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 192Ir 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.

Comparison of Monte Carlo calculations around a Fletcher Suit Delclos ovoid with radiochromic film and normoxic polymer gel dosimetry.

The Fletcher Suit Delclos (FSD) ovoids employed in intracavitary brachytherapy (ICB) for cervical cancer contain shields to reduce dose to the bladder and rectum. Many treatment planning systems (TPS) do not include the shields and other ovoid structures in the dose calculation. Instead, TPSs calculate dose by summing the dose contributions from the individual sources and ignoring ovoid structures such as the shields. The goal of this work was to calculate the dose distribution with Monte Carlo around a Selectron FSD ovoid and compare these calculations with radiochromic film (RCF) and normoxic polymer gel dosimetry. Monte Carlo calculations were performed with MCNPX 2.5.c for a single Selectron FSD ovoid with and without shields. RCF measurements were performed in a plane parallel to and displaced laterally 1.25 cm from the long axis of the ovoid. MAGIC gel measurements were performed in a polymethylmethacrylate phantom. RCF and MAGIC gel were irradiated with four 33 microGy m2 h(-1) Cs-137 pellets for a period of 24 h. Results indicated that MCNPX calculated dose to within +/- 2% or 2 mm for 98% of points compared with RCF measurements and to within +/- 3% or 3 mm for 98% of points compared with MAGIC gel measurements. It is concluded that MCNPX 2.5.c can calculate dose accurately in the presence of the ovoid shields, that RCF and MAGIC gel can demonstrate the effect of ovoid shields on the dose distribution and the ovoid shields reduce the dose by as much as 50%.

Pharmacokinetics and magnetic resonance imaging of biodegradable macromolecular blood-pool contrast agent PG-Gd in non-human primates: a pilot study.

The purpose of this study was to evaluate poly(L-glutamic acid)-benzyl-DTPA-Gd (PG-Gd), a new biodegradable macromolecular magnetic resonance imaging contrast agent, for its pharmacokinetics and MRI enhancement in nonhuman primates. Studies were performed in rhesus monkeys at intravenous doses of 0.01, 0.02 and 0.08 mmol Gd/kg. T(1)-weighted MR images were acquired at 1.5 T using fast spoiled gradient recalled echo and fast spin echo imaging protocols. The small-molecule contrast agent Magnevist was used as a control. PG-Gd in the monkey showed a bi-exponential disposition. The initial blood concentrations within 2 h of PG-Gd administration were much higher than those for Magnevist. The high blood concentration of PG-Gd was consistent with the MR imaging data, which showed prolonged circulation of PG-Gd in the blood pool. Enhancement of blood vessels and organs with a high blood perfusion (heart, liver, and kidney) was clearly visualized at 2 h after contrast injection at the three doses used. A greater than proportional increase of the area under the blood concentration-time curve was observed when the administered single dose was increased from 0.01 to 0.08 mmol/kg. By 2 days after PG-Gd injection, the contrast agent was mostly cleared from all major organs, including kidney. The mean residence time was 15 h at the 0.08 mmol/kg dose. A similar pharmacokinetic profile was observed in mice, with a mean residence time of 5.4 h and a volume of distribution at steady-state of 85.5 ml/kg, indicating that the drug was mainly distributed in the blood compartment. Based on this pilot study, further investigations on the potential systemic toxicity of PG-Gd in both rodents and large animals are warranted before testing this agent in humans.