A General Method to Calculate Dose to Spine in Opposing Anterior Posterior Treatment Setup Without CT

Abstract

The spinal cord is usually part of the treatment volume in the treatment of the thorax and abdomen areas using antero-posterior (AP) and postero-anterior (PA) ports. Thus it is necessary to know the dose the spinal cotd would receive. One needs to locate the spine to calculate the dose at that location. In the absence of CT, a set of orthogonal films is taken at simulation to aid in locating the spine. The relative point dose then is calculated using irregular field point dose calculation mode of the treatment planning system. The data needed for the calculation are the x and y coordinates of the spine in the coronal plane, air gap, and depth in tissue. Correct entry of these is thus important to obtain the dose accurately. Planning systems assume that the spine is close to the beam axis. There could be possible mislocation of the spine when this assumption is not valid. This presentation describes a procedure to obtain the needed numbers for a general location without any assumptions. To obtain x and y coordinates the suggestion is made to solve numerically using successive approximation series for the coordinates, the series continued until mutually consistent values are obtained. The other parameters, g and d, are obtained from the source to skin distance (SSD) above the spine point, with the use of multiple transaxial contours in association with the principle of linear interpolation. Besides providing the correct geometric parameters, this procedure also allows use of transaxial contours with the spine located on them to run isodose plans and thus have an additional way of obtaining the spine dose.