Role of inhomogeneity corrections in three-dimensional photon treatment planning. Photon Treatment Planning Collaborative Working Group.

The role of inhomogeneity corrections in three-dimensional (3-D) radiation treatment planning (RTP) was one of the issues addressed in a National Cancer Institute sponsored research contract. In eight selected disease sites, plans calculated with and without inhomogeneity corrections were compared. The one-dimensional Effective Path Length (EPL) method was used by all four participating institutions as the standard inhomogeneity correction method. However, the dose calculation algorithms were different, particularly in the treatment of blocking effects near the edge of the field. Evaluation tools such as dose-volume histogram, dose statistics, 3-D display of dose distributions and others were used in the comparison. Dose distributions were significantly altered by inhomogeneity corrections in the treatment plans for the lung tumors, and, to a lesser degree, for the breast and Hodgkin's diseases. Dose distributions for tumors of the head and neck region and in the abdomen were not significantly affected. The results should be regarded as particular to the EPL calculations. A treatment plan for the tumor of the larynx was calculated using both the EPL method and a 3-D scatter ray-trace Delta Volume method. For that particular site, inhomogeneity corrections were less important than correctly accounting for the effects of blockings on the scatter dose. Perturbations of electron transport were not accounted for by any of the methods used and not reflected in those sites where the effects were expected to be important. Fully quantitative evaluation of the role of inhomogeneity corrections in treatment planning requires an as yet unavailable all-encompassing accurate method of dose calculation.