Selection of brachytherapy applicators based on tumor size and shape for cervical cancer: simulation analysis of pear-shaped isodose dimensions

The purpose of study is to measure Point A pear-shaped isodose dimensions of the conventional intracavitary brachytherapy with various sizes of colpostats and analyze which size of tumor is the optimal for 3-D interstitial brachytherapy. CT simulation was performed with Fletcher type applicator using various sizes of colpostats (2.0, 2.5, and 3.0 cm diameter). The Manchester standard loading (dwell time) system was used to generate pear-shaped isodose envelopes with high-dose rate iridium-192 according to the colpostat sizes. The size of the pear-shaped envelope was measured at 5 different levels: A-level (center of the colpostats), B-level (top of the colpostats), C-level (between B and D), D-level (Point A), and E-level (1.0 cm above Point A). In this study, it was assumed that uterine tandem was located at the center of tumor. For width of pear-shape: At the A-level, 6.4, 7.3, and 8.0 cm for 2.0, 2.5, and 3.0 cm colpostats, respectively. At the B-level, 5.8, 6.4, and 6.8 cm for 2, 2.5, and 3.0 cm colpostats, respectively. At the C-level, 4.6, 4.8, and 4.8 for 2.0, 2.5, and 3.0 cm colpostats, respectively. At the D-level, 4.0 cm for all different size. At the E-level, 3.8 cm for all 3 different size colpostats. A-level was the largest dimension of pear-shape. However, it was located in the upper vagina below the main cervical mass. The center of the effective pear-shape size for tumor was between the C and D levels. For thickness, all 5 different levels were ranging 3.7 to 4.0 cm. For height, the length of height was dependent on the tandem length. Therefore, the pear-shape envelope was able to accommodate up to 4.0 cm diameter volume. According to our analysis of conventional pear-shape dimension, 3-D interstitial brachytherapy should be considered for tumors larger than 4.0 cm for symmetrical tumor.