On the beam hardening correction of the Transit-Guided Radiation Therapy attenuation model.

PURPOSE:The Transit-Guided Radiation Therapy (TGRT) technique is a novel technique aimed to quantify the position error of a patient by using the transit portal images (TPI) of the treatment fields. Despite of the promising preliminary results, about 4% of the cases would have led to position overcorrections. In this work, the TGRT formalism is refined to improve its accuracy and, especially, to decrease the risk of overcorrections. METHODS:A second free parameter accounting for beam hardening has been added to the attenuation model of the TGRT formalism. Five treatment plans combining different delivery techniques and tumour sites have been delivered to an anthropomorphic phantom. TPIs have been obtained under a set of random couch shifts for each field. For each TPI, both the original and the refined TGRT formalism have been used to estimate the underlying true shift. RESULTS:With respect the original formalism, the refined formalism: (i) decreased both the number (from 5% to 1%) and the magnitude of the overcorrections; (ii) lowered the detection threshold (from approximately 1 mm to <0.3 mm); (iii) largely improved the accuracy in tumour sites with large mass thickness variations; and (iv) largely improved the accuracy for true shifts below 5 mm. For true shifts above 5 mm, the accuracy was slightly impaired. CONCLUSIONS:The refined TGRT formalism performed globally better than the original TGRT formalism and it largely reduced the risk of overcorrections. Further refinements of the TGRT formalism should focus on true shifts above 5 mm.