Development of prototype shielded cervical intracavitary brachytherapy applicators compatible with CT and MR imaging

Purpose: Intracavitary brachytherapy (ICBT) is an integral part of the treatment regimen for cervical cancer and, generally, outcome in terms of local disease control and complications is a function of dose to the disease bed and critical structures, respectively. Therefore, it is paramount to accurately determine the dose given via ICBT to the tumor bed as well as critical structures. This is greatly facilitated through the use of advanced three-dimensional imaging modalities, such as CT and MR, to delineate critical and target structures with an ICBT applicator inserted in vivo. These methods are not possible when using a shielded applicator due to the image artifacts generated by interovoid shielding. The authors present two prototype shielded ICBT applicators that can be utilized for artifact-free CT image acquisition. They also investigate the MR amenability and dosimetry of a novel tungsten-alloy shielding material to extend the functionality of these devices. Methods: To accomplish artifact-free CT image acquisition, a "step-and-shoot" (S&S) methodology was utilized, which exploits the prototype applicators movable interovoid shielding. Both prototypes were placed in imaging phantoms that positioned the applicators in clinically applicable orientations. CT image sets were acquired of the prototype applicators as well as a shielded Fletcher-Williamson (sFW) ovoid. Artifacts present in each CT image set were qualitatively compared for each prototype applicator following the S&S methodology and the sFW. To test the novel tungsten-alloy shielding material's MR amenability, they constructed a phantom applicator that mimics the basic components of an ICBT ovoid. This phantom applicator positions the MR-compatible shields in orientations equivalent to the sFW bladder and rectal shields. MR images were acquired within a gadopentetate dimeglumine-doped water tank using standard pulse sequences and examined for artifacts. In addition, Monte Carlo simulations were performed to match the attenuation due to the thickness of this new shield type with current, clinically utilized ovoid shields and a Ir192 HDR/PDR source. Results: Artifact-free CT images could be acquired of both generation applicators in a clinically applicable geometry using the S&S method. MR images were acquired of the phantom applicator containing shields, which contained minimal, clinically relevant artifacts. The thickness required to match the dosimetry of the MR-compatible and sFW rectal shields was determined using Monte Carlo simulations. Conclusions: Utilizing a S&S imaging method in conjunction with prototype applicators that feature movable interovoid shields, they were able to acquire artifact-free CT image sets in a clinically applicable geometry. MR images were acquired of a phantom applicator that contained shields composed of a novel tungsten alloy. Artifacts were largely limited to regions within the ovoid cap and are of no clinical interest. The second generation A₃ utilizes this material for interovoid shielding.