Park et al. both processes leads to luminal stenosis and stent failure (3). The pathophysiology of PIH remains unclear, but biliary leakage into the shunt, exposed liver parenchyma and injured hepatic veins have been suggested as being the initiators of hyperplasia (1, 4). However, their significance is not unanimously accepted (5). Irradiation with using external X-ray beam or intralumi- nal brachytherapy was originally introduced for treating the stent stenosis, following arterial myointimal hyperpla- sia, that occurred after coronary or noncoronary arterial intervention, and this type of irradiation treatment has been successfully performed and reported upon (6 10). Based on the similar proliferating processes of myointimal hyperplasia and PIH, irradiation therapy was thought to be useful for limiting the PIH in TIPS. However, its utility for treating PIH in TIPS has not been validated. To the best of our knowledge, there has been only one experimental study by Lessie et al. that has evaluated the ability of intraluminal irradiation to treat PIH in TIPS (11). They applied irradiation for TIPS in a swine model and they reported that a low-dose (15.2 Gy) of beta irradiation, which has been commonly used for arterial stent treatment, did not significantly reduce the development of PIH in TIPS. They explained that this negative result could be caused by the differences in the origin of the proliferat- ing cells and their migration distance between myointimal hyperplasia and PIH, and an insufficient irradiation dosage was also a factor. We hypothesized that administering a higher dose of beta irradiation than the dose used in the coronary intervention to the hepatic parenchymal tract at the time of TIPS creation could inhibit hyperplasia and preserve stent patency. Therefore, the purpose of this study was to evaluate the ability of intraluminal irradiation with high- dose (30 Gy) Holmium-166 (166Ho) to reduce hyperplasia and prevent stenosis of TIPS in a portal hypertension- induced swine model. MATERIALS AND METHODS The experimental protocol of our study was used in all 12 domestic pigs (Yorkshire pigs, 2 months old and weighing 15 20 kg each), after approval by the animal care committees at this institution. All the pigs were in good physical condition prior to the experiment. The techniques and instrumentations used to create TIPS were similar to that used for humans. This investigation aimed to compare the results of the irradiated stent with the control stent at a follow-up interval of two weeks. Five pigs of the irradiated group underwent intraluminal irradiation of the intrahepatic parenchymal tract during balloon inflation of 128 TIPS, whereas the seven pigs in the control group underwent TIPS without irradiation. 166Ho for Irradiation Therapy 166Ho is a beta-emitting radioisotope. In our study, the targeted therapeutic dose for TIPS was set at 30 Gy. Prior to the experimental study in animals, we conducted an in vitro pilot study to determine the optimal concentration activity and exposure time of 166Ho for acquisition of the desired absorbed radiation dosage. The liquid form of 166Ho-(NO3)3 5H2O-DTPA was provided by the Korea Atomic Energy Research Institute (KAERI, Taejon, Korea). It was prepared by a neutron (n, ) reaction on abundant, non-radioactive 165Ho that was at a thermal flux state and by simple mixing the 166Ho with DTPA at room temperature. The produced 166Ho had a specific activity of 7.4 GBq mg 1 with a physical half-life of 26.8 hour, and it emitted high energy -particles with maximum energies of 1.85 MeV (51%) and 1.77 MeV (48%). A portion of rays and K-shell X-rays were also emitted. The range of absorbed -particles distribution was 8.6 mm at maximum and 1.2 mm on average (12, 13). GafChromic film (MD-55, Model Mo. 37 041, Lot #941206, ISP Technologies Inc., Wayne, NJ) was used for estimating the radiation dosage. A 60Co teletherapy beam and a 6 MV photon beam were used to obtain the dose optical density calibration curves for the GafChromic film. The exposed films were read by using a videodensitometer (Model # WP700, Weellhofer Co., Schwarzenbruck, Germany), with changing the radiation dosage. The angioplasty balloon catheter (10 mm-diameter 4 cm- length, Cook, USA) was filled with 3.16 mL of 166Ho solution (concentration activities: 1,850 MBq (50 mCi/mL) and 3,700 MBq (100 mCi/mL)) and the balloon was inflated to 3 atm pressure. The balloon was positioned in a solid water phantom, and GafChromic film was placed and adhered closely on the surface of balloon at an orthogonal direction. The exposure times for the desired absorbed dose (30 Gy) at the balloon surface were calculated with using the different concentration activities (1,850 MBq and 3,700 MBq) (Table 1). Finally, the balloon was filled with 166Ho at a concentration activity of 1,850 MBq and it was inflated to 3 atm for 4.5 minutes, delivering an absorbed surface dosage of 30 Gy, which was the dose we decided to use in the experimental study. Creation of Portal Hypertension and TIPS The animals were initially sedated with intramuscular injections of 2 mL ketamine hydrochloride (Ketalar, Yuhan, Seoul, Korea) and 2 mL xylazine hydrochloride (Rompun, Korean J Radiol 8(2), April 2007