Dynamics of pear-shaped dimensions and volume of intracavitary brachytherapy in cancer of the cervix: A desirable pear shape in the era of three-dimensional treatment planning

Purpose: To evaluate the dynamics of pear-shaped dimensions and volume of the intracavitary brachytherapy, and to define a desirable pear-shape in the era of three-dimensional (3D) treatment planning. Methods and Materials: Since Point A has been used for the dose specification, the pear shape defined the surface enclosed by Point A. This study utilized a new method of evaluating pear-shaped dimensions and its configuration. The pear shape was artificially divided into tandem and colpostat portions for evaluation of its changes. Width, height, and thickness at the tandem portion (W-t, H-t, and T-t) and at the colpostat portion (W-c, H-c, and T-c) were defined, respectively, on the frontal and sagittal plane, To evaluate the dynamics of the pear-shape configuration, 12 variations of applicator geometry and source loading were applied to generate the pear-shape isodose line and dose-volume histogram. Results: When the source strengths in the colpostats were reduced for optimization with the same dose to Point A dose, W-c, H-c, and T-c were decreased, whereas W-t, H-t, and T-t were increased without a change in the overall pear-shaped volume. When the separation of the colpostats was increased without a change in the source strength, W-c was increased, whereas H-c and T-c were reduced without a change in W-t, H-t, T-t and overall pear-shape volume. When the separation of colpostats and distal tandem source were increased, these changes at the colpostat portion were magnified. However, when both colpostat separation and its source strength were increased proportionally, W-c, H-c, and T-c were increased proportionally as well as its volume. Conclusion: The dose specification at Point A is less meaningful without a desirable pear shape encompassing the tumor around the cervix. In the era of 3D treatment planning, understanding the dynamics of the pear shape should improve the individualized dosimetry according to tumor size and location. The relationships between a desirable pear shape and its tumor coverage should establish a more reliable dose specification for cancer of the cervix. (C) 1997 Elsevier Science Inc.