Development of prototype shielded cervical intracavitary brachytherapy applicators compatible with CT and MR imaging
The first-generation prototype ovoid used for this study. (a) Computer aided rendering of the FGP . Major components labeled: (a) Movable shield, (b) shield carrier, (c) distal stop pins, (d) shield retainers, (e) threaded shaft (tread detail omitted), (f) proximal stop, (g) pinion gear assembly, (h) outer rotating sleeve, (i) flexible drive wire, and (j) proximal shield stop pin. The polysulfone cap that surrounds to ovoid is not shown. The shield’s position is adjusted by rotating the flexible drive wire. The rotational and translational movements of the shield are coupled. (b) Photographic for external dimension comparison of a sFW (top) and FGP ovoid (bottom). (c) Photograph of the FGP ovoid. Shown is the translucent ovoid cap that allows for visual confirmation of the shield orientation. Coin included for scale comparison.
The second-generation prototype ovoid used for this study. (a) Bird’s eye view of the SGP . The ovoid handles are covered by cylindrical caps that protect the handle mechanics during the sterilization process. (b) Photograph of the SGP handle sliders that independently control translation of the interovoid bladder and rectal shields. Circled is the locking mechanism that ensures shields are locked into their default orientation during treatment. (c) Photograph of the SGP ovoids and interuterine tandem. (d) Computer aided rendering of the SGP ovoid with cap removed. Visible is the bladder and rectal shields in their default positioning which duplicates the interovoid shielding of the sFW.
Determination of optimal CT bore tilt angle. Shown in the left image is a lateral projection of a scout film acquired of a single ovoid inserted into an imaging phantom. The phantom has been tilted to mimic the geometry of an applicator that has been inserted into a patient. Shown in the right image is a screen capture of a GE Lightspeed CT control GUI with the measurement tool applet selected and the angular measurement tool is circled. This tool has been used to measure the acute angle between the scanner table and the face of the shield. The optimal bore tilt is then determined using: .
Step 3 of the S&S method. In each image, the planes selected for image acquisition are indicated by diagonal lines. The movable shield has been outlined using a rectangle in each image for clarity. A single has been inserted into an imaging phantom that has been positioned to mimic the position of the applicator when inserted in vivo. The lateral scout image shown on the left shows the manual selection of the first series of image planes. Prior to the commencement of and during scanning, the movable shield is positioned as far distally as possible with respect to the patient. Once image planes cannot be defined that do not transverse the shield, the STP is defined (represented by an arrow). At this point, the scanner is paused and the shield is actuated to its most proximal position (right image). A second imaging series is then defined that completes image acquisition for the rest of the patient anatomy.
Definition of . Shown is a lateral projection of a CT image of a patient with an applicator inserted in vivo. The patient orientation is indicated by the cartoon shown at the lower right. In this example, .
Comparison of reconstructed CT images lying in planes passing through the bladder and rectal shields of a single sFSD, sFW ovoid, and the corresponding planes for the FGP ovoid inserted within a phantom that positions the ovoids in a clinically relevant orientation. The CT scans of the FGP were acquired using the S&S technique. The window and level of the images were adjusted to reduce artifacts while at the same time allowing visualization of the phantom, which acts as a surrogate for the patient body.
(a) Series of CT images in planes that intersect a majority of the SGP applicator system colpostats, S&S method not employed. The images were acquired in a proximal to distal direction with respect to an applicator’s orientation when inserted in vivo. The series progresses in a left to right, top to bottom manner in the figure. Metal artifacts are prevalent in images acquired in planes that intersect the shields of the SGP that are positioned in an orientation that is equivalent to the position of the sFW interovoid shields. (b) Series of CT images in planes that intersect a majority of the SGP applicator system ovoids, S&S method employed. The images were acquired in a proximal to distal direction with respect to an applicator’s orientation when inserted in vivo. The series progresses in a left to right, top to bottom manner in the figure. Metal artifacts are not prevalent in any image.
Depth-dose-rate per activity (cGy/mCi h) as a function of simulated shield thickness and material. Over the range of depths presented here ( from the distal surface of the modeled shield), lie the greatest differences in the attenuative properties of the MR-compatible HM3000 shield as a function of thickness. The dashed line indicates the depth at which the two-dimensional isodose comparisons shown in Fig. 9 were simulated.
Comparison of the MC-simulated isodose-rate [ for a source] distributions resulting from a thick Densimet-17 shield and a (left figure) and (right figure) thick HM3000 MR compatible shield in a plane from, and parallel to, the distal surface of the shields. Dose perturbations due to the shield can be seen in the lower quadrant of both isodose-rate plots.
MR images acquired of the shielded ovoid using a 2D T2-FSE (top row) and 2D T1-SE (bottom row) pulse sequences. The image planes correspond to the default location of the bladder [(a) and (d)] and rectal shields [(c) and (f)] in a sFW ovoid. Images (b) and (e) are taken in a plane that contains no shielding material and bisects the long axis of the ovoid.
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