Park et al. Follow-up Venography On follow-up venography, all of the five irradiated stents were patent (5/5, 100%) (Fig. 1A). Meanwhile, the 5 TIPS in the control group were completely occluded (5/6, 83.3%) (Fig. 2A) while the remaining TIPS was partially occluded (1/6, 16.7%) (Fig. 3A). All of shunt stenoses or occlusions developed in the hepatic parenchymal tract of TIPS, while both the hepatic vein and portal vein portions of the TIPS stents had relatively well-preserved lumens. Histological Analysis On histological analysis, the gross specimens in all cases demonstrated partial or complete covering of the stent by glistening white tissue (Fig. 2B). On portal venography, the predominant site of stenosis was within the parenchymal tract. Under microscopic examination, the granulation tissue was proved to be PIH, which was composed of numerous spindle-shaped myofibroblasts, extracellular collagen matrix, undifferentiated mesenchymal cells and inflammatory cells surrounding the stent wires (Figs. 1B, 2C, 2D, 3B). The specimens stained with both H-E and Masson-trichrome revealed that the layers of pseudointi- mal hyperplasia were sharply demarcated from the surrounding hepatic parenchymal tissue layer. The adjacent hepatic parenchyma appeared normal, except for minimal protrusion into the luminal surface. Masson- trichrome staining confirmed that the intercellular matrix within the PIH was predominately composed of collagen. In addition, a single cell layer that mimicked endothelial cells was below various amounts of fibrin thrombus deposits, which was the most superficial layer and this was seen on the luminal surface. These findings showed similar- ities with the PIH that occurred in human cases (3, 4). The low-powered magnified histologic specimens obtained from the irradiated stents revealed a thinner and more uniform layer of hyperplasia (Fig. 1B) than that in the nonirradiated control group (Table 2). No other histologic difference, such as the composition and order of arrange- ment, was evident between the two groups. Microscopic transverse sections through the portal or hepatic veins showed that the shunt lumen was continu- ously lined with a single layer of endothelial cell. Proliferation of smooth muscle cells was also noted between the stent and luminal surface, and this represented intimal hyperplasia. Compared with the PIH at the parenchymal level, the intimal hyperplasia at the venous levels had insignificant thickness and it was smooth in contour. No bile staining or bile duct injury was seen in any of the obtained specimens. 132 Table 2. Stent Patency and the Percentage of the Maximum Pseudointimal Hyperplasia (PIH) within the Parenchymal Tract of Each Animal Animal No. Pig 1 Pig 2 Pig 3 Pig 4 Pig 5 Pig 6 Pig 7 Pig 8 Pig 9 Pig 10 Pig 11 Pig 12 Study Group Control Control Control Control Control Control Control Irradiated Irradiated Irradiated Irradiated Irradiated Stent Patency Completely occluded Partially occluded Completely occluded Completely occluded Completely occluded Completely occluded Patent Patent Patent Patent Patent Maximum PIH (%) N/A* 73 68 83 80 77 75 21 52 27 23 41 Note. Maximum PIH (%) = thickness of PIH / estimated TIPS radius x 100%, * Pig 1. Periprocedural death due to peritoneal bleeding at post- TIPS 1st day. Statistical Analysis The average maximum PIH, which was estimated as a percentage of luminal stenosis per stent radius, was 32.8 13.27% in the irradiated group and 76.0 5.29% in the control group, respectively. Two-sample t-test analysis of the PIH in the irradiated versus the nonirradiated stents demonstrated a statistically significant difference in the maximum thickness of the PIH (p < 0.001). The mean portal pressure estimated during the procedure was 13.15 3.41 cmH2O and 26.78 5.80 cmH2O before and after the injection of NCBA, respec- tively (mean increase = 13.63 cmH2O), and this immedi- ately dropped back to 15.76 cmH2O after TIPS creation (mean reduction = 11.02 cmH2O), demonstrating the successful induction of portal hypertension by NCBA embolization, and this was followed by satisfactory reduction of the elevated portal pressure after stent insertion with statistical significance (t-test, p < 0.01). DISCUSSION Irradiation therapy has been reported to be very effective in preventing and reducing neointimal hyperpla- sia following coronary or noncoronary arterial interven- tion. The rationale for the use of irradiation to inhibit neointimal hyperplasia is the high susceptibility of actively proliferating cells to the lethal effects of ionizing radiation. Experimental studies have shown that external irradiation or low-dose (12 20 Gy) endovascular irradiation from various sources, i.e., high energy or emitters such as Iridium-192 (192Ir) (6), Yttrium-90 (90Y) (7), Phosphorus-32 Korean J Radiol 8(2), April 2007