Assessment of the influence of a carbon fiber tabletop on portal imaging

The purpose of this paper was to investigate beam attenuation caused by a carbon-fiber tabletop and its influence on portal image quality.The dose was measured by a Farmer type jonization chamber. The measurements of the portal image quality were performed with an EPID QC phantom for 6 MV beam for a specified field size (covering all test elements of the phantom completely −26×26 cm2 in the isocenter, SSD 96.2 cm) and various portal—isocenter distances. The beam attenuation factor was measured for Polkam 16 treatment table with a carbon fiber tabletop.Carbon fiber tabletop induces beam attenuation in vertical direction by a factor of 3.39%. The lowest maximum deviation to the regression line for linearity was measured for 40 cm portal—phantom distance. The lowest signal to noise ratio was observed for the portal—phantom distance of 30 cm. This factor dropped by 9% for images with a tabletop. The difference in high contrast: horizontal is 3.64; 0.32; 3.25 for 50 cm, 40 cm and 30 cm respectively and vertical—3.64%; 0.32%; 4.01% for 50 cm, 40 cm and 30 cm respectively. The visibility of the holes with the smallest diameters (1 mm) is the same for 50 and 40 cm while it is better for 30 cm, as can be expected due to the lower SNR.Carbon-fiber inserts, tabletops play a vital role in modern radiotherapy. One of the most important advantages of carbon-fiber tabletops is the lack of the gantry direction limitations. In this paper the attenuation of a carbon-fiber tabletop and its influence on a portal image quality were investigated. Dose attenuation effects, comparable to other measurements, were found. That effect influences dose distribution delivered to the target volume and can increase the time of irradiation needed to take a portal image. It has been found that the best conditions for taking portal image occur when the distance from the phantom (patient) to the portal is 40 cm and the portal is parallel to the tabletop. In such conditions one observes the best image quality parameters and the lowest influence of the carbon fiber tabletop. The insertion of the tabletop causes quality factors change, as can be expected, but these changes can be offset by software and have negligible influence for image quality.Measurements were made in the project: Accelerators & Detectors. Development of dedicated systems based on accelerators and detectors of ionizing radiation for medical therapy and in detection of hazardous materials and toxic wastes (Project co-financed by European Union via European Regional Development Fund).

► We make investigation of image quality factors changes caused by a carbon-fiber tabletop.
► We make images with and without tabletop and various portal—isocenter distances.
► We find tabletop beam attenuation in vertical direction—3.4%.
► We find best phantom (patient)—portal distance for taking portal image—40 cm.
► We find that the presence of tabletop have negligible influence for image quality.