830 Bult et al. (1.5–2.0% O2/air (1:1)). For post-administration imaging, the animals were sedated using dexmedetomidine at 50 mg kg−1 and ketamine at 15 mg kg−1. The animals were euthanized by intravenous injection of 600 mg pentobarbital. puncture wound in the liver was sealed with tissue glue (Histoacryl, B. Braun, Melsungen, Germany). Thereafter the abdominal muscle layer and skin layers were sutured in separate layers using Vicryl 3.0 (Johnson&Johnson Intl., St- Stevens-Woluwe, Belgium). Tumor Implantation Dose Preparation A subxiphoid laparotomy was performed by ventral mid- line incision, in order to expose the liver lobes. Three tumor pieces were implanted into the left lateral liver lobe with an intravenous catheter (Abbocath−T 16 G, Hospira, Donegal Town, Ireland). After implantation of tumor tissue, the Neutron irradiation of approximately 60 mg HoAcAcMS for 1 h resulted in an activity of approximately 600 MBq 166HoAcAcMS. The 166HoAcAcMS were suspended in 1.2 mL of sterile water containing 2% Pluronic® F68 and 10% ethanol abs. to obtain a final concentration of Fig. 2 Scanning electron micrographs of HoAcAcMS and 166HoAcAcMS after incubation for different times in phosphate buffer. (a–c) HoAcAcMS, (a) after 1 month, (b) after 3 months and (c) after 6 months of incubation. (d–f) 166HoAcAcMS neutron irradiated for 3 h at a neutron flux of 5×1012 n cm−2 s−1 (d) after 1 month, (e) after 3 months and (f) after 6 months of incubation. (g–i) 166HoAcAcMS neutron irradiated for 6 h at a neutron flux of 5× 1012 n cm−2 s−1 (g) after 1 month, (h) after 3 months and (i) after 6 months of incubation. The magnification in all images is 1000x, the bar represents 50 μm.