Inverse planning has stirred considerable excitement in the community. The objective is to make the dose conform to the tumor and spare other organs. Instead of resorting to the time-consuming inverse-planning process, we tried an extension of the conventional treatment planning technique. The beam orientation and wedge angles are chosen in the conventional rule-based manner. However, within each beam’s eye view, we use multiple field openings. One field opening conforms to tumor only with the appropriate margin. The other field openings are smaller, aside from blocking the rest of the tissue other than the tumor, they also block the beam from irradiating the organs to be spared which otherwise would be inside the beam if the first field opening is employed. As the number of organs to be spared increases, the number of field for each beam increases. The Cimmino simultaneous projection method was employed to obtain the optimized weighting for each field of each beam. We found that if the dose constraint for tumor and critical organ is reasonable, we are able to obtain a plan with limited number of beams that satisfy the specified dose objective.

The beam orientations are pre-selected. The figure shows a prostate case with rectum as the critical organ to be spared.

For each beam orientation, open and wedged beam, multiple beam openings are chosen. One opening includes the tumor volume with a margin while other openings also try to protect certain critical organ. The following figures illustrate the blocking. The organ to be treated is the prostate in color blue and the organ to be protected is the rectum in color dark cyan.

With different beam orientations and multiple beam openings for open and wedged beam, it is virtually impossible to manually optimize the weighting of each beam. Fortunately, to reach the specified dose limits for the tumor volume and the critical organ to be avoided is just a question of solving a set of inequality equations. The set of inequality equations are solved using Cimmino algorithm:

As a result, desired dose distribution can be obtained with the tumor getting treated with relatively high dose while sparing the critical organ. The concavity in the isodose line can be achieved and is evident in the figure.

Abstracts Submitted to 1999 Annual AAPM Meeting | Medical Physics Home