The differences were larger for the flattened beams and larger field sizes. The off-axis profiles showed similar trends. Field penumbras looked similar with and without the transmission detector. Comparisons in the profile central 80% showed a maximum average (maximum) profile difference between all field sizes of 1.0% (2.6%) and 1.4% (6.3%) for 100 and 80 cm SSD, respectively. The average measured skin dose increase at 100 cm (80 cm) SSD for a 10 x 10 cm2 field size was <4% (<35%) for all energies. For a 40 x 40 cm(2) field size, this increased to <31%(<= 63%). For the sample patient plans, the average skin dose difference was 0.53% (range, -6.6% to 10.4%). Conclusions: The transmission detector has minimal effect on clinically relevant radiation therapy beams for intensity modulated radiation therapy and volumetric arc therapy (field sizes 10 x 10 cm(2) and less). For larger field sizes, some perturbations are observable that would need to be assessed for clinical impact. (C) 2017 American Society for Radiation Oncology. Published by Elsevier Inc. All rights reserved.