Characterization of the effect of a new commercial transmission detector on radiotherapy beams

Publication date: Available online 9 April 2017
Source:Practical Radiation Oncology
Author(s): Joey P. Cheung, Angelica Perez-Andujar, Olivier Morin
PurposeTo evaluate the influence of a new commercial transmission detector on radiotherapy beams.Methods and MaterialsA transmission detector designed for online treatment monitoring was characterized on a TrueBeam STx linear accelerator with 6MV, 6FFF, 10MV, and 10FFF beams. Measurements of PDDs, inplane and crossplane off-axis profiles at different depths, transmission factors, and skin dose were acquired with 3x3cm², 5x5cm², 10x10cm², 20x20cm², and 40x40cm² field sizes at 100 cm and 80 cm SSD. A CC04 chamber was used for all profile and transmission factor measurements. Skin dose was assessed at 100 cm, 90 cm, and 80 cm SSD using a variety of detectors (Roos and Markus parallel-plate chambers, and OSLD). Skin dose was also assessed for various patient sample plans with OSLDs.ResultsThe PDDs showed small differences between the unperturbed and perturbed beams for 100 cm SSD (≤4 mm dmax difference, <1.2% average profile difference) for all field sizes. At 80 cm SSD, the differences were larger (≤8 mm dmax 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 cm and 80 cm SSD, respectively. The average measured skin dose increase at 100 cm (80 cm) SSD for 10x10cm² field size was <4% (<35%) for all energies. For 40x40cm² 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%).ConclusionsThe transmission detector has minimal effect on clinically relevant radiotherapy beams for IMRT and VMAT (field sizes 10x10cm² and less). For larger field sizes, some perturbations are observable which would need to be assessed for clinical impact.



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