Geometric error of cervical point A calculated through traditional reconstruction procedures for brachytherapy treatment

Brachytherapy used in local cervical cancer is still widely based on 2D standard dose planning with the prescription to point A, which is invisible on imaging and located at a high-dose gradient. In this study, the geometric location error of point A was investigated. It is traditionally reconstructed in the treatment planning system after carefully digitizing the point marks that were previously drawn on the orthogonal radiographs into the system. Two Cartesian coordinates of point A were established and compared. One was built up based on the geometric definition of point A and would be taken as the true coordinate, while the other was built up through traditional clinical treatment procedures and named as the practical coordinate. The orthogonal-film reconstruction technique was used and the location error between the practical and the true coordinate introduced from the variations of, first, the angle between the tandem and the simulator gantry-rotation-axis, and second, the interval between the tandem flange and the simulator isocenter, was analyzed. The location error of point A was higher if the tandem was rotated away from the gantry-rotation-axis or if the location of the tandem flange was set away from the isocenter. If a tandem with a 30-degree curvature was rotated away from the gantry-rotation-axis 10 degrees in the anterior-posterior (AP) view, and there was an 8.7 cm interval between the flange and the isocenter, the location error of point A would be greater than 3 mm without including other errors from simulator calibration, data input, patient setup and movements. To reduce the location error of point A calculated for traditional reconstruction procedures, it is suggested to move the couch or patient to make the mid-point of two points A near the isocenter and the tandem in the AP view parallel to the gantry-rotation-axis as much as possible.