CROSS-SECTIONAL ANATOMY RECONSTRUCTION AND APPLICATIONS TO RADIATION THERAPY TREATMENT PLANNING.

In the present practice of radiation therapy, a treatment regimen for a patient usually is developed by examination of dose distributions computed for one or more treat-ment plans proposed for the patient. Rarely are the dose distributions corrected for perturbations introduced by inhomogeneities such as lung and bone between the beam entrance surfaces and the treated region. These perturbations are not accounted for, even though they may affect the tumor dose by up to 20-30 percent in certain situations, primarily because the exact location and extent of the inhomogeneities within the patient are unknown. Various techniques have been proposed for delineation of patient cross-sectional anatomy and identification of inhomogeneities: (1) Visualization of cross-sectional anatomical displays from orthogonal roentgenograms projected in a special viewing device. This approach is complicated and provides no quantitive information for treatment planning; (2) Display of crosssectional anatomy on roentgenograms obtained directly by transverse tomography. The cost of transverse tomographic equipment capable of flexible patient position-ing and the poor quality of transverse tomographic images are the major limitations of this technique; (3) Representation of cross-sectional anatomy by obtaining individual patient contours and superimposing standard anatomical displays available in pictoral atlases. This method is no more than a crude approximation, because it offers no compensation fort anatomical variations among patients; however, limitations in these techniques have prevented their adoption in more than a few radiation therapy centers.