COMPARISONS OF SPATIAL COORDINATES OF SOURCES CALCULATED BY THREE METHODS BASED ON TWO RADIOGRAPHS IN BRACHYTHERAPY

Spatial coordinates of brachytherapy sources calculated by three methods, conventional geometrical formulas, the least squares method and approximation method with constant magnification, were compared with each other using a clinical case. The conventional geometrical formulas are theoretically correct, but sometimes induce greater errors than the ones produced by the approximation method. In particular, when a position is near the XZ plane consisting of two X-ray beam central axes and is not near the origin of spatial coordinates, the conventional geometrical formulas easily magnify the errors in the measured image coordinates on the radiographs. Because of small error propagation, the least squares method is very useful for clinical applications to calculate the position using all measured image coordinates.