Examination of a dose evaluation method for floor-mounted kV X-ray image-guided radiation therapy systems.

Floor-mounted kV X-ray image-guided radiation therapy systems (such as ExacTrac) are used to treat tumors in movable body regions such as the head and neck. ExacTrac requires exposure dose control based on image acquisition. However, placement of the aluminum plate perpendicular to the dosimeter is difficult due to the oblique X-ray beam path. This study provides an evaluation of the entrance surface, organ, and effective doses delivered to the head and neck during image-guided radiation therapy. A semiconductor detector (Unfors Xi) and radiophotoluminescent glass dosimeter (RPLD) were used as dosimeters with entrance surface dose calculation (Sdec) and exposure estimation (PCXMC) software. When the Unfors Xi was placed in front of the flat panel detector (FPD), an accurate entrance surface dose was obtained by considering the X-ray attenuation due to the influence of the distance from X-ray tube to FPD.The couch absorption was 32% and incident air kerma at the patient entrance point was 0.094 mGy for Unfors Xi. The entrance surface doses were 0.12 and 0.122 mGy for the Sdec and RPLD, respectively. The thyroid organ doses were 0.044 and 0.058 mGy for the RPLD and PCXMC, respectively. The effective doses of PCXMC for ExacTrac and cone-beam computed tomography were 0.0068 and 0.31 mSv, respectively. The exposure dose of ExacTrac was approximately 2% of that of cone-beam computed tomography. The results demonstrated that the proposed software-based method is effective for the assessment and management of ExacTrac exposure.