Background Intra-arterial radioembolization with yttrium-90 microspheres ( 90 Y-RE) is an increasingly used therapy for patients with unresectable liver malignancies. Over the last decade, radioactive holmium-166 poly(L-lactic acid) microspheres ( 166 Ho-PLLA-MS) have been developed as a possible alternative to 90 Y-RE. Next to high-energy beta-radiation, 166 Ho also emits gamma-radiation, which allows for imaging by gamma scintigraphy. In addition, Ho is a highly paramagnetic element and can therefore be visualized by MRI. These imaging modalities are useful for assessment of the biodistribution, and allow dosimetry through quantitative analysis of the scintigraphic and MR images. Previous studies have demonstrated the safety of 166 Ho-PLLA-MS radioembolization ( 166 Ho-RE) in animals. The aim of this phase I trial is to assess the safety and toxicity profile of 166 Ho-RE in patients with liver metastases. Methods The HEPAR study (Holmium Embolization Particles for Arterial Radiotherapy) is a non-randomized, open label, safety study. We aim to include 15 to 24 patients with liver metastases of any origin, who have chemotherapy-refractory disease and who are not amenable to surgical resection. Prior to treatment, in addition to the standard technetium-99m labelled macroaggregated albumin ( 99m Tc-MAA) dose, a low radioactive safety dose of 60-mg 166 Ho-PLLA-MS will be administered. Patients are treated in 4 cohorts of 3-6 patients, according to a standard dose escalation protocol (20 Gy, 40 Gy, 60 Gy, and 80 Gy, respectively). The primary objective will be to establish the maximum tolerated radiation dose of 166 Ho-PLLA-MS. Secondary objectives are to assess tumour response, biodistribution, performance status, quality of life, and to compare the 166 Ho-PLLA-MS safety dose and the 99m Tc-MAA dose distributions with respect to the ability to accurately predict microsphere distribution. Discussion This will be the first clinical study on 166 Ho-RE. Based on preclinical studies, it is expected that 166 Ho-RE has a safety and toxicity profile comparable to that of 90 Y-RE. The biochemical and radionuclide characteristics of 166 Ho-PLLA-MS that enable accurate dosimetry calculations and biodistribution assessment may however improve the overall safety of the procedure. Trial registration ClinicalTrials.gov NCT01031784