The aim of this study was to assess the dependence of the normal liver tissue threshold dose on the volume exposed and the catheter geometry-dependent dose gradients for single-fraction high-dose-rate brachytherapy of malignant liver lesions. A total of 50 patients with malignant liver tumors treated with CT-guided high-dose-rate ¹⁹²Ir brachytherapy were included. Dose planning was performed using a three-dimensional CT data set acquired after percutaneous applicator positioning. Magnetic resonance imaging (MRI), performed 6 and 12 weeks after therapy, was analyzed retrospectively. All MRI data sets were merged with 3D dosimetry data. The border of hyperintensity on T2-weighted images (edema) and of hypointensity on T1-weighted images (impaired hepatocyte function) were analyzed to assess the radiation effect. The threshold isodose surface of the volume exposed was calculated from the 3D dosimetry data. The relationships between irradiated volume and threshold isodose surface as well as dose gradient and threshold isodose surface were evaluated over time. The median threshold dose of the volume exposed, characterized by hepatocyte dysfunction and edema, was ≈13 Gy 6 weeks after irradiation and ≈16 Gy at 12 weeks. We found a significant correlation between the normal liver tissue threshold dose and volume exposed (P < 0.0001). The 12-week threshold dose was estimated between ≈14 Gy for 500 cm³, ≈16 Gy for 100 cm³, and ≈18 Gy for 10 cm³ of irradiated volume. The results indicate that the dose gradient has no effect on the threshold liver dose. There was a significant shift of the threshold doses from regions of lower to regions of higher-dose exposure in the course of follow-up (P < 0.0001). Thus the normal liver tissue threshold dose is dependent on the volume exposed but not on the dose gradient.