The use of trans-applicator intracavitary ultrasonography in brachytherapy for cervical cancer: phantom study of a novel approach to 3D image-guided brachytherapy

Purpose: To assess the feasibility of applying trans-applicator intracavitary ultrasonography to image-guided brachytherapy for cervical cancer.

Material and methods: For this experiment, a phantom was created and included a polyethylene tube, intended to simulate a tandem applicator, which was inserted into chicken meat and embedded in agar, along with magnetic resonance imaging (MRI)-compatible ovoid applicators. Three-dimensional images of the phantom were obtained using computed tomography (CT), MRI (T2-weighted), and intracavitary ultrasonography sectional images acquired at 1 mm slice intervals. Intracavitary ultrasonography images were acquired from within the simulated tandem applicator using a radial transducer. Magnetic resonance imaging and intracavitary ultrasonography images were manually registered onto CT images. The chicken meat was contoured as the target volume independently on the CT, MRI, and intracavitary ultrasonography images, and the Dice similarity coefficient was used to compare the target volumes. The dose distributions of a sample brachytherapy plan were also evaluated.

Results: Computed tomography, MRI, and intracavitary ultrasonography all visualized the three-dimensional phantom volumes. Intracavitary ultrasonography images depicted the meat with high echoic signals and a border clearly distinguishable from the surrounding agar. The Dice similarity coefficient values for the target volumes on CT vs. MRI, CT vs. intracavitary ultrasonography, and MRI vs. intracavitary ultrasonography were 0.966, 0.965, and 0.971, respectively, indicating similar contouring with the three modalities. Among the modalities, the differences in D₅₀, D₉₀, D₉₈, and D₁₀₀ values were 1.8%, 2.9%, 3.7%, and 2.9%, respectively.

Conclusions: Three-dimensional reconstructed trans-applicator intracavitary ultrasonographic images clearly depicted meat tissue within the phantom, and could thus be used for brachytherapy planning. This study proves the concept of trans-applicator intracavitary ultrasonography for intracavitary brachytherapy. Further research such as development of intracavitary ultrasonography system, 3D reconstruction method, ultrasonography-compatible applicators, and ultrasonography-based target concept is warranted to assess the potential clinical application.