A CT-aided PC-based physical treatment planning of TBI: a method for dose calculation

As for conventional radiotherapy, one of the basic requirements in Total Body Irradiation (TBI) is to know accurately the dose delivered to the entire body. Both the dosimetry and the treatment planning need to be improved. Physical, technical and dosimetrical aspects of TBI have been widely discussed in the literature. However, to our knowledge, no planning systems specifically designed for TBI are commercially available. This article describes a CT-aided PC-based planning system (TBI-Plansys) and its dose calculation algorithm, which applies scatter and inhomogeneity corrections, developed for the TBI technique currently in use at our centre (AP/PA irradiation with patient positioned on his side).

A description of the material and method followed in the dosimetrical procedure is included as it constitutes the basis of the proposed dose calculation algorithm (more than 2D). A Windows programming environment has been used to develop the software.

TBI-Plansys uses patient CT data and indicates absolute and relative dose distributions along midline (at reference points), the transversal axis at the specification point and on transverse sections. The system also calculates the appropriate thicknesses of bolus and shielding to modify undesired dose distributions. TBI-Plansys has been checked against two other well-established systems (beam-zone method and our in vivo semiconductor probe-based system). The checks showed good accuracy with dose differences less than 1% and 3% for homogeneous and inhomogeneous tissues, respectively.

CT calculations by TBI-Plansys allow us to detect undesired distributions which may go unnoticed by calculations at only some specific points. The system has shown clear advantages for routine clinical use as it generates more detailed and accurate information than manual calculations and diminishes the time requirements.