Optimization of photon dose distributions with compensation

A homogeneous dose distribution according to the demands of the ICRU-publication 50 often can only be achieved by the use of compensators. Because of the expense those are seldom applied. The purpose of this work is to find practical methods for the production and verification. Two procedures for the production of compensators using 3 different materials and suitable dose verification methods were investigated. The first procedure uses a laser system to get the patient contour, from which the compensator can be calculated. In a second method a 3D planning system calculates the dose modulators on the basis of CT-slices. The production is done by the use of a computer driven milling machine either via a founding form or direct milling. Mixtures of a polymer and lead powder, a mixture of tin granules and wax and the commercially available alloy MCP-96 were used. Dose verification was done using film-, TL- and the three-dimensional MR Fricke gel dosimetry as well as a diode array scanner. Though both methods can be used, the CT-based procedure proved to be more appropriate. Among the materials the direct milled MCP-96 compensator is favorable with respect to the handling, mechanical properties and inhomogeneous radiation attenuation. The dose verification has been done in an Alderson phantom for mantlefield and head-and-neck irradiation techniques. Here the dose modulation yielded an improvement of the homogeneity. The dose maxima normalized to the dose reference point could be reduced from 127% to 103% respectively 122% to 104%. Verifications of compensators for patient treatments confirmed the good results from the phantom measurements. The demonstrated investigations show a practicable way for the clinical application of compensators. The necessity for the use of them can be derived from the verified decrease of the dose maxima.