The determination of virtual source position using convergent arcCOS method for scanning-passive scatter beam in carbon ion therapy

We developed a convergent arcCOS (cACOS) technique capable of dealing with the virtual source position delivered by different carbon ion energies from the pattern of scanning-passive scatter beam in this study.

A homemade large-format CMOS sensor and Gaf Chromic EBT3 films were used for the virtual source position measurement. The Gaf films were embedded in a self-designed rectangular plastic frame to tighten the films and set up on a treatment couch for irradiation in the air with the film perpendicular to the carbon ion beam at the point of nominal source-axis-distance (SAD) as well as upstream and downstream from the SAD. The horizontal carbon ion beam with 5 energies at a machine opening field size was carried out in this study. The virtual source position was determined with a convergent arcCOS method and compared with the linear regression by back projecting the FWHM to zero at a distance upstream from the various point of source-film distance.

The determination of virtual source position using convergent arcCOS method agrees with the method by back projecting the FWHM to zero, and for higher carbon ion energy has an obvious longer distance from the SAD since the more carbon ion beam energy, the less spreading affected by the horizontal and vertical magnetism, therefore, the distance of virtual source positions is decreased from SAD with high to low energy.

We have developed a technique capable of dealing with the virtual source position with a convergent arcCOS method to avoid any manual measurement mistakes in scanning-passive scatter carbon ion beam. The method for investigating the virtual source position in the carbon ion beam in this study can also be used for external electrons and the proton.