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trast outlines a large mediastinal collection of contrast as well as a right lower lobe abscess secondary to perforation of the proximal end of the esophageal endoprosthesis.

a distal anchoring point for the prosthesis [36] (Fig. 2.13). Very proximal lesions, such as those near the cricopharyngeus muscle (upper esophageal sphincter), are also prone to malpositioning [47].

Stent migration may be detected by serial examinations showing progressive or abrupt changes in the position of a previously placed stent. Stents have been noted to pass through the entire length of the GI tract until they are expelled through the rectum [48]. Migration may be secondary to inadequate anchoring, especially at the proximal edge of the stricture, or the lack of sufficient radial force exerted by an expandable stent.

Even though coating is not applied to the anchoring ends of coated metal stents, plastic and coated Wallstents have migration rates reported to be as high as 25% [45,49]. Z stents are prone to migration, especially when placed across the GE junction [40,50]. When this occurs, the stent may have to be removed via a gastrostomy, or it may be necessary to insert a second, overlapping stent [47].

Figure 2.13. Migrated Celestin tubes. Supine film of the upper abdomen revealing two Celestin tubes that have migrated from the more proximal esophagus. Note the pneumomediastinum just lateral to the flange of the proximal tube, which lies partially dislodged in the distal esophagus.
Figure 2.14. Tumor ingrowth inside a stent. Two films from a barium esoph-agogram revealing an irregular filling defect at the proximal end of an endo-prosthesis. This represents tumor ingrowth from a carcinoma.

Tumor ingrowth may occur only in uncoated stents, or in cases of covered stents damaged during deployment [46]. The Ultraflex and uncovered Wallstents are most prone to tumor ingrowth [40]. Rates of tumor ingrowth may be as high as 36% for uncovered Nitinol stents [36]. Tumor ingrowth is usually detected approximately 3 months after stent insertion [51]. It may be detected by progressive narrowing of the inner diameter of the stent with irregularity of the stent lumen.

Tumor overgrowth of the stent may occur at the proximal or distal end of the stent, or at both ends (Fig. 2.14). Overgrowth may result in obstruction, or it may leave the patient prone to obstruction from an impacted food bolus. To prevent this complication, the stent should be long enough to extend 1- to 3 cm both proximal and distal to the gross tumor margins [36]. Occasionally, a marked inflammatory response to the presence of the stent may mimic neoplastic overgrowth [40]. The use of coated stents helps to decrease the rate of this hyperplastic tissue response. When tumor overgrowth occurs, it may be treated with an additional, interlocked stent, or laser ablation [47].

Multiple factors may lead to the impaction of a food bolus. These include inadequate chewing and food processing and improper selection of food, as well as an inadequate lumen within the stent (Fig. 2.15).

Figure 2.15. Obstructed endoprosthesis. (A)

Frontal film of the chest reveals an esophageal metallic endoprosthesis in situ. (B) Frontal film from a contrast esophagogram on the same patient reveals marked narrowing and irregu-

Figure 2.15. Obstructed endoprosthesis. (A)

Frontal film of the chest reveals an esophageal metallic endoprosthesis in situ. (B) Frontal film from a contrast esophagogram on the same patient reveals marked narrowing and irregu-

larity to the lumen of the endoprosthesis. This represents clogging from food in debris. (C) CT image of the same patient at the level of the carina reveals almost complete obliteration of the endoprosthesis lumen.

Food impaction has been noted more frequently in rigid stents [52]. It is less common in self-expanding metal stents, reflecting their larger internal diameters. The finding of one or more lobulated filling defects at the proximal end, or within a stent, should suggest an impacted food bolus. The differential diagnosis includes tumor in- or overgrowth, as well as stent migration uncovering the primary lesion. Endoscopic removal of the offending material is the usual treatment [47].

A more immediate, and potentially fatal complication is tracheal compression, which may necessitate stent retrieval, intubation, or placement of a tracheal stent [40]. The sudden onset usually precludes imaging.

Esophageal-airway fistulas are a common indication for esophageal stent placement. When left untreated, most patients expire within a month of diagnosis [53]. Esophageal stenting can result in a 90% closure rate for these fistulas [47]. However, continued filling of an airway fistula can be considered to be a complication (or failure) of stent placement. This lack of closure may be due to passage of contrast through the stent lumen, around the distal end of the prosthesis, and then, in a retrograde fashion, along the external wall of the stent until the fistula is reached [47]. Alternatively, if the esophagus proximal to the lesion is dilated to a diameter that exceeds the largest stent external diameter (28F), contrast will leak around the proximal end, leading to filling of the fistula [47]. Stent malpositioning or migration may also uncover a previously occluded fistula.

Infolding of the stent lumen is another possible complication. It is more commonly seen in patients with Ultraflex stents, and to a lesser extent, the Z stent [40]. It may be secondary to tumor growth exceeding the radial strength of the stent or to defective expansion during deployment [36]. Balloon dilatation may correct the problem, although in at least one case, the passage of the endoscope itself dilated the stent [36].

As MR imaging of the thorax increases in usage, caution must be used when a patient has an esophageal stent in situ. The European version of the Wallstent is made with titanium and does not pose a problem during imaging [54]. However, the U.S. version of the same stent is made with stainless steel and is ferromagnetic. Therefore it is theoretically susceptible to both attractive forces and torque while in the bore of a magnet. The Z stent is similarly affected. A useful guideline, similar to that used for patients with endovascular stents, is to wait 6 weeks after deployment before imaging. This allows sufficient fibrotic reaction to relatively fix the stent in place.

Another factor to be considered is the type of magnet to be used for imaging. In most closed-bore systems, the static field is aligned with the long axis of the patient. Because an esophageal stent generally is also aligned along the same axis, there should be relatively little torque applied to the stent. However, some open-air magnets have their static magnetic field aligned perpendicular to the patient's long axis. In these units, a ferromagnetic stent may be subject to considerable torque and possible dislodgment. Studies performed in both types of unit may be subject to image degradation secondary to ferromagnetic artifact due to stainless steel, but not titanium-based stents [40].

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