Technical considerations in the use of 3-D beam arrangements in the abdomen

The practical utilization of 3-D treatment planning to reduce doses to normal tissues in the abdomen is illustrated for irradiation of hepatic masses using fields with central axes rotated out of the transverse plane. The beams were arranged to go through the minimum amount of normal liver tissue, while exiting above or below a kidney. Although these beam arrangements were not coplanar with standard transverse body sections, they were designed for dose delivery through use of standard Megavoltage equipment. The planning process for these techniques illustrates the need for and use of several tools usually associated with 3-D treatment planning systems. Beam's eye-view planning with perspective display of the relevant anatomy in the projective beam geometry is required for designing the placement of focused blocks for these oblique fields. Three-dimensional volumetric dose calculations are required to evaluate dose distributions. Additionally, port-film-type radiographs, digitally reconstructed from the CT dataset, are found to be useful in understanding the correctness of simulation and verification films. The reduction in dose to normal tissues over that achievable using standard plans with beams entering the patient at right angles to the central axis of the body is illustrated using dose-volume histograms. These techniques have allowed the initiation of a radiation dose escalation protocol for tumors involving the liver and porta hepatis.