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 insertedin 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 CTimage 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 CTimage 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. CTimage sets were acquired of the prototype applicators as well as a shielded Fletcher–Williamson (sFW) ovoid. Artifacts present in each CTimage 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. MRimages 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 HDR/PDR source.Results:
Artifact-free CTimages could be acquired of both generation applicators in a clinically applicable geometry using the S&S method. MRimages 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 CTimage sets in a clinically applicable geometry. MRimages 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 utilizes this material for interovoid shielding.
This work was partially supported by the Nucletron Corporation (Nucletron B.V., Veenendaal, The Netherlands).
II.A.1. CT-amenable attributes of the prototypes
II.A.2. S&S CTimaging method
II.A.3. Experimental CT acquisition setup
II.B.1. Development of MR-compatible interovoid shielding
II.B.2. Matching the dosimetric properties of the sFW and MR-compatible interovoid shielding
II.B.3. MRimaging parameters
III. RESULTS AND DISCUSSION
III.B.1. Dosimetric matching of the sFW and MR amenable interovoid shielding
III.B.2. MRimaging of the novel tungsten-alloy shields
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